'{-L'NO'f. STATE GEOLOGICAL SURVEY

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STATE OF ILLINOIS

STATE GEOLOGICAL SURVEY

FRANK W. DE WOLF, Director

Cooperative Goal Alining Series

BULLETIN 17

SURFACE SUBSIDENCE IN ILLINOIS

RESULTING FROM

COAL MINING

BY

LEWIS E. YOUNG

ILLINOIS COAL MINING INVESTIGATIONS

( Prepared under a cooperative agreement between the Illinois State- Geological Suxv« v.

the Kngineering Experiment Station of the University oi Illinois, and

the U. S. Bureau of Mines.)

I'KINTKO BY AU'I IIOKITY OF THE STATE OF ILLINOIS

ILLINOIS STATE GEOLOGICAL SURVEY

UNIVERSITY OK ILLINOIS

URBANA

L916

CONTENTS

PAGE

Chapter I — Introduction 11

Area investigated . , 11

Scope and object of report , 11

Acknowledgments 13

Illinois coal field and production data 15

Chapter II — Geologic conditions affecting subsidence 18

Description of Illinois "Coal Measures" 18

Strata associated with Illinois coal beds 21

Cleat 26

Faults and rolls 27

General relations to subsidence 27

Coal No. 2 27

Coal No. 5 27

Coal No. 6 28

Coal No. 7 29

Chapter III — Damage caused by removal of coal 30

Contrasting effects of longwall and room-and-pillar methods 30

Surface evidences of subsidence 30

Surface cracks and displacements 30

Pit holes or caves 34

Sags 42

Agriculture and surface subsidence 50

Importance of agriculture 50

Extent and value of farm lands 50

Value of coal lands 54

Nature of damage to agricultural lands 56

Restoring agricultural lands 58

Transportation and surface subsidence 61

Railroads 61

Wagon roads 64

Streams and canals 64

Buildings and improvements affected by subsidence 65

Buildings 65

Streets, pipe lines, and sewers 69

Water supply 71

Municipal waterworks 71

Reservations of coal 72

Chapter IV — Subsidence data by districts 73

Introductory statement 73

District 1 75

District II 78

District III 78

District IV 79

District V 81

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PAGE

District VI 83

District VII 8S

District VIII 86

Chapter V— Protection of surface 88

General considerations °°

Pillars 88

Crushing strength of coal °°

Angle of break and angle o f d raw °9

Safe depth 9 1

Theory as previously advanced 93

Increase in volume of rock by breaking 93

Compressibility of broken rock 94 !

Shaft pillars 10°

Room pillars

Filling methods.

103

Gobbing 103

Hydraulic filling 104

Griffith's method of filling 104'

Artificial supports ^5

Chapter VI— Investigations of subsidence 106

European 10°

United States '. 106

Suggested Illinois investigations 106

Considerations 106

Typical mines 107

Monuments and surveys 107

Underground work 107

Possible benefits 10°

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ILLUSTRATIONS

PLATE PAGE

I. General geological sections for southwestern Illinois 18

II. General geological sections in coal field of Williamson and Franklin counties 20

III. Graphic sections in Longwall District 22

IV. Cross-sections CD and EF showing structure across the southern and north-

ern parts of the Danville field 24

FIGURE

1. Map of Illinois showing extent of the "Coal Measures" 12

2. Map of Illinois showing distribution of thick, medium, and thin coal 14

3. Map showing area underlain by principal coal seams 16

4. Graphic section showing persistent nature of limestones in the McLeansboro

formation 20

5. Diagrammatic illustration showing angle to break and angle of subsidence. ... 31

6. Crack in the soil caused by room-and-pillar mining at Streator 32

7. Plan of two panels of Franklin County mine showing relation of subsidence

to underground workings 33

8. Crack due to subsidence over north panel shown in figure 7 34

9. Cracks due to subsidence over south panel shown in figure 7 35

Cracks and breaks due to shear 36

Buckling of frozen sod due to compression along the axis of a sag 36

Surface breaks in a field near Carterville 37

Surface breaks over a mine near Harrisburg 37

14. Plan showing workings of a mine and location of subsidence movements ad-

j acent to a dike 38

Plan of a mine showing details of movement near a dike IS

Breaks in a pasture over an abandoned mine west of Danville 39

Side-hill breaks near Cuba caused by room-and-pillar mining 39

Large area covered by pit holes in the suburbs of a town in central Illinois. . . 40

Cave-in south of Dewmaine 40

Detail of a cave on a hillside near Streator 41

|21. Side-hill breaks over shallow workings near Streator 41

22. Plan of a 40-acre tract in southern Illinois showing relation of approximately

60 pit holes to underground workings 42

23. Topographic map of the Coal City-Wilmington area 43

24. Pond in a sag due to longwall mining at Coal City 44

25. Plan of a mine in Perry County showing relation of sags to underground

workings 45

Telephone poles tipped toward a sag over a mine in Franklin County 46

Swamp in a typical sag in Randolph County 47

Pond in a sag near Clifford, Williamson County 48

Flooding of a large tract due to subsidence in Franklin County 48

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FIGURE PAGE

30. Pond due to subsidence at Westville, Vermilion County 49

31. Open space in a cornfield south of Danville 49

32. Drowned-out area of corn near Nokomis 49

33. Map showing the relation of different thicknesses of coal to values of farm

lands in 1910 51

34. Map showing location of tile drains laid by a mining company to drain sags. . 58

35. Pit hole filled with rubbish at Electric mine near Danville 59

36. Pit holes and cracks over shallow mine near Streator 60

37. Pit holes near Streator filled with mine rock and soil 60

38. Railroad track in Franklin County lowered by subsidence over room-and-

pillar mine 62

39. Plan showing relation of railroad in figure 38 to mine workings 63

40. Repaired railroad trestle in Franklin County 64

41. House near Coal City lowered 9 inches at one corner 65

42. Brick house near Danville abandoned on account of danger by room-and-

pillar mining 66

43. Plan showing relation of house in figure 42 to the pillar in the mine 67

44. Houses in Springfield for which claims were paid by a mining company 68

45. Small house near Streator beside which a pit hole has formed 68

46. Flooded streets, broken sidewalks and foundations, and damages to plaster-

ing resulting from subsidence in Franklin County 69

47. Barn of mining company lowered 4 feet at one end 69

48. Broken sidewalk caused by subsidence in Franklin County 70

49. Map showing division of State into districts 74

50. Cracks in the immediate roof of a longwall mine in the La Salle area 90

51. Stress diagram of an ideal homogeneous cantilever 91

52. Stress diagram of a low-down neutral surface 92

53a. Diagrammatic illustration through the gob and parallel to the face of a long- wall mine 96

b. Plan showing pack walls and loose gob between roadways and cross entries

in the same mine 96

54. Diagrammatic illustration showing the flow of roof shale under pressure in

a mine near Peoria 98

55. Diagrammatic illustration showing the size of shaft pillars for given depths

as recommended by various mining engineers

56. Diagrammatic illustration showing the extent of subsidence resulting from

the removal of adjacent pillars 102

99

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TABLES

PAGE

1. Output of Illinois coal by coal seams 15

2. Output of Illinois coal by depth of shaft 17

3. Character of roof and floor of the commercial coal beds throughout Illinois.. 24

4. Value of farm lands, April 15, 1910 53

5. Value of surface and of coal rights by counties in Illinois 55

6. Assessed value of coal rights and of agricultural lands by counties 56

7. Districts into which the State has been divided for the purpose of investigation 73

8. Alphabetical arrangement of coal-producing counties 75

9. Longwall mines in Illinois 76

10. Data on subsidence in District IV 80

11. Data on subsidence at typical mines in District V 83

12. Data on subsidence at typical mines in District VI 84

13. Data on subsidence at typical mines in District VII 85

14. Compression tests of Illinois coals 89

15. Volumes of different materials after crushing as compared with volumes "in

the solid" 94

16. Compressibility of different materials after having been crushed 94

17. Compressibilty tests upon crushed materials made by the United States Bureau

of Mines 95

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SURFACE SUBSIDENCE IN ILLINOIS RESULTING FROM COAL MINING

By Lewis E. Young

CHAPTER I— INTRODUCTION Area Investigated

During the summer of 1914 there was made a preliminary study of surface subsidence resulting from coal mining operations in Illinois. Mines were visited in 24 of the 52 counties in which shipping mines are located. These 24 counties produced 94 per cent of the coal mined in the State in 1913. Within these 24 counties are 324 of the 371 shipping mines listed in 1913 and 293 of the 469 local mines. The counties visited were Bureau, Christian, Clinton, Franklin, Fulton, Grundy, Jackson, La Salle, Livingston, Macon, Macoupin, Madison, Marion, Montgomery, Peoria, Perry, Randolph, St. Clair, Saline, San- gamon, Vermilion, Washington, Will, and Williamson. In all but three of these counties coal mining has resulted in subsidence of such in- tensity as to result in substantial damage to surface property.

Data were secured from 108 companies operating 149 separate plants, and in addition, the field notes of the Cooperative Investiga- tion upon 100 Illinois mines supplied data on 8 mines not visited for this particular purpose.

Scope and Object of Report

Prior to this preliminary survey no work upon the subsidence problem in Illinois had been undertaken by any scientific bureau. For over a quarter of a century there has been litigation between the operators of coal mines and the owners of the surface when surface subsidence has been attributed to coal mining operations. There is a need for more definite knowledge concerning (1) the extent to which the complete removal of coal may disturb the overlying rock strata, (2) the percentage of coal that may be mined without disturbing the surface, (3) the methods of mining that should be employed in order to minimize the surface movement following the removal of the coal,

(4) the methods of protecting the surface by artificial supports, and

(5) the best methods and policies to be employed in conserving the mining and agricultural resources of the State.

At the present time there is more or less unrest in certain coal mining districts in Illinois on account of minor damage to the surface.

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map or

ILLINOIS

Fig. 1.— Map of Illinois showing extent of the "Coal Measures."

INTRODUCTION 13

At a number of points mine operators are being forced to pay claims for damages much in excess of the actual cost of restoring the injured property. There is an urgent need for definite data upon the experience of operators in the various districts, so that those who mine coal in the future may be able to recover the maximum percentage of the coal with the least damage to the surface or with the briefest inter- ference with the use of the surface property.

It is undoubtedly to the interest of the State to conserve the coal supply by securing as complete extraction as possible from the areas in which mining is now being carried on. Almost one-half of the coal is being left in the ground and made unavailable for future mining, and in some places a large part of this abandoned coal is left because the mine operators fear that by removing a larger per- centage they may cause surface subsidence and be required to pay damages out of proportion to the real value of the surface. In many places the surface may be restored at little expense. But until the prevailing conditions are known accurately and the essential facts are understood and studied, the State cannot consider or propose meas- ures to relieve the mining industry of this severe burden.

With these problems and difficulties in view, this preliminary survey was made, and the data collected show in an impartial manner the situation as it exists in Illinois at the present time.

Acknowledgments The writer wishes to acknowledge his indebtedness to Mr. G. S. Rice, Chief Mining Engineer, U. S. Bureau of Mines ; Professor H. H. Stoek, head of the Department of Mining Engineering, University of Illinois; and Mr. F. W. DeWolf, Director of the Illinois State Geo- logical Survey, under whose combined direction the work of the in- vestigation has been carried on. Mr. S. (..). Andros of the De- partment of Mining Engineering, University of Illinois, lias been very helpful. The various State and county mine inspectors have been uniformly courteous in furnishing much useful in- formation. Messrs. II. I. Smith and J. R. Fleming, Assistant Min- ing Engineers, U. S. Bureau of Mines, have made valuable suggestions and have furnished numerous photographs for which acknowledg- ments are made in proper place. Mr. R. Y. Williams, formerly mining engineer, U. S. Bureau of Mines, furnished much of the data on Saline County. To the following representatives of the coal mining industry in Illinois the author is deeply indebted for many courtesies and for close cooperation : Mr. W. M. Cole, Mining Engineer, Spring Valley Coal Company; Mr. J. Quaid, General Superintendent, Big Creek Coal Company; Mr. C. II. Nicolet, Engineer, Matthiessen and Hegeler Coal Company; Mr. R. D. Brown, Chief Engineer, O'Gara

Fig. 2. — Map of Illinois showing distribution of thick, medium, and thin coal. (After Bement.)

INTRODUCTION 15

Coal Company; Mr. R. Williams, Mining Engineer, Saline County Coal Company ; Mr. R. Forrester, Superintendent, Paradise Coal Com- pany; Mr. A. J. Moorshead, President, Madison Coal Corporation; Mr. G. E. Lyman, Chief Engineer, Madison Coal Corporation; Mr. John P. Reese, Superintendent, Superior Coal Company; Mr. S. F. Jorgensen, Chief Engineer, Superior Coal Company ; Mr. T. P. Brew- ster, General Manager, Mount Olive and Staunton Coal Company; Mr. James Dubois, Superintendent, Dering Coal Company; Mr. Thomas Moses, General Superintendent, Bunsen Coal Company; Mr. M. F. Peltier, Chief Engineer, Peabody Coal Company; Mr. J. A. Garcia, Mining Engineer; Mr. D. J. Carroll, Chicago, Wilmington & Franklin Coal Company.

Illinois Coal Field and Production Data

Estimates made by the Illinois State Geological Survey show that the known coal areas of Illinois contained approximately 175,000,000,- 000 tons of coal in beds not less than three feet in thickness. The coal-bearing formations underlie part or all of 86 counties, an area of about 36,800 square miles (fig. 1). It may be noted that 674 square miles are underlain by several coal beds over 3 feet thick or an aggre- gate thickness of 15 feet; 3,883 square miles are underlain with an aggregate thickness of 11 feet; 12,546 square miles with an aggre- gate thickness of 7 feet; and 10,184 square miles with 3 feet of coal. Figure 2 shows approximately the extent of the thick, medium, and thin coal; and figure 3 shows the area underlain by the principal coal seams.

Practically the entire output of Illinois coal in 1913 was pro- duced from 5 seams, as shown in the accompanying table.1

Table 1. — Output of Illinois coal by coal scams (Year ended June 30, 1913)

Tons

Coal No. 1 500,000

Coal No. 2 5,650,000

Coal No. 5 13,500,000

Coal No. 6 41,400 000

Coal No. 7 750,000

Total from 5 seams 61,800,000

n^otal for the State as given by Illinois Coal Report, 1913, was 61,846,204 tons.

Fig. 3.— Map showing area underlain by principal coal seams. (After Bement.)

INTRODUCTION 17

The greater portion of the output comes from shafts 100 to 400 feet in depth. Table 2 shows the relation of tonnage to depth.1

Table 2. — Output of Illinois coal by depth of shaft (Year ended June 30, 1913)

shaft Number of shafts Output

Tons

489 6,500,000

159 17,400,000

73 11,500,000

46 11,500,000

33 8,750,000

18 2,750,000

12 2,350,000

6 720,000

2 180,000

1 105,000

1 75,000

Depth

of

Feet

0-

99

100-

199

200-

299

300-

399

400-

499

500-

599

600-

699

700-

799

800-

899

900-

999

1,000-1,099

Total 61,830,000

CHAPTER II— GEOLOGICAL CONDITIONS AFFECTING SUBSIDENCE

It is evident that whatever movement may result from under- ground mining will be influenced greatly by the nature of the rock over- lying the coal seam being worked. As noted in Table 2, the greater part of the output of Illinois coal is from shafts not exceeding 400 feet in depth, and practically 50 per cent of the output is from shafts not over 300 feet deep. Throughout the greater part of the State the coal beds lie practically flat.

Description of Illinois "Coal Measures"

The following extracts1 give a concise statement of the essential facts concerning the coal-bearing formations. Plate I shows general geological sections for southwestern Illinois, Plate II shows sections in the Williamson-Franklin field, Plate III shows sections in the coal fields of northern Illinois, and Plate IV shows sections in the Dan- ville field.

The coals of Illinois exist as widespread beds or as local pockets among layers of shale, sandstone, and limestone which together make up the Pennsyl- vanian series. There are five coals known to be important enough to have special names, and numerous other beds occur at various depths. The maximum thickness of Pennsylvanian rocks is known to be at least 2,200 feet, though it is not to be assumed that a single bore hole could penetrate all of the various forma- tions at the place of extreme thickness.

The Pennsylvanian series has been divided for convenience of description into three formations which present different characteristics as to time of deposi- tion, physical composition, and economic importance. The divisions from the bottom upwards are the Pottsville, Carbondale, and McLeansboro formations.

POTTSVILLE FORMATION

The lowermost formation of the Pennsylvanian in Illinois is composed chiefly of massive sandstones interrupted by thinner beds of shale and beds or pockets of coal and fire clay.

In Illinois this formation carries plant fragments which indicate, according to White," that deposition took place during late Pottsville time of the Appala- chian coal basins. The early sediments are thinner to the north and west and are nearly or quite absent over much of the State. Massive sandstones in the Rock Island region are of Pottsville age, but later than the first sediments of southern Illinois. There are a number of occurrences of coal in the Pottsville in southern counties but at present they have no commercial importance, because

Preliminary report on organization and method of investigations, Illinois Coal Mining Investigations, pp. 48-51, 1913.

2White, David, 111. State Geol. Survey Bull. 14, p. 293, 1908.

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ILLINOIS STATU GEOLOGICAL SURVEY

COOPERATIVE COAL MINING SERIES BULLETIN 17, PLATE I

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CONDITIONS AFFECTING SUBSIDENCE 19

of local or pockety character. Coal of No. 1 of Rock Island and Mercer counties is similarly restricted in area but is thick enough to be valuable. The topmost sediments of the Pottsville in Illinois include the Cheltenham fire clays, which have been traced through the counties between St. Louis and Rock Island, and thence eastward to Ottawa. The Pottsville closed just before the deposition of coal No. 2. (Murphysboro, La Salle, Third Vein, etc.)

CARBONDALE FORMATION

The second division of the Pennsylvanian series extends from the base of coal No. 2 up to the top of coal No. 6. (Herrin, Belleville, Blue band, etc.). It represents a time interval comparable to the Allegheny formation of the eastern states, though the close of Allegheny time in Illinois has not been definitely deter- mined and may include some of the strata lying above the Carbondale as here defined.

The Carbondale is composed chiefly of shale and lesser amounts of sand- stone, coal, and limestone. It includes all the coals mined for commercial ship- ment except the Rock Island (No. 1) bed, and the Danville (No. 7) bed. This formation extends over practically the whole coal-area, but its upper beds are absent along the rim and its lower beds are not well known in the central part of the basin. The thickness varies considerably, being from 200 to 240 feet in the La Salle region, 200 feet at Peoria, 300 feet at Mattoon, and 285 feet or more in the southern counties of the coal field.

MCLEANSBORO FORMATION

The topmost division begins at the top of coal No. 6 and extends up to the highest Pennsylvanian rocks of the State (fig. 4). With the exception of the Danville coal (No. 7) which at present is included in this formation, there are no coals of known importance, although a thin bed in Shelby County is mined for local use. The formation is dominated by beds of shale and sand- stone, among which are found some thin limestones. The Carlinville (Shoal Creek) limestone occurs about 275 feet above the base of the formation and is persistent over a considerable area. The greatest thickness of the formation seems to be in the vicinity of Hamilton and White counties where coal No. 6 lies approximately 1000 feet below the surface. A somewhat less reliable record at Olney indicates a depth of 1155 feet for this coal.

SPOON-SHAPED STRUCTURAL BASIN

The strata beneath the surface deposits of Illinois to a great extent lie horizontal, but locally dip as much as 350 feet to the mile. When all the evi- dence from mine shafts and bore holes is studied it is evident that the State is an immense spoon-shaped basin with the tip in the extreme northwest counties and the bowl in the region of Wayne, Edwards, Hamilton, and White counties. The long axis of the "spoon" passes near Olney in Richland County and Loving- ton in Moultrie County, and the dip in the central part of the basin towards this axis is commonly as low as 10 feet per mile.

Thus, an east-west section from Springfield eastward to Cerro Gordo in Piatt County has a dip of 300 feet in 50 miles or 6 feet per mile. Similiarly, the dip eastward from Iuka in Marion County to Olney in Richland County is 400 feet in 40 miles or 10 feet per mile.

20

SURFACE SUBSIDENCE IN ILLINOIS

The warping of the strata along the southwestern and southern rim of the basin has been much more pronounced and has been somewhat relieved by the Shawneetown fault, which extends as a narrow belt from western Kentucky into Illinois at Shawneetown and has been traced at least 15 miles farther west. This fault causes the strata on the north to be about 1400 feet lower than those on the south. North of the fault and along the border of the active coal field, the dip from Cottage Grove in Saline County northward to Eldorado averages 115 feet per mile. Similarly from Marion northward to West Frankfort it aver- ages 50 feet per mile and locally is double this amount. In the hills 5 miles

New Haven

Shoal Creek

Coal No.8

Coal No.7 |g Top

Shoal Creek

Coal No.8

Coal No.7 Coal

New Haven

Shoal Creek

Coal No.8

Coal No.7

Shoal Creek

700

600

600

300

30 |s^t Coal No.7 6

■pic 4.— Graphic sections showing persistent nature of limestones in the Mc- Leansboro formation.

northwest of Murphysboro the dip is eastward at a rate of 350 feet per mile. The same pitch exists also in the area one mile east of Duquoin.

Although the Illinois basin is approximately spoon shaped, there are minor folds and terrace-like flats on the flanks. The most notable is the La Salle anti- cline, which is pronounced at La Salle and also in the oil fields of Clark, Craw- ford, and Lawrence counties. Doubtless it extends as a persistent feature with a rather uniform direction between these distant areas. The west side, facing

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ILLINOIS STATE GEOLOGICAL SURVEY

COOPERATIVE COAL MINING SERIES BULLETIN NO. 17, PLATE II

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2 Sec. 30, T. 4 S..R.2 E.

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3 Sec. 8, T. 10 S..R. 1 E.

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4 Sec. 35, T. 5 S.,R. 4. E.

5 Sec. 19.T.6 S..R.3 E.

6 Sec. 32. T. 7 S..R. 1 E.

FEET 800

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91

54 Coal No.5

Graphic sections of borings showing the general character of the McLeansboro formation in District VI and Shelby County

CONDITIONS AFFECTING SUBSIDENCE 21

the axis of the basin, is much steeper than the east flank. Another pronounced anticline has been traced from the region 5 miles west of Murphysboro to Du- quoin and thence northward to Centralia and Sandoval. This Duquoin anti- cline is also steeper on the side facing the axis of the basin, in this case the east side. Folds and faults of appreciable size other than those mentioned have been discovered, and doubtless they will be found to be numerous, as detailed surveys proceed.

The structural relations in Illinois are on the whole unusually favorable to coal mining. The first effect is of course to make the coal around the border of the field more easily available than that in the deeper portion, but the extreme depth necessary to reach the most important coal (No. 6, Herrin, Blue band) probably will not greatly exceed 1000 feet, and the shaft at Assumption is already operating successfully to approximately this depth.

Strata Associated with Illinois Coal Beds

It is important that attention be directed to the thick strata of limestone, shale, and sandstone, which lie above the various important coal seams, as these may affect the rate and amount of subsidence when underlying seams are mined.3

As previously noted coal No. 1 lies in the Pottsville formation, but the important beds of sandstone and of conglomerate lie below this seam. The Illinois strata in the Pottsville above this seam are largely fire clays and shales.

The Carbondale formation extends from the base of coal No. 2 to the top of coal No. 6. In northern Illinois the boundary between the Carbondale and the overlying McLeansboro formation is not as clearly marked as in southern Illinois. The Carbondale formation in southern Illinois is composed chiefly of shale but includes some sand- stones and limestones. The thickness varies from 200 to 300 feet. In several typical bore holes, both limestones and sandstones are shown, but the thickest single beds of each are about 8 feet. In Jackson County, however, the Vergennes sandstone lies from 20 to 40 feet above coal No. 2. This is persistent but irregular in thickness (15 to 45 feet) and varies from a sandstone to a sandy shale. It has been de- scribed as micaceous, loose, and friable and can not be regarded as a bed that will be strong enough to check subsidence of the overlying beds.4

Overlying coal No. 6 is the McLeansboro formation in which only coal No. 7 is of commercial importance. The following beds of this formation are reasonably persistent, whereas some of their inter- vening beds vary greatly in character.

8For detailed reports on the geology of the districts see the Cooperative Bulletins Nos. 10, 11, 14, and 15.

4Shaw, E. W. and Savage, T. E., U. S. Geol. Survey Geol. Atlas Murphyshoro-Herrin folio (No. 185), p. 7, 1912.

22 SURFACE SUBSIDENCE IN ILLINOIS

The well-marked lithologic units of the McLeansboro (in District VII) may be enumerated as follows5 :

5. New Haven limestone, 200 to 250 feet above Carlinville limestone (thickness about 25 feet).

4. Shoal Creek limestone, about 100 feet above the Carlinville (thick- ness 12 to 25 feet).

3. Carlinville limestone, from 200 feet to a little more than 300 feet above coal No. 6 (average thickness 7 feet).

2. A bed of pink, red, or variegated shale, variable in thickness, aver- aging 35 to 50 feet above coal No. 6 (seldom exceeds 15 feet in thickness).

1. A hard limestone overlying or slightly above coal No. 6 (averages 7 feet in thickness).

The New Haven limestone is quite persistent, as indicated graph - ically in the records from Moultrie, Shelby, Montgomery, and Fayette counties in District VII. Mr. G. H. Cady reports it is present over parts of Franklin County from 500 to 550 feet above coal No. 6. It is a solid bed given in most logs as 25 feet thick.

The Shoal Creek limestone is from 12 to 25 feet thick and lies 275 to 350 feet above coal No. 6 in District VII. It is described as lacking the homogeneity of the Carlinville and in places consists of a series of more or less argillaceous limestone layers.6 This limestone apparently does not occur continuously over District VI.

"The Carlinville limestone is one of the most widely distributed in the 'Coal Measures' of Illinois. It has been traced from north of Carlinville, Macoupin County, southeast to the Indiana line in Gallatin County. In the type localities this limestone is, according to Udden, 'generally bluish gray, compact, close textured, and very hard, break- ing into irregular, splintery pieces. It averages about seven feet in thickness'. In most of District VII the interval between this limestone and coal No. 6 averages from 275 to 325 feet."7 In District VI this stratum occurs from 250 to 300 feet above coal No. 6 but is thin and has not been mentioned in many of the records.8

In parts of Saline and Gallatin counties there occurs a hard sand- stone, known locally as "Anvil Rock", a sandstone about 10 feet above coal No. 6. In several of the small mines on coal No. 6 in Gallatin

5Kay, Fred H., Coal resources of District VII: 111. Coal Mining Investigations Bull. 11, p. 23, 1915.

6Lee, Wallace, acknowledgements to, Illinois Coal Mining Investigatons Bull. 11, p. 26, 1915.

7Kay, Fred H., Coal resources of District VII: 111. Coal Mining Investigations Bull. 11, p. 25, 1915.

8Cady, G. H., Coal resources of District VI: HI. Coal Mining Investigations Bull. 15, p. 34, 1916.

1

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11 1 INOIS STATE GEOLOGICAL SURVEY

COOPERATIVE COAL MINING SERIES BULLETIN NO. 17. TLATE III

13

10

6

McLeansboro <!

#.

I Pottsvill

=!S i Niagara wee I

I

Lower Magneslan pr-r;

LEGEND

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LEGEND

ROCK

111 ll

E3

E=l

Fireclay

Sandy shale

ES

Graphic sections in the Longwall District. Scale : 1 inch equals 200 feet

3

CONDITIONS AFFECTING SUBSIDENCE

23

County, it serves as the immediate roof. There the limestone lying above coal No. 6 is described as the limestone cap rock. Throughout District VII this limestone is generally not less than 2 feet thick, although in several small areas it is entirely missing. The range com- monly is from 5 to 10 feet. In some places the cap rock is underlain by black "slate" or shale a few feet in thickness, and in others by a gray shale known as "white top."9

In District VI the cap rock occurs over a large part of the district within 25 feet above coal No. 6. It is described as a compact, heavy- bedded limestone, commonly dark or almost black when fresh. It may be as thick as 11 feet, but averages 4 or 5 feet. Over the western part of District VI the cap rock is missing, or is at a greater distance above the coal.10

A log given as typical of the eastern part of District VII shows, in a total depth of 658 feet 6 inches to the top of coal No. 6, a total thickness of 24 feet of limestone and 15 feet of sandstone.

In the Longwall District the strata of the Pennsylvanian series are not so persistent as in southern Illinois. Graphic sections in the Longwall District are shown on Plate III. So far as is known no single limestone stratum, except the Lonsdale limestone, is traceable from one point to another in the Longwall District, and even the Lons- dale limestone is not readily identified in drill records. It contains a large amount of argillaceous material and is easily mistaken for shale in drilling.11 "The lower 5 feet. consists of a firmly cemented, largely organic limestone in beds varying in thickness from 6 inches to 1 foot 6 inches. Above these firm beds there are 15 feet of a slightly argilla- ceous and more flaggy rock, in which concretionary structures can nearly always be detected."12 It occurs near the base of the upper sec- tion of the McLeansboro formation and 50 to 75 feet above coal No. 7.

In the upper section of the McLeansboro and about 400 feet above coal No. 2 or 175 feet above coal No. 7 occurs the La Salle limestone, varying from 25 to 30 feet in thickness. "It has a very local distribution, being confined to a belt about two miles broad and extending parallel to its outcrop along the anticline from Bailey Falls on the south to the NE. yA sec. 28, T. 34 N., R. 1 E., three miles north of La Salle. The belt is much wider in the middle than at either q\\(\.

"Kay, Fred H., Coal resources of District VII: 111. Coal Mining Investigations Bull. 11, p. 24, 1915.

10Cady, G. II., Coal resources of District VI: 111. Coal Mining Investigations Bull. 15, p. 30, 1916.

"Carly, G. H., Coal resources of District I: 111. Coal Mining Investigations Bull. 10, p. 41, 1915.

"Udden, J. A., Geology of the Peoria quadrangle, Illinois: U. S. Geol. Survey Bull. 506, p. 39, 1912.

24

SURFACE SUBSIDENCE IN ILLINOIS

The same horizon extends farther westward, but the lithological change is considerable.13 The extent to which these beds affect subsidence in the Longwall District has not been determined.

From the foregoing it seems that there are few, if any, persistent beds that are hard enough and thick enough over large areas to in- fluence greatly the problem of subsidence.

By the Cooperative Coal Mining Investigations the State has been divided into districts as shown in figure 49. In the following tabulation of notes on roof and floor, the data submitted in the reports upon the several districts are used together with such supplemental data as have been secured later.

Table 3. — Character of roof and floor of the commercial coal beds throughout Illinois

Coal No. 1

Districts and counties

Roof

Floor

III

In Mercer and Rock Island counties, hard black shale 2 to 5 inches thick; limestone cap rock, 1 to 4 feet.

Light gray micaceous fire clay which heaves badly when wet. In places an irregular band (3 to 6 inches) of carbonaceous shale or sandstone lies imme- diately below the coal.

Coal No. 2

II

III

Gray shale, replaced in places by a black shale about 3 feet thick.

Gray shale up to 36 feet thick. In some places, dark-colored shale which is hard to support.

Hard black shale generally not over 1 foot thick, overlain by a limestone cap rock 3 feet thick.

Dark-gray fire clay up to sev- eral feet thick. In some parts of the La Salle field a hard sandstone lies directly beneath the coal.

Bottom bench of coal is a layer of bone 2 to 3 inches thick. In most places floor is sandstone; in sections is shale or fire clay.

Gray fire clay containing nod- ules of iron pyrites.

13Cady, G. H., Coal resources of District I: 111. Coal Mining Investigations Bull. 10,

PP. 36-39, 1915.

25

I

I

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ILLINOIS STATE GEOLOGICAL SURVEY

COOPERATIVE COAL MINING SERIES BULLETIN NO. 17, PLATE IV

1 Sec.12.T17 N..R.13W

2 Sec.8.T17 N..R.12 W.

3 Sec.4,T17 N..R.12 W.

4 Sec.34.T.18 N..R.12 W

5 Sec.36.T18 N..R.12 W.

6 Sec.35.T.18 N..R.12 W

7 Sec36.T18 N..R.12 W.

8 Sec.36,T 18N..R.12 W.

9 Sec.30,T 18N..R.11 W.

10 Sec.29,T18N.,R.11 W.

11 Sec.28,T.18N.,R.11 W.

12 Sec.22.T18 N..R.11 W.

13 Sec.24,T18 N..R.11 W.

Columnar Section E F

1

Sec.31,T.20 N..R.12 W.

8 Sec.29,T.20N.,R.11 W.

2

Seo.32,T.20 N..R.12 W.

9 Sec.21,T.20 N..R.11 W.

3

Sec.32,T.20 N..R.12 W.

10 Sec.22,T.20 N.R.11 W

4

Sec.33,T.20 N..R.12 W.

11 Seo.14,T.20 N..R.11 W.

5

Sec.34,T.20 N..R.12 W.

6

Sec.35,T.20 N..R.12 W.

7

Sec.36,T.20 N..R.12 W.

(£3 Dri,t

Hfl Shale gg] Black slate

[H Clod §jjjj Coal

l-'y'vl Sand or sandstone |^OJ Fire clay |PEk] Sand and shale

Cross-section CD showing structure across the southern part of the Danville field Cross-section EF showing structure across the northern part of the Danville field

CONDITIONS AFFECTING SUBSIDENCE

25

Coal No. 5

Districts and counties

Roof

Floor

IV

Gray fire clay 1 to 4 feet thick, underlain b y sandstone o r sandy shale.

Varies from a gray shale to black "slate", sandstone and lo- cally limestone; "white top" roof in certain areas.

Black sheety shale up to 35 feet thick. A limestone cap rock over the shale. Where shale is thin, the cap rock be- comes the roof.

Light gray to black shale, in Fire clay generally. In some

Gray fire clay.

some areas laminated with coal for a distance of 3 feet above seam. Shale of immediate roof is weak.

areas the clay is sandy and heaves badly when wet.

Coal No. 6

VI

Franklin

Williamson

VII

Clinton

Christian

Macoupin Madison

Coal is left as immediate roof. Upon the coal is a thin bed of draw slate, and within 25 feet above the coal is usually a lime- stone cap rock 4 to 10 feet thick.

Coal is left generally as roof. In a number of mines the lime- stone cap rock is missing or higher than in Franklin County.

Limestone cap rock, 5 to 15 feet thick. In places black shale between limestone and coal.

Black shale overlain by lime- stone ranging from 1 to 20 feet. In some mines shale between coal and limestone.

Black shale with limestone cap rock.

Gray or black shale of varying thickness overlain by limestone ranging in thickness from a few feet to as much as 30 feet. In some places limestone rests on the coal.

Gray fire clay 2 to 8 feet thick underlain by a sandy limestone. Heaves in only a few mines.

Gray fire clay 2 to 4 feet thick underlain by limestone. The floor heaves badly in several mines.

Clay, 18 inches to 8 feet in most places on shale.

Clay of variable thickness.

Clay averaging about 1 foot. Beneath the clay there is gen- erally limestone.

26

SURFACE SUBSIDENCE IN ILLINOIS

Table 3. — Character of roof and floor of the commercial coal beds

throughout Illinois — Concluded

Coal No. 6 — Concluded

Districts and counties

Roof

Floor

Marion

Limestone cap rock, about 15 feet thick.

Clay of varying thickness.

Montgomery

Limestone cap rock.

Clay of variable thickness.

St. Clair

Black shale and limestone.

Thin clay on limestone.

Perry,

Randolph, and

Washington

Black shale under limestone to the west of Duquoin anticline. To the east, the same limestone is 100 feet above coal.

Clay of variable thickness.

Shelby and Moultrie

Shale and limestone.

Shale, clay, limestone.

Sangamon

Irregular shale and limestone.

Thin clay.

VIII

Roof is variable. Immediate roof generally a grayish-black shale about 6 feet thick. Diffi-

Generally a grayish fire clay varying in thickness up to sev- eral feet. Heaves badly when

cult to support in many areas. Many irregularities.

wet. A bed of limestone be- neath the fire clay.

Coal No. 7

VIII

Gray silicious shale 35 or more feet thick. Immediate roof is generally darker than the upper beds of shale.

Black shale up to several feet thick under a gray shale as much as 50 feet thick in places.

Gray fire clay 2 to 3 feet thick lying on sandstone. Black shale in places forms the floor.

Immediate floor is a thin bed of clay which heaves badly. Be- neath is a layer of harder clay 5 to 15 feet thick. Some streaks of coal in the clay.

Cleat

It has been suggested by some mine operators that subsidence may be avoided if in room-and-pillar mines the rooms are driven face on the cleat. The theory is that the immediate roof and the overlying strata are jointed in the same direction as the cleat, and that most of the slips occurring in the roof also run in the direction of the cleat. It is held that if the rooms are properly turned, the pillars will catch up these slips and prevent any local movement which may be the beginning of a general movement causing subsidence.

At the present time there are not sufficient data collected to prove or disprove this statement. At several mines where attention is paid

CONDITIONS AFFECTING SUBSIDENCE

to the cleat no subsidence has occurred, but there are a number of mines with rooms opened on the cleat in which serious subsidence has resulted.

Faults and Rolls

GENERAL RELATIONS TO SUBSIDENCE

As will be noted later, the movement of the overlying beds follow- ing the mining of coal may be influenced greatly by both the character and structure of the beds themselves. If faults occur, the ability of beds to arch and thus to support the surface may be destroyed. More- over, the effects of subsidence may be localized if faults are numerous, for the faulted beds over the mined-out area will fall in blocks and will not produce long sags with the usual draw or pull.

In almost all the districts of the State faults, slips, and rolls would interfere more or less with the application of formulae and theories of subsidence. In certain districts, as will be noted, these irregularities are much more marked. Moreover, the occurrence of slips, rolls, horses, and of other irregularities in the coal bed itself increases the amount of barren material thrown into the gob and thereby the amount of filling is increased and the amount of subsidence may be reduced. When the rolls or horses are not broken by mining they serve as pillars and help support the overlying strata if slips do not occur along the line of the rolls or horses.

COAL NO. 2

District /.—The geologists report no important faults, but nu- merous small displacements have been noted in the coal seam. Both thrust and normal faults have been noted (sec figures 18, 19, 20, Bull. 10, Illinois Coal Mining Investigations). At many points the coal is thinner and the roof rolls down.

District II.— "In all the mines of this district numerous small faults occur, and horses usually of a hard dark-gray, micaceous sand- stone are found in the vicinity of the faults."14

COAL NO. 5

District /.—"A peculiar condition of the roof and coal known lo- cally as 'white top,' exists on the west side of the M. & Ik mine (see figure 21, Bull. 10, 111. Coal Mining Investigations) on the cast side of the Cherry mine (middle bed), and, according to report, in the Cahill mine. At Cherry this consists of a white to gray sandstone or

"Andros, S. O., Coal mining practice in District II: Illinois Coal Mining Investi- gations Bull. 7, p. 10, 1914.

28 SURFACE SUBSIDENCE IN ILLINOIS

sandy, gray shale which replaces the usual gray and black shale of the roof and permeates the coal down to a band of clay found about 14 inches from the floor. Large pieces of white sandstone are found scattered through the bed so that the whole resembles a conglomerate. Slickenside surfaces are common throughout the 'white top' areas, and the roof is commonly rough and broken, so that it is very difficult to keep the roads clear. The impurities at some of these places exceed one-half of the total thickness of the bed, and render the coal worth- less."15

District IV. — Numerous clay veins extend from the roof across the coal seam.16 There are also many small faults, slips, and rolls. Mr. J. A. Udden17 has applied the term "fractures" to the dislocations occurring in the Peoria district. These fractures "are believed to be the result of physical processes altogether different from those causing faults. In many places they have a close resemblance to true faults, but the direction of the dislocation is normally horizontal instead of normally vertical."18

In the Springfield region numerous so-called "horsebacks" occur. "These are more or less irregular and branching fissures, filled with clay or shale, extending downward from the overlying beds into or through the coal. They range in width from 2 or 3 inches to 3 or 4 feet, the walls not being very nearly parallel, and are considerably and abruptly wider in the coal than in the overlying roof shale."19

District V. — Coal No. 5 is faulted at a number of places in the district and contains many rolls. In several places an igneous intrusive penetrates the coal bed and in others it lies in a sheet at some distance above the coal. Squeezes and subsidence have undoubtedly been in- fluenced greatly by these irregularities in this district. Detailed records of subsidence adjacent to faults and dikes are given in Chapter V.

coal no. 6

District VI. — Reference has previously been made to the cap rock in this district. In some places there are rock rolls extending down into the coal. Minor faults occur, but they do not interfere with

15Cady, G. H., Coal resources of District I: 111. Coal Mining Investigations Bull. 10, P. 78, 1915.

10Andros, S. O., Coal mining practice in District IV: Illinois Coal Mining Investiga- tions, Bull. 12, fig. 4, 1915.

"Udden. J. A., Geology and mineral resources of the Peoria quadrangle, Illinois: U. S. Geol. Survey Bull. 506, p. 68, 1912.

lsAndros, S. O., Coal mining practice in District IV : Illinois Coal Mining Investiga- tions Bull. 12, fig. 5, 1915.

19Shaw, E. W. and Savage, T. E.. U. S. Geol. Survey Geol. Atlas, Tallula-Springfield folio (No. 188), p. 3, 1913.

CONDITIONS AFFECTING SUBSIDENCE 29

mining to a large degree. A detailed statement of faults in the dis- trict is given in the bulletin on the geology of the district.20

District VII. — Minor faults or slips in coal No. 6 seam have been noted in Clinton, Macoupin, Madison, Marion, Montgomery, St. Clair, Perry, Randolph, Washington, and Sangamon counties. North of Centralia the so-called "Centralia fault" has a displacement of 110 feet. East of the Duquoin anticline numerous small faults occur, the largest having a downthrow of 20 feet.21

District VIII. — There are numerous rolls in coal No. 6 in the Danville region. Stringers and lenses of shale extend down from the roof into the coal. In places there are minor faults. A detailed study of the irregularities in the coal bed has been made by Mr. F. H. Kay.22

coal no. 7

District /.—Coal No. 7 is quite irregular and contains horsebacks and rolls which add considerably to the volume of material thrown into the worked-out rooms.23

District VIII.— Numerous horsebacks or rolls occur in this bed in the Danville district but they are not so extensive as in coal No. 6.24

20Cady, G. H., Coal resources of District VI: Illinois Coal Mining Investigations Bull. 15, pp. 82-87, 1916.

21Kay, F. H., Coal resources of District VII: Illinois Coal Mining Investigation Bull. 11, pp. 196 and 197, 1915.

22Kay, F. H., Coal resources of District VIII: Illinois Coal Mining Investigations Bull. 14, pp. 42-49, 1915.

23Cady, G. H., Coal resources of District I: Illinois Coal Mining Investigations Bull. 10, p. 85, 1915.

2*Kay, F. H., Coal resources of District VIII: Illinois Coal Mining Investigations Bull. 14, p. 53, 1915.

CHAPTER III— DAMAGE CAUSED BY REMOVAL OF COAL

Contrasting Effects of Longwall and Room-and-Pillar

Methods

It may be said that in general the surface effects of longwall min- ing are more uniform than those resulting from room-and-pillar and panel working. This is due principally to the gradual sinking of the roof in longwall mining where the same gradual movement generally extends through the overlying rocks toward the surface. Upon the surface the evidences of subsidence will be a gentle sag, and where the longwall face is stopped there may be a more or less well-defined terrace outlining in a general way the area that has been undermined. Except in a shallow working and in faulted areas probably no large surface cracks or breaks will occur, and if these do occur they will usually close after the longwall face has advanced some distance beyond the point in the mine vertically below.

On the other hand, where coal has been mined by other systems, the area over which the settling may extend is generally smaller, and serious cracks and breaks are more likely to result.

Surface Evidences of Subsidence

In general, it may be said that the principal ways in which sur- face movement is shown are by : (1) surface cracks and displace- ments, (2) pit holes or caves, and (3) sags. The sags and holes fre- quently hold accumulations of surface water.

surface cracks and displacements

As previously noted, serious cracks have not been observed fre- quently in the longwall district of Illinois. One of the reasons that no surface cracks are evident is the presence of a heavy blanket of glacial drift from 25 to 200 feet thick lying over the "Coal Measures" in this district. At various points where the workings have been shal- low, the rock cover thin, and the surficial beds saturated with water, serious breaks have resulted accompanied by a rush of sand, mud, and water into the mine. The resulting break upon the surface is however not typical of longwall mining, ?nd this type of break is as likely to occur in room-and-pillar mining. In Europe at various points in the longwall coal mining districts, cracks several inches wide have been observed above longwall workings over 1,000 feet deep.

(30)

DAMAGE BY COAL REMOVAL

31

The surface cracks that have been observed have been due to ten- sion and to shear. Where the coal has been removed over a large area in room-and-pillar mining, the roof if unsupported sinks gradually, and if it does not break will in time rest upon the floor. In longwall mining, the roof settles upon the gob and compresses it so that in time it occupies a volume equal to only a small fraction of the volume of

Fig. 5. — Diagrammatic illustration showing angle of break and angle of subsidence (after Wachsmann).

the excavation, depending upon the nature and the amount of the fill- ing. As the immediate roof sinks the overlying beds also sink, and stresses are set up in the various beds successively from the immediate roof to the surface rocks (fig. 5). If each stratum is regarded as a beam, a portion of the beam will be in tension and a portion in com- pression. As the ordinary rock of mine roofs is much weaker in ten-

32

SURFACE SUBSIDENCE IN ILLINOIS

sion than in compression, cracks due to tension will appear before the evidences of compression. Tension cracks may be found in the mine roof in the zone where tension is greater than the tensile strength of the rocks, and similarly, upon the surface tension cracks may be found in the rocks or in consolidated surficial beds in the tension zone. Where there has been an extensive sag over a worked-out area, tension cracks are likely to occur around the perimeter of the sag. Figure 6 shows a tension crack from 4 to 10 inches wide occurring over a room- and-pillar mine in the Streator district.

Fig. 6. — Crack in the soil, caused by room-and-pillar mining at a shallow depth at Streator.

In a mine on coal No. 6 in Franklin County a number of squeezes have resulted in surface subsidence. The shaft is 550 feet deep and the coal 9 to 10 feet thick, from 1 to 2 feet being left as roof coal. The mine is worked in panels, room centers being 40 feet and pillars 18 feet (fig. 7). No pillars have been drawn. In two panels there has been movement which has affected the surface. At "A" the crack is 8 to 10

DAMAGE BY COAL REMOVAL

33

inches wide and at "B" there are two cracks, one of which is 2 to 3 inches wide (figs. 8 and 9).

Where the mine roof fails by shear along the pillars or supports, the entire overlying mass may be dropped into the excavation, and a more or less sharp break may appear on the surface. Where the fallen mass is large, a series of breaks may occur, as shown in figure 10. In the opinion of a number of prominent American engineers the strata above the immediate roof frequently fail by shear.

As previously noted the mine roof is likely to fail first in tension, and readjustment is likely to precede failure under compression. If

SIS

fii

TTrnn

PPfPfi'*

DOCK

r

Fig. 7.— Plan of two panels of a Franklin County mine, 550 feet deep. Subsidence caused surface cracks as indicated. Over the north panel at A the crack was 8 to 10 inches wide, and at B were two smaller cracks.

great lateral pressure and a free or unconfined surface exist, buckling may result, as illustrated in the heaving of the rock floor in deep mines and tunnels. One of the best illustrations of this type of movement occurred in the Simplon tunnel in Europe.1

Where there is a sag over an extensive area or subsidence over an excavation of considerable height, the surface of the central position of the basin may be subjected to severe stresses in compression. This will result in the elevation of this central area relative to the adjacent portion of the subsiding ground, although the entire mass may be sink- ing. If the uppermost stratum is rock or hard material, such as frozen

'Lauchli, E., Tunneling, fig. 92, p. 95, 1915.

34

SURFACE SUBSIDENCE IN ILLINOIS

soil, this compression may result in buckling, and small anticlines may result, as shown in figure 11.

PIT HOLES OR CAVES

Where shallow mining is carried on, falls of mine roof are fre- quently followed by surface subsidence causing pit holes or caves.

Fig 8.— Crack, 8 to 10 inches wide, due to subsidence over the north panel shown in figure 7. The view was taken from A looking west. (Photo by J. R. Fleming, U. S. Bureau of Mines.)

If the rock cover is not thick and the surficial beds are heavy and loose, the mine openings may be filled by a rush of surficial material, so that the pit hole may have a much greater volume than the single mine chamber in which the break occurred. Such pit holes may appear di- rectly over many worked-out rooms and may destroy entirely the value of the surface for agriculture and for building sites. When subsidence

DAMAGE BY COAL REMOVAL

35

has ceased, the surface may be restored by filling. Pit holes of various types are illustrated in figures 12 to 18.

When the surface beds are cemented material, frequently called "hard pan," an arch may tend to form across from one pillar or wall to another. Falls may extend close to the surface and be concealed

Fig. 9.— Cracks over south side of panel shown in figure 7. The view was taken from B looking east. (Photo by J. R. Fleming, U. S. Bureau of Mines.)

by a thin covering of a resistant bed. Eventually this last bed may break, and the fall may show the large cave beneath, as illustrated by figure 19. Where the soil is covered by a heavy sod, the dangerous :ondition of the fields may be concealed for a time owing to the tenacity 3f the sod. Heavy sheets of sod may be seen (fig. 20) hanging over the rim of breaks, and instances have been reported in which for a :onsiderable time simply a sod roof has concealed caves several feet in diameter.

36

SURFACE SUBSIDENCE IN ILLINOIS

FlG 10— Cracks and breaks due to shear. View along the rim of a large pit hole caused by an extensive movement into a shallow mine. In the right fore- ground the mass of earth has dropped 18 inches.

Fig 11— Buckling of frozen sod due to compression along the axis of a basin or sag. The movement occurred over a large excavation in a shallow mine. The sod in the foreground was lifted as much as two feet.

DAMAGE BY COAL REMOVAL

37

^% M

Fig. 12.— Surface breaks in a field near Carterville. The shaft is 120 feet deep, and a 7-foot bed of coal is being mined. The roof is slate and the over- burden clay; no quicksand has been noted. The breaks are about 20 feet in diameter and from a few feet to as much as 20 feet deep.

Fig. 13.— Surface breaks extending over a room in a mine near Harrisburg where the coal is 7 feet thick and 80 feet deep. The rooms are 24 feet wide and have 24-foot pillars.

58

SURFACE SUBSIDENCE IN ILLINOIS

Fig. 14.— Plan showing workings of a mine and location of subsidence movements adjacent to a dike.

Fig. 15.— Plan of a mine showing a few details of movement near a dike.

DAMAGE BY COAL REMOVAL

39

Fig. 16. — Breaks in a pasture over an abandoned mine west of Danville where the coal is 6 feet thick and 100 feet deep.

Fig. 17. — Side-hill breaks near Cuba caused by room-and-pillar mining of a 5-foot bed of coal from 50 to 75 feet deep.

40

SURFACE SUBSIDENCE IN ILLINOIS

Fig. 18.— Large area covered by pit holes in the suburbs of a town in cen- tral Illinois where a 5-foot bed of coal is being mined at a depth of 75 to 100 feet. Trees are falling into the pit holes ; the houses are being used.

pIG 19.— Cave-in on land south of Dewmaine where a 9-foot bed of coal is being mined at a depth of 100 feet. The opening is 12 feet in diameter and 15 feet below the surface ; the width is approximately 25 feet. The hard pan arches up from the shale.

DAMAGE BY COAL REMOVAL

41

Where the breaks occur on hillsides, a series of resulting move- ments may be clue to the natural slope of the ground. The more or less concentric rings formed by these breaks are shown in figure 21.

Fig. 20. — Detail of a cave on a hillside near Streator; note the hanging sod.

A serious condition is illustrated in figure 22 where the surface is owned by one mining company as a site for company houses, and the coal is owned and mined by another company. The owner of the coal is reported to be mining from 8 to 1 1 feet of coal at a depth of 100

-Side

1 lAl L i

i

m

1 *^ifc&**

illow workings near Streator.

feet by the room-and-pillar method. The owner of the surface se- cured an injunction restraining the mining of coal under the buildings shown in the illustration. The court would not restrain mining under

42

SURFACE SUBSIDENCE IN ILLINOIS

the other portions of the tract as any damage resulting might be the basis for a suit.

SAGS

Where the workings are covered by thicker and stronger beds of rock, and the area excavated is larger, the surface effects of subsidence may be sags instead of breaks, caves, or pit holes. During part of the

C5CT-0— =

O

o

\r

Hl o°

Scale 100 200 300 feet

L

r

Fig. 22.— Plan of a 40-acre tract in southern Illinois, showing relation of approximately 60 pit holes to the underground workings.

year these sags may contain pools of water, and if they are considerably below the previous drainage levels, they may be inundated continually. In the longwall field of northern Illinois it is necessary to provide artificial drainage for much of the land which is otherwise suited for agricultural purposes. Figure 23 shows the nature of the topography

DAMAGE BY COAL REMOVAL

43

in the Coal City-Wilmington area. It is evident that subsidence re- sulting from mining operations may seriously derange the artificial drainage systems which are installed. Figure 24 shows a pond formed over a sag due to longwall mining near Coal City.

Fig. 23. — Topographic map of the Coal City- Wilmington area.

In a portion of a mine in Perry County (fig. 25) opened on coal No. 6 at a depth of 350 feet the coal averages 9 to 11 feet, but the upper bench of 3 feet is not mined. Six sags but no surface breaks have resulted from the mining. In general the pillars have not been drawn. In one place where pillars have been drawn an area 600 feet in diameter sagged, the maximum depression being 4 feet; the subsi-

44 SURFACE SUBSIDENCE IN ILLINOIS

dence of the surface began twenty-four hours after the movement was observed underground. Another area 800 feet in diameter with a maximum depression of 3 feet was noted at the same mine.

At a mine in Franklin County where the coal seam is 9 feet thick, one foot of the top coal is left in mining. The shaft is approxi- mately 450 feet deep. The roof is gray shale, and no limestone occurs immediately over the coal. The bottom is fire clay varying from a few inches to 5 feet. A number of squeezes have occurred, principally where the rooms have been turned on 45-foot centers with 15-foot pillars. The rooms were carried 250 feet and it was planned to leave a 20-foot barrier between panels of rooms. In places the room necks have been turned wide, and the room cross-cuts have been driven about 20 feet wide. The main entries are protected by a barrier pillar of 110 feet, the cross-entries have been driven on 25- and on 50-foot centers.

Fig. 24. — Pond in a sag due to longwall mining at Coal City, where a 3-foot bed is being removed at a depth of 135 feet.

Several squeezes had taken place where pillars had been drawn, particularly on one panel in the northeastern section of the mine. In November, 1914, an extensive movement began underground in the panel adjoining on the east a panel that had previously squeezed, and surface subsidence resulted. Mining had been completed in this panel 2 to 3 months previous. The movement continued several weeks, and a large fall occurred on the night of December 9, 1914. Falls continued for 4 hours, and the following morning there were evi- dences of surface movement. A railroad bridge had subsided 18 inches, concrete sidewalks cracked in some places (see figure 48) or broken by buckling in others, telephone poles were tipped (see figure 26), and foundations of houses were cracked. Later when the snow

DAMAGE BY COAL REMOVAL

45

thawed, the lateral extent of the movement was evidenced by the ponds which covered a large area (see figure 46). From the data at hand no evidence showed that at the end of two weeks the depressions were more than 18 inches deep. However, at the end of three months the subsidence at places was as much as 4 feet.

1711 IU!

-<r vt- ii— -w — ii

i lvjc jcri cdc dc5c5 cj&cjczjc 5c5 c :

T- -if — inr

NUv_

j'JL_-

PC Subsidence

J

-'jn!v-

j|

---3,-".':

J^--

£r- No. |5f=

_ ;T

5 !-•'--.

-L,_ a,--- ,V

\-J

:3!jc:

^

--''p.''.

nil ni- 1 cn'J •- l.U> •:

Fig. 25.— Plan of a mine in Perry County showing relation of sags to under- ground workings.

The subsidence occurred over an area of 44 rooms in a panel in which the entry pillars were 20 feet wide. The cut-throughs in the rooms were staggered 60 feet apart and were 20 feet wide. Where the

46

SURFACE SUBSIDENCE IN ILLINOIS

room barrier on the west had been broken through, the squeeze worked over into the zone which had previously subsided. Where the barrier pillar was still intact, it was sufficient to check the movement, and directly above on the surface tension cracks appeared extending in the same general direction as the barrier pillar. There was a gentle sag to the east between the room barrier and the entry pillar. As this pillar was small and broken by cross-cuts, the squeeze rode over it,

Fig. 26. — Telephone poles tipped toward a sag over a Franklin County mine where an 8-foot coal is being removed at a depth of 450 feet.

and some subsidence resulted, although the deepest portion of the sag was transversely across the panel of rooms. The underground movement covered an area approximately 500 by 1,000 feet.

In April, 1915, at the same mine, squeezes occurred in two ad- joining panels each of which consisted of 32 rooms. The rooms were

DAMAGE BY COAL REMOVAL

47

turned with wide necks and were worked out quickly. No pillars were drawn. About 3 months after the rooms had been finished, a severe squeeze began. The easterly panel was badly crushed in 3 days, and the squeeze rode over the small barrier to the next panel on the west. Falls began in the west panel on the fifth day and continued for about one week. At the end of two weeks no evidence showed that any considerable surface movement had occurred. In July, 1915, the next panel east caved, and in a few days several sidewalks were cracked and tension cracks appeared in the dry earth.

In many districts room-and-pillar mining has caused extensive sags which have eventually become swamps. The trees, unsuited to the changed conditions, have been killed and the entire area has become

•> ia'TjE:. ?-.*'•" ukJ

Fig. 27. — Swamp in a typical sag in Randolph County where a 6- foot coal is being mined at a depth of 150 feet. The rooms are on 50- foot centers; pillars 18 feet.

a waste. Somewhat typical illustrations of such swamps are shown in figures 27 to 31.

Frequently swamps of this nature are formed in the spring within fields and interfere with the plowing and planting. During the sum- mer these ponds may dry up, and as a result there may be within fields large untilled and wasted areas.

In figure 31 is shown a Vermilion County cornfield containing an unworked area at least 200 feet square. Similar conditions have been noted in many of the coal districts (fig. 32). Not only may occa-

48

SURFACE SUBSIDENCE IN ILLINOIS

sional ponds be created, but at times large sections may be lowered beneath the flood levels of river bottoms. There has been considerable litigation in northern Illinois where coal is being mined in the Illinois River bottom. It has been claimed by property holders that lands

Fig. 28. — Pond 4 feet deep and covering an area of about an acre, due to subsidence near Clifford, where a coal bed about 7 feet, 6 inches is being mined at a depth of 140 feet. Considerable quicksand occurs in the vicinity. The rooms are driven 20 to 22 feet wide on 40-foot centers, and no pillars are drawn.

Fig. 29. — Flooding of a large tract in Franklin County caused by mining of a 8-foot coal at a 460-foot depth. Rooms were carried 30 feet wide on 45-foot centers ; no pillars were drawn.

were inundated on account of the increased amount of water flowing into Illinois River due to the discharge through the Chicago Drainage Canal. On the other hand the officers of the Chicago Sanitary Dis-

DAMAGE BY COAL REMOVAL

49

Fig. 30.— Pond caused by subsidence at Westville, Vermilion County, where 6 feet of coal was removed by room-and-pillar mining at a depth of 210 feet. Levels at surface show maximum depth of sag to be 4.7 feet. (Photo by R. Y. Williams.)

y.

■«■**>-

v

Fig. 31.— Open space south of Danville about 200 feet square in cornfield, indicating the area is not tilled because water stands over the sag in the spring.

Fig. 32. — Drowned-out area of corn near Nokomis where the 8-foot coal bed is worked at a depth of about 625 feet.

50 SURFACE SUBSIDENCE IN ILLINOIS

trict claimed that the lands were flooded because coal-mining operations lowered the lands beneath the former drainage levels.

Agriculture and Surface Subsidence importance of agriculture Illinois is preeminently an agricultural and manufacturing state. To the manufacturing interests an ample and continuing fuel supply is of prime importance. In few states are both agriculture and manu- facturing developed to so great an extent, and in practically no other state do the coal beds underlie so extensive and so fertile farm lands. In a large part of the Appalachian and the western coal fields the sur- face is almost valueless for agricultural purposes, but where the over- lying surface is tilled it is of much less value than the farm lands of Illinois.

EXTENT AND VALUE OF FARM LANDS

In the accompanying tables are given data from the United States States Census for 1910 showing the percentage of the area in farm lands, the average value of the land per acre, and also the value of the coal produced in 1910.

The farm lands of Pennsylvania constituted 64.8 per cent of the area of the State, whereas only 68.2 per cent of the farm land was improved, or 44.2 per cent of the total area of the State. In the lead- ing bituminous county of Pennsylvania (Table 4) 62.6 per cent of the land was in farms, and in Washington County, which, among the coal counties ranks high in the fertility of soil, the percentage of total area in farms was 91.3. In the three counties important as anthracite producers, the percentage of area in farms ranges from 43.5 to 47.2. The average value of land per acre in Pennsylvania was $33.92 ; in the five leading bituminous counties (excluding Allegheny2) the range was from $22.01 to $55.21 ; whereas in the anthracite counties the range was from $25.03 to $33.68.

In the leading coal-producing county in West Virginia only 37.8 per cent of the land was in farms, and 13.6 per cent of the farm land was improved or 5.14 per cent of the area of the county. The average value of the land per acre was $33.42. In one important coal-producing county 93.1 per cent of the land was in farms, yet the average value was only $43.81 per acre.

In Kentucky 86.3 per cent of the land was in farms, and 64.7 per cent of the farm land was improved or 55.84 per cent of the total

2The census reports the average value of land per acre in Allegheny County to have been $146.21. This high value is due to the fact that Pittsburgh and Allegheny are located in this county.

Fig. 33. — Map showing relation of different thicknesses of coal to the values of farm lands as given in the 1910 census.

52 SURFACE SUBSIDENCE IN ILLINOIS

area.

The average value of the land was $21.83 per acre. In one of the leading coal-producing counties of Kentucky 81.2 per cent of the area is in farms, and 29.2 per cent improved, or 23.71 per cent of the total area. The land in this county has an average value of $11.72.

In Illinois 90.7 per cent of the area was in farms, and 86.2 per cent of the farm-land area was improved, or the improved land was 78.18 per cent of the area of the State. The average value of the land per acre was $95.02. In Sangamon County, an important coal pro- ducer, 87.33 per cent of the land was improved, which is 9 per cent better than the average for the State. The average price of land in Sangamon County was $138.30 per acre, a value of $43 more than the average for the State. The average price in Vermilion County, another important coal producer, was $138.85. The average price for La Salle County was $142.92. In the southern coal counties the farm lands are less valuable, but in Franklin and Williamson counties the percentage of land improved is greater than in the best counties of any of the states previously mentioned, whereas the average prices per acre, $38.43 and $30.61 respectively, compare favorably with the average prices of lands in the other states.

With the "Coal Measures" covering 66.9 per cent of the entire area of Illinois, and over 90 per cent of the area of the State in farms, the average value of which is at present greater than the average price of coal rights, the problem in Illinois of protecting the surface, par- ticularly agricultural lands, is much different and much more important than the problem in most of the other coal-mining states. As pre- viously noted, over 15,000 square miles are underlain by coal beds over 4 feet thick, and at least 10,000 square miles additional carry a bed 3 feet thick. Beds no thicker than 3 feet are being mined in Illinois and in other states ; and it is only a matter of time until the problem of lowering the surface of one-half the State by an appreciable amount must be considered, if at least a fair extraction of the coal is desired. The average value of land per acre in the counties in the coal districts of Illinois is shown in Table 4 and on figure 33.

DAMAGE BY COAL REMOVAL

53

Table 4. — Value of farm lands, April 15, igio, (U. S. Census)

States and counties

Approx. land area

Illinois 35

Bureau

Christian

Clinton

Franklin

Fulton

Gallatin

Greene

Grundy

Henry

Jackson

Jefferson

Knox

La Salle

Livingston

Logan

Macon

Macoupin

McDonough . . .

McLean

Madison

Marion

Marshall

Menard

Mercer

Montgomery ...

Moultrie

Peoria

Perry

Randolph

Rock Island ....

St. Clair

Saline ,

Sangamon ,

Shelby

Stark

Tazewell

Vermilion ,

Warren

Washington ....

White

Will

Williamson

Woodford

Acres

,867,520

563,840

448,000

309,120

284,800

565,760

216,320

329,600

277,120

527,360

376,320

385,920

455,040

733,440

667,520

394,880

374,400

550,400

376,320

762,240

471,680

364,160

253,440

202,880

345,600

440,960

216,320

407,040

288,640

375,680

271,360

424,320

255,360

560,640

494,080

185,600

414,080

589,440

349,440

359,040

324,480

540,160

287,360

337,920

Land in farms

Acres 32,522,937 524,455 422,520 280,440 222,578 506,222 162,693 308,579 249,984 504,927 305,759 336,340 424,381 662,755 646,551 381,478 356,946 511,225 353,776 733,161 408,487 335,624 232,456 192,910 326,311 426,398 207,249 353,206 234,915 323,237 237,936 364,523 213,831 520,999 461,878 175,719 374,528 534,385 326,653 329,135 285,027 498,651 227,642 316,064

n^O-S

Value of

coal*

production

in 1910

90.7

93.0

94.3

90.7

78.1

89.5

75.2

93.6

90.2

95.7

81.2

87.2

93.3

90.4 !

96.9

96.6

95.3

92.9

94.0

96.2

86.6

92.2

91.7

95.1

94.4

96.7

95.8

86.8

81.4

86.0

87.7

85.9

83.7

92.9

93.5

94.7

90.4

90.7

93.5

91.7

87.8

92.3

79.2

93.5

86.2 87.9 96.4 87.2 86.8 70.6 86.0 79.3 91.6 90.6 71.7 85.2 81.6 91.3 97.5 95.7 95.9 80.2 85.7 96.0 86.8 85.5 84.2 91.7 83.2 89.4 94.2 80.3 80.2 76.8 77.7 84.9 85.9 94.0 92.1 91.4 87.7 93.6 86.5 82.6 92.1 89.2 84.4 87.4

$95.02

114.53

123.63

44.59

38.48

88.18

48.60

72.52

124.50

112.03

31.27

34.62

112.69

142.92

161.76

156.49

161.29

69.74

116.89

171.85

70.53

39.45

123.92

122.04

104.63

73.49

154.95

107.67

30.62

36.11

87.97

81.57

39.88

138.30

88.22

123.10

144.21

138.85

129.80

34.02

55.44

104.08

30.61

154.27

52,405,897

1,488,070

1,322,162

1,092,752

2,312,342

2 253,307

85,000

14,330

968,563

225,018

776,363

15,000

54,174

2,032,002

262,056

469,657

387,713

3,479,049

61,194

29,470

4,222,078

801,117

466724

464,375

343,115

1,907,006

3 800

1,042,478

1.411,553

1,065,969

109,433

5,763,249

2,713,514

5,014,237

179,291

53,056

210,824

2,691,574

5,086,928

22,500

27,172

126,362

5,086,928

121,131

Mineral Resources of United States for 1910, U. S. Geol. Survey, 1911.

54 SURFACE SUBSIDENCE IN ILLINOIS

Table 4.— Value of farm lands, April 15, igio, (U. S. Census)— Concluded

1 States and counties

Approx. land area

Land in farms

en

v rt c3

£"3-9

Per cent of farm land improved

Average value of land per acre

Value of

coal*

production

in 1910

Kentucky

Henderson

Hopkins

A cres 25,715,840 278,400 349,440 302,080 373,760 498,560 28,692,480

464,000 458,886 730,880 508,800 551,680 664,960

288,640 570,880 497,280

25,767,680 339,840 268,800

15,374,080 426,880 266,240 550,400 341,120 201,600 382,080

Acres 22,189,127 231,677 298,263 245,210 338,211 481,370 18,586,832

308,342 228,004 271,094 318,475 503,923 493,491

134,160 269,486 216,348 19,495,636 270,581 122,874 ; 10026,442 110,142 247,835 252,402 128,784 173,529 139,134

86.3 83.2 85.4 81.2 90.5 96.6 64.8

66.5 49.7 37.1 62.6 91.3 74.2

46.5 47.2 43.5 75.7 79.6 45.7 65.2 25.8 93.1 45.9 37.8 86.1 36.4

64.7 86.7 60.7 29.2 55.1 26.3 68.2

79.4

57.2 59.5 66.4 85.7 74.9

43.8 51.1 65.8 50.6 60.7 41.8 55.1 48.8 84.6 53.8 13.6 76.0 46.9

21.83 36.08 20.28 11.72 9.97 8.82 33.92

146.21 32.59 22.01 55.21 48.03 42.03

33.68 28.64 25.03 20.24 34.58 22.79 20.65 24.72 43.81 23.45 33.42 39.91 27.15

14,405,887

239,332

2 202,299

Muhlenberg

Ohio

Pike

Pennsylvania

Bituminous —

Allegheny ........

2,503,371

746,611

869,501

313,304,812

20,359,650 17,566,903

Clearfield

Fayette

Washington

Westmoreland . . . Anthracite —

Lackawanna

8,048,056 31,210,480 17,567,634 22,389,051

36,868,765 52,759,185

Schuylkill

Virginia

Tazewell

Wise

28,998,199 5,877,486 1,169,981 3,274,809

West Virginia

Fayette

Harrison

Kanawha

McDowell

56,665,061

10,135,369

3,814,791

6,518055

12,767,998

4,165,737

Raleigh

3,309,67c1

^Mineral Resources

of United St

ites for 1910,

u. s. c

i'eol. Sur

vey, 191

1.

VALUE OF COAL LANDS

Although it is impossible to give an average value for coal lands in Illinois, yet it may not be out of place to indicate at what prices coal lands and coal rights are being sold. From these data some idea of the relation between the present value of the surface and of the coal can be obtained, and possibly a better idea of the prospective value of Illinois coal may be secured by a study of these data.

In a report3 on the value of coal land made by the United States Geological Survey in 1910, the following royalty rates per ton of mine-

:!Ashley, G. H., Valuation of public coal lands: U. S. Geol. Survey Bull. 424, p.

1910.

DAMAGE BY COAL REMOVAL

55

Table 5. — Value* of surface and of coal rights by counties in Illinois

County

Value of coal per acre

Number of coal bed

Average surface value, census of 1910

Bond ! $ 25

Bureau 10-100

Christian 10-50

Franklin 35-100

Fulton 15-100

Gallatin 20-25

Grundy 10-25

Henry 135

Jackson 25- 75

La Salle 10-100

Livingston 10- 50

Logan 20-50

Macoupin 15- 50

Madison 10-40

Marion 20

Marshall 15

McLean 15

Menard 25-30

Montgomery 25- 50

Morgan 20-30

Peoria 20-50

Perry 25

Putnam 15

Randolph 25

St. Clair 10-100

Saline 50-150

Sangamon 20-100

Scott 10- 40

Shelby 10-25

Vermilion 100-150

Warren 15

Will 15

Williamson 50-150

Woodford 15

6

2

6

6

5

5

2

6

2,6

2,5

6

5

6

6

6

2

5

6

6

6

5

6

2

6

6

5

5,6

2

6,5

6,7

1,2

2

6

2

$ 45.43

114.53

123.63

38.48

88.18

48.60

72.52

112.03

31.27

142.92

161.76

156.49

69.74

70.53

39.45

123.92

171.85

122.04

73.49

124.28

107.67

30.62

104.69

36.11

81.57

39.88

138.30

83.21

88.72

138.85

129.80

104.08

30.61

154.27

*These prices are not offered as an authoritative basis for valuation but indicate in a general manner the prices at which coal has been sold or at which it is held in some of the important counties.

run coal were given for Illinois : northern Illinois, 5 cents ; southern Illinois, 2 cents ; La Salle district, 10 to 25 cents per ton of screened coal.

In 1914, the leasing rates were reported in various counties as follows : Franklin, 3 cents ; Fulton, 3 to 4 cents ; Henry, 12^2 cents ;

56

SURFACE SUBSIDENCE IN ILLINOIS

Jackson, 3 to 5 cents; La Salle,4 10 to 15 cents; Peoria, 8 cents; Perry, 3 cents ; Rock Island, 20 cents ; Vermilion, 3 cents ; and Williamson, 3 cents.

Table 5 shows the range of prices for coal rights, depending upon the proximity to operating shafts and developed lands.

The United States Geological Survey5 gives the following sale prices for 1910: Illinois, $10-150; Grundy district, $40-110; Rock Island district, $50-75; Springfield district, $10; and southern Illinois, $25-50.

In response to an inquiry regarding the assessed value of coal lands in Illinois in 1913, the various assessors furnished the data given in the accompanying table.

Table 6. — Assessed valuation of coal rights and of agricultural lands

by counties

County

Assessed value of coal rights in 1913

ssed valut arm lands

including rovements

coal

Remarks

Highest

Lowest

Average

Asse of f not imp and

Christian ....

$ 5.00

$ 5.00

$ 5.00

$77.18

Includes improvements, but

Fulton

35.00

35.00

35.00

60.00

La Salle

7.00

1.53

4.89

20.51

Livingston . .

10.00

10.00

10.00

27.50

Reduced for 1914.

Logan

90.00

Coal rights not assessed.

Madison ....

7.00

7.00

7.00

22.00

Menard

7.00

7.00

7.00

20.00

Mercer

18.00

11.00

14.50

11.00

Average of land for which coal right is assessed separately.

Perry .

....

30 00

Improved ; coal rights not assessed.

Putnam

3 50-5.00

Randolph ....

11.00

8.00

8.69

23.46

Full value.

St. Clair

60.00

3.00

25.00

50.00

Not including E. St. Louis.

Saline

10.00

3.00

6.00

25.00

Including improvements.

Scott

Coal rights not assessed.

Washington .

5.00

3.30

3.80

8 09

NATURE OF DAMAGE TO AGRICULTURAL LANDS

As previously noted the removal of the coal may cause (1) the caving of the surface with the formation of cracks and pit holes; (2)

4In the La Salle district most of the coal mined is owned by the mining companies, and very little is leased.

"Ashley, G. H., Valuation of public coal lands: U. S. Geol. Survey Bull. 424, p. 35, 1910.

DAMAGE BY COAL REMOVAL 57

the sagging of the surface, resulting in the derangement of natural and artificial drainage; (3) the fracturing of beds containing or preserv- ing water supply; or (4) damage to surface improvements. With the proper care on the part of the mine operator some of these damages may be prevented or reduced. (See Chapter V.)

In the main it may be said that damage to farm property is only temporary, and the land and property can be restored. On the other hand when a portion of the coal is left in the ground as support for the surface, it is irretrievably lost, at least according to our commercial standards of the present time. In discussing mining wastes in Illinois, Mr. G. S. Rice6 said :

"If it were possible to systematize mining (longwall) so that the land near- est the water courses was first undermined and then in succession the land farther 'away, the damage done to farming would be minimized. However, until the agricultural land of the United States becomes insufficient to fill the needs of the population, which would be reflected in a continual increase of price for farming land, the money loss from temporarily destroying the surface in places is rela- tively small as compared with the selling price of the coal mined under the same. Taking the average value of the surface at $125.00 per acre, if 80 per cent be rendered worthless, the immediate money-loss would be $100 per acre. A seam 6 feet thick would contain per acre 11,000 tons of coal in place, yielding at 90 per cent, 9,900 tons. The damage done by practically destroying the sur- face would be only 1 cent per ton. If the land prices should rise two or three times above the value stated, this loss would still not prohibit mining."

It may be suggested that under average conditions in Illinois at present 45 per cent of the coal is left in the ground in certain portions of the State largely to prevent surface subsidence. If an additional 35 per cent were recovered and damage to the surface should thereby result to the extent suggested by Air. Rice, the increased output per acre, 3,850 tons, may be charged with the surface damage, assuming that ordinary mining costs remain the same (they would probably be reduced) with the increased tonnage. At a price of $125 per acre, the cost per ton for surface damage would be 2.6 cents. Many mining op- erators figure on this basis, and their experience has been that as a rule they are obliged to pay damages greatly in excess of the actual depreciation in value of the property.

The suggestion has been made frequently that the mining company should purchase the surface as well as the coal right, and that as soon as the coal has been mined completely under each tract or farm, the prop- erty should be restored as nearly as possible and sold. Under such conditions the surface would propably be as valuable when restored after the coal had been mined as when first purchased The income from the use of the surface for agricultural purposes would have

"Rice, G. S., Mining wastes and mining costs in Illinois: Til. State Geol. Survey Bull. 14, p. 218, 1909.

58

SURFACE SUBSIDENCE IN ILLINOIS

been sufficient to pay the interest on the money invested in the land. If the company suffers any loss through the depreciation of the surface it would be small as compared with the damages that would be paid to the surface owner under conditions such as now exist in the coal-mining districts.7

RESTORING AGRICULTURAL LANDS

Where the removal of the coal causes local sags or depressions, water may accumulate to such an extent that artificial drainage must be provided. If the sags are of small area the problem may not be particularly difficult or expensive.

Figure 34 shows the position of 3 sinks with regard to the

""1 GG ODoooaaCj dd c3QDaQDDD,JQoaC

f" jnaao o

r»owr?nofV«nnfi

innnnnnjin

|Qrp

ifl'J! WitfnS

2l)o^ t^iiaOaZiiia^lSufl/lfllliiLii

-' 0 o o ■;:?, o o ci o o coi ! L.I r^o<:>f:K:^ooc-7c-jM=xa) 1 fl!

Fig. 34. — Map showing location of tile drains laid by a mining company to drain sags caused by mining a 7-foot coal at a depth of 330 feet.

workings of a room-and-pillar mine. At a depth of 330 feet a 7-foot coal was mined. Rooms were carried 30 feet wide with 30-foot pillars. The floor is fire clay, and it is thought the squeezes were due largely to the soft bottom. In order to remove the water in the ponds form- ed after the subsidence, the mining company laid 4,800 feet of tile at a cost of $647.57. The sags were from 2 to 3 feet deep. The coal was mined from 3 to 5 years before the subsidence occurred.

7Most of the mining companies are not now amount necessary to purchase the surface.

a position to invest the additional

DAMAGE BY COAL REMOVAL

59

At a longwall mine in the northern part of the State it was found advisable to construct a sump and to install a pumping plant in order to drain a pond which was caused by surface subsidence. As a gen- eral rule, it is found more economical to lay drain tile at possibly greater first cost.

Where breaks or pit holes result from mining operations, if the surface is valuable, it will usually pay to fill the holes partly with re- fuse and then to surface with a layer of soil sufficiently deep to support vegetation. The Agricultural Department of the University of Illinois recommends that the soil cover should be not less than 4 feet in depth. If the soil in its natural state is less than 4 feet thick over the entire area, it would seem equitable to make the soil layer of the filled area of equal thickness with that of the undisturbed area.

Fig. 35.— Pit hole filled with rubbish at the Electric mine near Danville.

In figure 35 is shown what frequently occurs in the mining district — the dumping of all kinds of rubbish into the cavities without regard to the ultimate dressing of the surface with soil. Figure 36 shows the practice in an area adjacent to Streator where the holes are filled partly with mine rock. Figure 37 shows a field near Streator in which the holes have been filled and then dressed with soil. The darker area in the foreground shows the fresh filling. The hole filled was 20 feet in diameter and averaged about 6 feet in depth.

Occasionally a subsided area extends across a road. In many places water collects and forms a pond that extends over the road.

60

SURFACE SUBSIDENCE IN ILLINOIS

In order to make the road passable it may be filled to grade with the material most easily accessible.

In order to prevent surface water from entering pit holes and thence flowing into the mine below, it frequently becomes necessary to construct dikes around the largest holes and those holes located

Fig. 36. — Pit hole and cracks caused by mining at a shallow depth near Streator. Mine rock near the hole has been hauled for filling; the surface will then be dressed with soil.

Fig. 37. — Pit holes near Streator caused by room-and-pillar mining filled with mine rock and surfaced with soil. The dark lines border the area of filling.

in the deepest part of the sags or in the course of the drainage of storm water. A dike 3 feet high built around a pit hole 60 feet in diameter and 28 feet deep has been constructed near Dewmaine.

It may be stated in general that, except where the "Coal Measures" are overlaid with thick beds of quicksand or other material that will flow

DAMAGE BY COAL REMOVAL

61

easily, there will be little irreparable damage to the surface on account of coal-mining operations, particularly when the coal is removed com- pletely. If a considerable portion of the coal is left permanently in pillars, the surface will probably be thrown into hummocks and sags, with occasional breaks and ponds. A complete removal of the coal will leave the surface in a much better condition for farming purposes. If the rock cover is thin and the overburden has a tendency to flow into the mine, special precautions must be taken or the surface will be con- siderably broken, due to the flow of sand into the workings.

At present attempts at filling the mine workings by flushing seem to be impracticable for the greater portion of the Illinois coal fields owing to the scarcity of material suitable for filling. If surface material is taken, a considerable area will be rendered unfit for agricultural purposes due to the removal of the soil. In portions of the State, however, it is possible that in the future market con- ditions may warrant higher mining costs, and under such conditions hydraulic stowing of crushed material from surface quarries may be feasible.

Transportation and Surface Subsidence railroads

The practice of separating the mining right from the title to the surface has frequently resulted in mining beneath roads, streets, and railroads without any consideration of the protection of the surface. In many instances the deed for the mining rights specifies that the coal may be removed completely and without a liability for damage to the surface. In other instances in which damage to the surface has resulted, an effort has been made to remove as large a portion of the coal as possible without injury to the surface. Certain rail- roads have permitted mine entries to be driven across the right-of- way and have forbidden the opening of rooms within a specified distance of the center of the track. In several cases mining has been carried on according to the regulation of the railroad, but the removal of coal outside the reserved area has resulted in "draw" or "pull" which has threatened the railway tracks. Railways have been constructed across tracts which have previously been under- mined by the room-and-pillar system and on which no subsidence has occurred. In time the pillars have weakened, and the added burden and the vibration due to the passing trains have resulted in the sinking of the tracks. In the opinion of a number of experienced railroad engineers the problem of subsidence as affecting railroads

62 SURFACE SUBSIDENCE IN ILLINOIS

is much different from that affecting other types of property, due principally to the intermittent and moving loads.

Few instances have been recorded in Illinois of the loss of life or injury to patrons or employes of railroads resulting directly from subsidence due to mining beneath the right-of-way.

Figure 38 shows the partially regraded railway track over a mine in which 9 feet of coal has been worked at a depth of 425 feet. In the distance the work train can be seen as filling material is being

';:■

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Fig. 38.— Railroad track in Franklin County lowered by surface subsidence resulting from room-and-pillar mining of a 9-foot coal at a depth of 425 feet. The maximum subsidence was 4 feet. The track has been partially repaired.

unloaded for the regrading of a switch. Figure 39 shows the nature and extent of the workings beneath the surface tracks which have subsided. The rooms were carried 30 feet wide by 300 feet long; pillars were 20 feet wide. The squeeze covered an area 1,200 feet square, and the depression at the surface was in places 4 feet. The underclay is quite soft in parts of the mine.

One of the large railroad companies reports that in the La Salle district it has been necessary to raise and surface the main line track every year, the total subsidence being estimated at about 3 feet. It is reported by two railroad companies that subsidence result- ing from longwall mining in the vicinity of Decatur has necessitated regrading and filling amounting to 3 to 4 feet.

Room-and-pillar mining in southern Illinois has caused consider- able damage to railroad tracks. Near Duquoin a track to a mine

DAMAGE BY COAL REMOVAL

63

subsided 3 feet. A branch line of a railroad in Williamson County was damaged by a sink hole 6 feet in diameter and 20 feet deep. This occurred over the abandoned workings of a mine 90 feet deep. Im- portant tracks have subsided amounts varying from a few inches to a few feet at numerous places. Important bridges have been threat-

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ened; one pier of a bridge is reported to have subsided 18 inches but without much tilting (fig. 40).

It is now the practice of the railroad companies whose lines extend through mining districts to secure annually from their division engineers complete reports showing the extent of mining beneath the right-of-way.

64

SURFACE SUBSIDENCE IN ILLINOIS

WAGON ROADS

Generally the title to the coal beneath wagon roads is not reserv- ed by the district maintaining the roads. Frequently where there is a likelihood that the roads may be damaged by mining operations an agreement exists between the mining company and the local officers to the effect that any damage to any road will be repaired by and at the expense of the mining company.

In the Longwall District of northern Illinois a general subsidence following the advance of the longwall face in many places results in the inundation of the roads during part of the year. At a number

Fig. 40. — Repaired railroad trestle in Franklin County. One end subsided 18 inches more than the other, the total having been reported as about 4 feet. Timbers were placed at A, B, C, and D in order to reduce the sag of the track over the trestle.

of places the mining companies responsible for the subsidence have constructed adequate ditches paralleling the sunken road in order to remove the water. It is not uncommon for the mining companies, as important users of the roads, to haul mine refuse and ashes to fill the road to the desired grade.

STREAMS AND CANALS

Mining has been carried on beneath several navigable streams and canals, but in no instance has the water been let into the mine, nor has subsidence caused any appreciable damage to the water course

DAMAGE BY COAL REMOVAL

65

— at least not to the extent of interfering with its usefulness for the passage of watercraft.

Buildings and Improvements Affected by Subsidence

buildings

The subsidence of the surface over a large area may be so uniform and so gradual as to cause no serious damage. In parts of the longwall field mining has been reported to have had no appreci- able effect upon the frame buildings at the surface. The effect is less if the mining face advances rapidly and across the shorter axis of the building. As previously noted, the building is in a ten- sion zone as the mining face approaches and passes under the building, and the section of the building under which the coal has been removed first tends to tilt toward the mine and to tear itself free from the remainder of the building. If the building is constructed of masonry,

Fig. 41. — House near Coal City lowered 9 inches at one corner, caused by mining a 3-foot coal at a depth of 125 feet.

serious cracks may form and afterward close when the mining face has advanced completely beyond the building.

When the advance of longwall mining has been stopped under or adjacent to a building, the surface is likely to be tilted enough to throw the building out of plumb. Figure 41 shows a house in the Coal City district which was thrown out of plumb by the stop- ping of the longwall face near the house. The coal beneath had not been mined. The seam of coal is 3 feet thick and lies at a depth of 125 feet. One corner of the house was almost one foot lower than the other corners.

66

SURFACE SUBSIDENCE IN ILLINOIS

The removal of part of the coal by room-and-pillar mining is more likely to damage buildings seriously than is the complete removal of the coal by the longwall system. This difference results from the probable formation of pits and from the unequal subsidence that may tear to pieces a building that happens to be located over a pillar. If it is located in the middle of a small sag, the compression or squeezing may be so great as to destroy the building.

Figure 42 shows the part of a brick house still standing above a pillar in a mine near Danville. Approximately 6 feet of coal was taken out at a depth of 200 feet, The position of the house with regard to the mine workings is shown in figure 43.

Fig. 42. — Brick house near Danville abandoned on account of danger by room-and-pillar mining of a 6-foot coal at a depth of about 200 feet.

Figure 44 shows a portion of a row of houses in Springfield These brick houses were damaged by subsidence resulting from room- and-pillar mining. The coal is 5 feet 9 inches thick and lies at a depth of approximately 200 feet. A pillar 10 feet wide was left along the street and this apparently was responsible for the crack- ing of the houses. If all the coal had been removed the houses probably would have settled uniformly. The houses have been re- paired at the expense of the mining company.

In figure 45 is shown a small frame house in the suburbs of Streator. A pit hole 10 by 20 feet and 5 feet deep has been formed along one side of the house.

In southern Illinois has recently occurred a movement affecting a large area in one of the coal-mining towns. Eight feet of coal

DAMAGE BY COAL REMOVAL

67

was being mined by the room-and-pillar method at a depth of ap- proximately 450 feet. Rooms were carried 30 feet wide with 15-foot pillars. Figure 46 gives a general view showing the flooded streets resulting from the subsidence. The maximum depth of the sag was about 3 feet. The foundations of houses were cracked, and in a

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number of houses the plaster fell. Another depression at the same mine resulted in lowering one end of the company barn over 4 feet. The barn has been repaired, but one end is still approximately 15 inches lower than the other. The tilting of the barn is shown in figure 47.

68

SURFACE SUBSIDENCE IN ILLINOIS

FIG_ 44.— Houses in northwest Springfield for which claims were paid by a mining company for damages caused by mining a coal 5 feet 9 inches thick at a depth of about 200 feet.

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IB

Fig. 45.— Small house near Streator beside which a pit hole 10 by 20 feet and 5 feet deep has formed.

DAMAGE BY COAL REMOVAL

69

STREETS, PIPE LINES, AND SEWERS

Mining within the limits of cities and towns and the construction of towns upon lands which have previously been undermined have been attended with more or less danger owing to the subsidence of the streets. Where coal at shallow depth has been mined by the room-and-

Fig. 46. — Flooded streets, broken sidewalks and foundations, and damages to plastering resulting from subsidence in Franklin County. Figure 48 shows the broken sidewalk at A. (Photo by H. T. Smith, U. S. Bureau of Mines.)

\ /.

Fig. 47. — Barn of mining company lowered 4 feet at one end. partially restored.

It has been

pillar system, a sudden collapse of the overlying beds into the worked- out rooms may result. Passing teams have at several times been reported to have dropped with the surface into a pit hole. No fatal accidents are known. Where the coal is at greater depth, or where the coal has been mined by the longwall system, the movement will

70

SURFACE SUBSIDENCE IN ILLINOIS

probably not be attended by the formation of holes into which build- ings or creatures may fall.

The gradual sinking of the surface may do serious injury to pavements, sidewalks, car tracks, pipe lines, and sewers. Numerous instances of such damage have been reported but as repairs are usually made immediately there has been little opportunity to secure photo- graphs illustrating this type of damage.

Figure 48 shows a broken sidewalk caused by subsidence. Mining was being carried on by the room-and-pillar method at a depth of 450 feet on an 8-foot seam of coal.

g. 48.— Broken sidewalk caused by subsidence in Franklin County; the location is shown in figure 46. (Photo by H. I. Smith, U. S. Bureau of Mines.)

In order to secure reliable information regarding the extent of mining operations within the towns of the State, and the nature and amount of damage which has been attributed to mining, a general letter was sent in August, 1914, to the mayor of each of 80 more or less typical incorporated towns and cities in the coal districts. Replies8 were received from 56.

8The towns reporting were Auburn, Belleville, Bloomington, Braidwood, Breese, Car- bondale, Carterville, Christopher, Colchester, Coulterville, Duquoin, Edinburg, Fairbury, Farmington, Galesburg, Geneseo, Ilarrisburg, Hillsboro, Jacksonville, Kewanee, Lewistown, Lincoln, Litchfield, Lovington, Macomb, Marion, Marissa, Mascoutah, Mattoon, Minonk, Morris, Mount Olive, Moweaqua, Murphysboro, Nashville, Nokomis, Norris City, Odin, O'Fallon, Pana, Peoria, Peru, Pinckneyville, Pontiac, Riverton, Salem, Seneca, Shelbyville, Sorento, Springfield, Spring Valley, Staunton, Streator, Virden, West Frankfort, and Witt.

DAMAGE BY COAL REMOVAL 71

The questions and answers were as follows:

1. Has coal ever been mined within the city limits? Yes, in 48 towns

of 56 replying.

2. Has coal ever been mined under the streets and alleys? Yes, in 39

towns of 56 replying.

3. Has any damage resulted to the streets and alleys on account of the

mining of the coal? Yes, in 5 towns of 56 replying; no, in 49.

4. Does the city now own or control the right to mine coal under any or all

of the streets and alleys? Yes, in 17 of 56 replying; no, in 35.

The replies to question No. 3 received from the five cities and towns reporting damage to streets and alleys were as follows:

1. Yes.

2. Not recently, but in former years some subsidences caused some trou-

ble. These have been fixed except one that gives some trouble, but nothing serious.

3. A little.

4. Some cave-ins.

5. Yes. Sink on North Main Street and property adjoining it; also in

east and west part of city.

The mayor of one town in which a 7-foot seam of coal lies at a depth of less than 450 feet reports that "the city has deeded the

right to mine coal under all its streets to the Coal Company,

and they have been mined as far as Main Street." The coal company reports that 50 per cent of the coal is left unmined, the rooms being from 30 to 35 feet wide on 55-foot centers. The mining company has not been released from surface damage. There has recently occurred in this town a movement which damaged a railroad right-of- way.

Another mayor wrote as follows : "The right to mine coal under streets and alleys was voted to - - Coal Company several years

ago." At this place the coal is 7 feet thick and is over 600 feet below the surface.

A city in the Longwall District of northern Illinois receives a yearly rental for the privilege of mining coal under the streets. A city ordinance granted this privilege conditional upon the payment of the rental.

WATER SUPPLY

At a number of places in the State it has been reported that wells have ceased to furnish the usual supply of water owing to the cracks and fissures resulting from subsidence. In some instances water-bearing rocks have been shattered, and in others gravel beds and catchment basins have been tilted or disturbed so that they no

72 SURFACE SUBSIDENCE IN ILLINOIS

longer serve as reservoirs for water. However, in a number of in- stances after the subsidence movement has stopped, the surficial beds have become compact and have again furnished water in quantities nearly, if not quite, as great as previously.

MUNICIPAL WATERWORKS

The protection of city waterworks is a matter which merits serious attention. At several points, notably at two cities of over 50,000 population, mine workings are advancing toward the waterworks. In these two large cities the undermining of the wells, reservoirs, and plants will cause serious damage.

Reservations of Coal

The topic of reservations may logically be considered under the discussion of protection of the surface and directly in connection with pillars. It has been more or less customary for the owners of farm lands when selling the coal right to reserve a tract of the coal under the dwelling, the well and cistern, the barn, and any important farm buildings adjacent to the dwelling. The advisability of leaving a small tract of coal for the protection of the surface is seriously questioned. If the coal is not thick and can be removed rapidly and completely it may be much more economical to have the coal tak- en out and to make such minor repairs as may result from subsidence. The presence of faults and beds of quicksand would complicate matters.

Moreover under some conditions the angle of draw may be so great that for the depth at which the coal occurs the size of reservation necessary for adequate protection would be entirely out of proportion to the value of the objects on the surface for which protection is desired.

There are no statutes in Illinois forbidding mining under any particular type of structures, public utilities, or other buildings, al- though there are such statutes in some states. When it can be shown that irremediable damage would result to property used for public purposes, or when mining might seriously threaten life through damage to property used for a public purpose, an injunction may be secured restraining mining in the area where subsidence is feared.

CHAPTER IV— SUBSIDENCE DATA BY DISTRICTS Introductory Statement

As the reports upon the mining practice and upon the geology have assembled the data by districts, it has been thought advisable to review the data on subsidence by districts so that they may be correlated with the geological and mining data.

The location of the various districts is shown in figure 49. Table 7 gives the districts of the State by counties and Table 8 gives the counties arranged alphabetically.

Table 7.— Districts into which the State has been divided for the purpose of

investigation

Coal

Method of mining

Counties

I

II III

IV

V VI

VII

2 1 and 2

6 (East of Duqtioin anti- cline)

6 (West of Duquoin anti- cline)

6 and 7 (Danville)

Longwall

Room-and-pillar Room-and-pillar

Room-and-pillar

Room-and-pillar Room-and-pillar

Room-and-pillar Room-and-pillar

Bureau, Grundy, La Salle, Marshall, Putnam, Will, Woodford.

Jackson.

Brown, Calhoun, Cass, Fulton, Greene, Hancock, Henry, Jersey, Knox, Mc- Donough, Mercer, Morgan, Rock Is- land, Schuyler, Scott, Warren.

Cass, DeWitt, Fulton, Knox, Logan, Macon, Mason, McLean, Menard, Peo- ria, Sangamon, Schuyler, Tazewell, Woodford.

Gallatin, Saline.

Franklin, Jackson, Perry, Williamson.

Bond, Christian, Clinton, Macoupin, Madison, Marion, Montgomery, Moul- trie, Perry, Randolph, Sangamon, Shel- by, St. Clair, Washington.

Edgar, Vermilion.

(73)

utiles

Fig. 49. — Map showing division of State into districts.

SUBSIDENCE DATA

75

Table 8. — Alphabetical arrangement of coal-producing counties

County

Bond . . . Brown . Bureau . Calhoun Cass . . . Christian Clinton . Edgar . . Franklin Fulton . Gallatin Greene . Grundy Hancock Henry . . , Jackson . Jersey . . . Knox . . . La Salle. Logan . . . Macon . . Mason . . Macoupin Madison Marion . .

Coal seam

District

6

2 2 2

2,5

6

6 6,7

6 1,2,5

5 1,2

2 1,2 1,2 2,6 1,2

5

2

5

5

5

6

6

6

VII

III

I

III

III, IV

VII

VII

VIII

VII

III, IV

V

III

I

III

III

II, VI

III

IV

I

IV

IV

IV

VII

VII

VII

County

Marshall . . . McDonough McLean . . . Menard ....

Mercer

Montgomery Moultrie . . .

Peoria

Perry

Putnam

Randolph . . . Rock Island. St. Clair ....

Saline

Sangamon . . Schuyler . . .

Scott

Shelby

Tazewell . . . Vermilion . . ,

Warren

Washington .

Will

Williamson . , Woodford . . .

Coal seam

District

2

I

1,2

III

5

IV

5

IV

1,2

III

6

VII

6

VII

5

IV

6

VI, VII

2

I

6

VII

1,2

III

6

VII

5

V

5,6

IV, VII

1,2,5

111,1V

1,2

III

6

VII

5

IV

6,7

VIII

1,2

III

6

VII

2

I

6

VI

2,5

I, IV

These districts do not contain quite all the mines operating in Illi- nois because there are a few which do not fall into the arrangement such as the Assumption mine, 1004 feet deep, operating in coal No. 1 at Assumption in Christian County and a few small room-and-pillar mines in coal No. 2 in the longwall held. From the mines included in the eight districts of the Coal Mining Investigations, however, there is produced 98.3 per cent of the tonnage of the State, and 97.6 per cent of all the employes in coal mines in Illinois work in these districts.

District I

Practically all the longwall mines of the State are included in District I. In Table 9 is given a list of longwall mines arranged ac- cording to depth and showing the average thickness of coal worked. It will be noted that the majority of the mines are being worked through shafts more than 300 feet deep. Of the 36 shafts only 2 are more than 600 feet deep, and these are outside of the longwall district proper.

76

SURFACE SUBSIDENCE IN ILLINOIS

Reference has previously been made to the character of roof and floor in District I. Attention may be called again to the extension over a part of the district of a heavy bed of limestone 25 to 30 feet thick and lying 375 to 400 feet above coal No. 2 and 175 feet above coal No. 7. Data are not available to show to what extent this bed influences the surface movement resulting from longwall mining, but it is reported that where the limestone bed is known to occur, the sinking of the surface does not follow so soon after the coal has been removed as in those areas where the limestone is known to be missing

Table 9. — Longwall mines in Illinois

Post office

rt*o

<4- 0

(/)

Operator and mine

address of mine

O u

"oIj <u

ss

y m

&3H

g"8

Feet

Ft. in.

1. Murphy, Linskey & Kasher

Braidwood

2

61

3 3

2. Wilm. C. Mg. & Mfg. Co., No. 6

Torino

2

100

3 6

3. Big Four Wilm. C. Co.

Coal City

2

100

3 4

4. Wilm.-Star Mg. Co., No. 7

Coal City

2

126

3 ..

5. G. & J. Coal Co., No. 1

Seneca

2

130

3 ..

6. Fulton Co. C. Mg. Co., No. 1

Sparland

2

165

2 8

7. Chic. Wilm. & Fr. Coal Co., No. 1

S. Wilmington

2

186

3 2

8. Chic. Wilm. & Fr. Coal Co., No. 3

S. Wilmington

2

187

3 2

9. 111. Zinc Co., No. 3

Peru

2

336

3 6

10. Spring Valley Coal Co., No. 1

Spring Valley

2

339

3 4

11. 111. Zinc Co., No. 1

Deer Park

2

364

3 3

12. La Salle Co. Carbon Coal Co.

La Salle

2

390

3 4

13. Spring Valley C. Co., No. 4

Seatonville

2

393

3 4

14. Spring Valley C. Co., No. 5

Dalzell

2

421

3 4

15. La Salle Co. Carbon C. Co., No. 1

La Salle

2

440

3 4

16. Spring Valley C. Co., No. 3

Spring Valley

2

457

3 4

17. Oglesby C. Co.

Oglesby

2

464

3 6

18. 111. Third Vein C. Co.

Ladd

2

468

3 6

19. Roanoke C. Co.

Roanoke

2

480

2 8

20. St. Paul C. Co., No. 2

Cherry

2

485

3 6

21. St. Paul C. Co., No. 1

Granville

2

486

3 ..

22. B. F. Berry C. Co., No. 1

Granville

2

505

3 ..

23. Rutland C. Co., No. 1

Rutland

2

511

2 10

24. Toluca C. Co., No. 1

Toluca

2

512

2 10

25. McLean Co. C. Co.

Bloomington

2

525

3 6

26. LaSalle Co. Carbon C. Co., No. 5

La Salle

2

545

3 6

27. Minonk C. Co., No. 2

Minonk

2

550

2 8

28. Mfrs. & Consumers C. Co.

Decatur

5

560

4 6

29. Decatur C. Co., No. 2

Decatur

5

612

4 6

30. Assumption C. Mg. Co.

Assumption

1&2

1004

4 ..

SUBSIDENCE DATA 77

and where there is the same total thickness of overlying sedimentary rocks.

Data from 10 shafts ranging from 50 to 550 feet in depth showed the average subsidence as follows in percentage of thickness of the coal mined :

Depths up to 200 feet, subsidence averaged 55 per cent. Depths from 200 to 400 feet, subsidence averaged 50 per cent. Depths from 400 to 550 feet, subsidence averaged 39 per cent. Sufficient data are not available to warrant the use of the foregoing figures as the basis of estimates of general subsidence in the Longwall District.

For all depths the amount of subsidence depends largely upon the quality and quantity of material stowed in the gob. It should be ob- served that as the depth increases, the weight of the overlying rock increases in proportion, and the material in the gob in deeper mines would probably be compressed more than the gob in shallower mines. With the same quality and quantity of material in the gob, all other underground conditions being the same, there may be as great a vertical movement of the surface when coal is mined at 600 feet as when the same thickness is mined at 200 feet. Some companies do not report that subsidence has occurred at their mines, and some question whether it actually occurs on their property. This probably is an honest statement resulting from the lack of established monu- ments and the failure to take elevations from time to time to show actual changes in the surface. In the greater part of the district, however, it is generally acknowledged that some subsidence results, but it is claimed that the surface movement is inappreciable in most places and does not cause damage to property on the surface.

As previously noted one of the most common forms of damage is by the flooding of lands by creation of sags or the sinking of areas below the previous drainage channels or flood plains (see figure 24). Some damage to buildings has been done but only little has been serious (see figure 41). Brick buildings have suffered more than frame buildings. No brick building has been irremediably damaged. Foundations have been cracked, but where all the coal has been removed and the longwall face has advanced well beyond the structure, the cracks have partly closed. Several long buildings have been badly cracked when the longwall face has not been advanced rapidly as the coal beneath the building was being removed. In three towns the occasional trouble with water and gas mains is attributed to subsidence resulting from mining.

In this district, one of the oldest mining districts in the State, there have been relatively few lawsuits on account of surface damage resulting from subsidence. Most of the claims for damages have

78 SURFACE SUBSIDENCE IN ILLINOIS

arisen on account of the flooding of lands, and these have usually been adjusted out of court.

District II

This district comprises Jackson County. The commercial coals lie at shallow depths, the deepest shaft being 165 feet. The coal varies in thickness from 3 feet 6 inches to 9 feet, and one small mine is operated on a bed 12 feet thick. All the mining is done on the room-and-pillar plan. The percentage of coal recovered, as reported for four representative mines, ranges from 44 to 55, the average being 48.5. Due to a thin cover over a large territory and considerable sand in the surface beds, a great deal of damage to the surface has occurred in certain areas. It may be said that in general the territory damaged has not been particularly valuable for farming purposes. Where the coal lies at greater depths, gentle sags from 1 to 3 feet deep may occur.

Little litigation in this district has resulted from subsidence. It is generally recognized that the mining of the coal at shallow depths must cause surface damage, and the coal is relatively much more valuable than the surface in Jackson County.

District III

This district includes room-and-pillar mines on coals No. 1 and No. 2 in the northwestern part of the State. The mines of this district were not visited in connection with this investigation, but some data upon subsidence have been collected in connection with other investigations. The commercial coal beds lie at shallow depths, a large proportion of the mines being opened by slopes and drifts. In general the room-and-pillar system of mining is employed.

The deepest mine examined in the district during the cooperative investigation is 210 feet deep, but the most of the hoisting shafts are not over 100 feet deep. In several of the mines where under- ground conditions are favorable, the longwall system can not be used because the lowering of the surface and the breaking of the thin rock cover is likely to permit the inflow of water and sand.

In parts of the district conditions permit of systematic pillar drawing and, where a large percentage of the pillar coal is removed, surface subsidence occurs. Owing to the thick surficial beds,1 con- siderable draw is likely to result from extensive falls. In several places the movement of surficial material has extended laterally a considerable distance, and in one case reported farm buildings, not undermined, have suffered damage.

JThe log of one of the shafts records 125 feet of surficial material.

SUBSIDENCE DATA 79

Where pillars are drawn and extensive falls occur, cracks 2 to 12 inches wide have appeared on the surface within a period of two weeks to three months. The depression following the removal of 44 inches of coal has varied from 12 to 20 inches, the average being 40 per cent of the thickness of the coal. Where the cover is thin and the surficial beds are deep, pit holes will form instead of sags. Ow- ing to the hilly nature of the country little damage has resulted by the formation of sags.

Comparatively little litigation has been due to subsidence in this district, most of the claims for damages having been adjusted by the interested parties.

District IV

This district includes the room-and-pillar mines on coal No. 5 in the north-central part of the State. The surface in District IV is in part flat, and in part hilly. The important coal bed outcrops in the northern and western counties and reaches a depth of 600 feet in parts of Macon County. Coal is mined extensively at a number of points ; in all there are about 250 mines, shipping and local, in the district. The average thickness of the coal is given as 4 feet 8 inches.2 In most of the mines either the room-and-pillar system or a modified panel system is used. However, in several mines the longwall system is employed.

In but few of the room-and-pillar mines are the pillars drawn, and the average extraction for the room-and-pillar mines of the dis- trict is 54 per cent. In one-half the mines included in the coopera- tive investigation, squeezes have occurred. "Where there have been so many squeezes under comparatively shallow cover surface, subsidence is to be expected. Surface cracks and subsidence seem to be related to the absence of limestone cap rock. Where sandstone is the cap rock subsidence is more marked."3

In the special study of subsidence in this district 15 mines were visited, and in all some subsidence was apparent or reported by the mining company.4 In but few places has serious damage resulted. Owing to the incomplete removal of the coal over any large areas, in most of the room-and-pillar mines of the district the pillar coal pre- vents subsidence in the form of extensive and regular sags. Where

2Andros, S. O., Coal mining practice in District IV : 111. Coal Mining Investigations Hull. 12, p. 15, 1915.

3Idem, p. 29.

4Tt should he stated that it was the general plan to visit mines at which it was re- ported that suhsidence had occurred. The 15 mines visited were more or less typical mines, hut it should not he inferred that suhsidence has occurred at all the mines of the district.

80 SURFACE SUBSIDENCE IN ILLINOIS

subsidence occurs over the parts of mines in which all the pillar coal has not been recovered, the surface may be broken by pit holes or caves or may be thrown into irregular hummocks and sags. Where the coal is worked near the outcrop, extensive areas are badly broken by caves (see figures 15 and 17). Where the surficial material is wet, it may run into the rooms when the roof breaks. The wet material may slip and run until the sides of the cave are on an angle of 25 to 30 degrees. Where possible some of these holes have been filled and now show on the surface as sags because of the settling of the filled ma- terial.

Data on 10 fairly typical mines are given in the accompanying table. Except for the two shallow mines, the extraction averages about 55 per cent.

Table 10. — Data on subsidence in District IV

Depth of shaft

Thickness of coal

Evidences of subsidence

Feet

Inches

35

58

Surface cracks and breaks, pit holes.

75

60

Surface cracks and breaks, pit holes.

160

54

Sags 18 inches deep ; some cracks.

175

72

Sags 24 to 36 inches.

185

54

Surface rolling ; some cracks and sags.

200

72

Sags 18 to 36 inches.

200

69

Sags 12 to 18 inches.

240

69

Sags 12 to 18 inches.

245

72

Sags 12 to 18 inches.

250

69

Sags 12 inches.

Comparatively little litigation has been the result of subsidence in this district except where the mines are located adjacent to large towns. A number of the mining companies operating in the suburbs of towns which have grown toward the mining location have been obliged to pay claims for damages to residences (see figure 44). One of the large companies reports that claims for damages have been so excessive that it has been found expedient to leave over 40 per cent of the coal to prevent subsidence. The president of this company claims that coal in this vicinity is worth $200 per acre to a developed mine and $100 per acre to a new mine. On this basis the coal left in the ground is worth from $40 to $80 per acre mined.

In the river bottoms the principal damage following mining has been the formation of sags that have been filled with flood water in the spring. A number of these cover extensive areas made unfit for cultivation except in years of light rainfall.

SUBSIDENCE DATA 81

When the coal right is not owned by the mining company, and the coal is mined on a royalty basis, the mining company does not usually assume liability for damage to the surface. Moreover, it is to the financial interest of the owner of the coal to have the extraction as complete as possible. Considerable coal is mined on this basis in this district, and therefore relatively little friction exists between the agricultural and the mining interests.

District V

The principal mines of Saline and Gallatin counties are opened upon coal No. 5. There are 21 shipping and 12 local mines in the district. In Saline County the average thickness of the bed is 5 feet 4 inches, and the depth from 25 to 400 feet.5

The problem of surface support, as well as that of the complete removal of the coal, is made more difficult by the occurrence of horses and faults. Mining is conducted by the room-and-pillar system, and the percentage of coal gained in the mines examined in the cooperative investigation ranged from 58.6 to 81.5, the average of seven mines being 67.1.

In the mines along the outcrop, considerable damage results to the surface by breaks and pit holes (see figure 13). When the coal lies at greater depths, cracks and sags may occur over the squeezes and extensive falls.

In figure 14 are shown two areas, A and B, which subsided as a result of squeezes adjacent to dikes. A 5-foot coal bed is mined at an approximate depth of 300 feet. The depressions are about 2 feet deep and are flooded part of the year.6 In another mine in the same district, due to the mining of a 6-foot coal at a depth of 265 feet, sub- sidence occurred adjacent to a fault. The break on the surface ap- peared 6 hours after the mine examiner had passed through the workings below. The surface subsided 2 feet 6 inches.

Mr. R. Y. Williams reports that in another mine in the same dis- trict a squeeze occurred December 23 to 26, 1914. The area affected underground covered about three acres. The squeeze occurred on the northeast side of a dike. Figure 15 shows the mine workings in relation to the dike. A number of pillars had been drawn where the coal was about 8 feet thick and where the cover was approximately 150 feet. The logs of holes No. 2 and No. 3 are given below.

BAndros, S. O., Coal mining practice in District V : 111. Coal Mining Investigations Bull. 6, p. 9, 1914.

flIt is estimated that tile could be laid for M mile for $400.

82

SURFACE SUBSIDENCE IN ILLINOIS

The rooms were 28 feet wide, and the pillars 12 feet. In order to stop the squeeze, cogs 6 to 8 feet square were built of 4- to 6-inch timber and filled with gob. Some of the cogs were 10 to 12 feet long by 6 to 8 feet wide. On the surface a few cracks appeared, but as the country is rolling it was impossible to observe the movement of the surface without the taking of measurements.

Log of drill hole No. 2 (Elevation at surface 341.24)

Description of strata

Thickness

Depth

Surface

Sand shale

Fire clay

Sand rock

Blue shale

Coal

Blue shale

White sand rock

Blue shale

Black slate

Coal

White sand rock

Blue shale

Coal

Sand rock

Ft.

18

20

1

10 29 2 2 3 14 4

3

42

6

in.

Ft. 18 38 39 49

in.

1

78

1

11

81

83

86

100

9

104

9

1

104

10

3

108

1

150

1

4

156

5

10

157

3

Log of drill hole No. 3 (Elevation at surface 285.09)

Description of strata

1 Thickness Depth

Surface

Ft.

14 6 5 5 4

15 3 3

52 9

1

n.

Ft. 1 14

20

25

30

34

49

52

55 107 116

n.

Sand shale

Sand rock

Blue shale

Black slate

Blue shale

Iron bowlder

Coal

Gray shale

Coal

0

2

SUBSIDENCE DATA

83

Table 11. — Data on subsidence at typical mines in District V

Depth of

shaft

Thickness of coal

Evidences of subsidence

Feet

Inches

150

72

Sag 24 inches deep.

225

72

Sag 30 inches deep (near a fault).

300

60

Sag 24 inches deep (near a fault).

400

56

Sag 18 inches deep.

Little litigation in this district has been due to surface subsidence. Owing to the hilly nature of the surface, the sags cause practically no damage to farm lands.

District VI

This district includes the mines opened on coal No. 6 east of the Duquoin anticline. There are 78 shipping mines in this district varying in depth from shallow mines along the outcrop to shafts a little over 700 feet deep. The average thickness of the coal is 9 feet 5 inches, the range being from 7J/2 to 14 feet.7 In the year ended June 30, 1914,

folio (No. 185), 1912.

the mines of the district produced about 23.5 per cent of the tonnage of the entire State. In 1914 the extraction reported by the operators was 56 per cent of the coal in the bed.8

Most of the coal is free from horses, rolls, and faults. The nature of the roof and floor, as well as of the overlying strata, has been discussed in Chapter II.

Subsidence has occurred at many mines, both at those along the outcrop and at some of the deepest mines of the district. "With present dimensions when rooms have been driven 200 to 300 feet there is a large area of unsupported cap rock. If an attempt is made to draw pillars under such conditions a squeeze is usually started which often rides over room and entry pillars and sometimes affects large acreage. In one mine a squeeze covered 85 acres ; in another 80."8

Data on subsidence were secured at 30 shipping mines. It has been impossible to formulate any definite statement in regard to the average amount of subsidence that has occurred or that may be ex- pected for different depths and when specific percentages of coal are extracted. The statement is made generally throughout the district that if 40 per cent of the coal is left in room pillars there will be no surface subsidence. This statement should undoubtedly be qualified,

7Shaw, R. W. and Savage, T. E., U. S. Geol. Survey Geol. Atlas, Murphysboro-Herrin RAndros, S. ()., Coal mining practice in District VI: 111. Coal Mining Investigations

Bull. 8, p. 15, 1914.

84

SURFACE SUBSIDENCE IN ILLINOIS

and the maximum width of room specified for various depths as well as the percentage of coal to be left in pillars.

At a few places levels have been run on the surface both before and after subsidence. When the mining companies have reported subsidence and have given the depth of a depression on the surface, the depth given has usually been an estimate or a measurement of the depth of the deepest part of the sag below the general level of the surrounding territory.

The most reliable of the data collected in the district are con- densed and presented in Table 12.

Table 12. — Data on subsidence at typical mines in District VI

Thickness

Depth of shaft

of coal

Evidences of subsidence

Feet

Inches

100

100

Breaks and pit holes.

115

96

Sags 60 inches deep and pits.

118

72

Sags 60 inches deep.

119

90

Pit holes.

120

108

Sags 48 to 60 inches deep and pits.

140

90

Sags 40 inches deep and pits.

140

108

Sags 60 inches deep and pits.

147

108

Sags 48 inches deep and pits.

150

90

Sags 48 inches deep and pits.

190

90

Sags 24 inches deep.

215

84

Sags 36 inches deep.

220

84

Sags 36 inches deep.

325

108

Sags 36 inches deep.

350

96

Sags 48 inches deep.

409

96

Sags 30 inches deep.

417

108

Sags 48 inches deep.

443

96

Sags 30 inches deep.

460

96

Sags 48 inches deep.

517

96

Sags 36 inches deep.

Little effort has been made to restore the surface where pit holes have been formed (see figures 12 and 19). This is due probably to the much smaller valuation of this land for agricultural purposes than the land in the northern part of the State, where it is not uncommon to fill the holes and to restore the general slope of the surface. As the country is much more hilly less damage results from sags and from the formation of ponds and swamps than in the prairie lands of District I.

Considering the amount of subsidence, there has been compara- tively little litigation. Claims for damage to the surface are generally

SUBSIDENCE DATA

85

adjusted out of court. It must be expected that in this district of thick coal, where coal mining is probably the leading industry, more or less injury to the surface must be endured if the coal resources are to be properly utilized. Moreover, below the coal bed now being worked are other beds which undoubtedly will attract attention after coal No. 6 has been worked out, and this deeper mining may also be expected to cause surface movement in part of the district.

District VII

This district includes all mines operating in coal No. 6 west of the Duquoin anticline, and north as far as an east-west line about 6 miles south of Springfield. The 150 shipping and 46 local mines of this district produce nearly 40 per cent of the entire tonnage of the State. The thickness of the coal varies from 2y2 to 14 feet, the average being 7 feet. The coal outcrops on the western side of the district and is deepest in Christian County where one shaft has opened it at a depth of 730 feet.

The coal is mined by the room-and-pillar and panel systems. The aver- age per cent of recovery is 55 ; that is 45 per cent of the coal in the bed is left in the mine and probably will not be recovered in the future. Because of the large area of this district, the failure to bring to the surface a proper percentage of the coal in the bed is a matter for serious consideration. A contributing cause of this waste of natural resources is the fear of bringing about surface subsidence and attendant damage suits. It would probably be economy for the

Table 13. — Data on subsidence at typical mines, District VII

Depth of shaft

Thickness of coal

Evidences of subsidence

Feet

Inches

70

72

Sags 60 inches deep and pit holes.

85

72

Pit holes.

160

84

Sags 40 to 50 inches deep.

190

90

Sags 48 inches deep.

300

75

Sags 24 inches deep.

325

100

Sags 12 to 60 inches deep.

328

100

Sags 42 inches deep.

330

84

Sags 30 inches deep.

385

84

Sags 20 inches deep.

400

93

Sags 24 inches deep.

408

78

Sags 30 inches deep.

430

90

Sags 14 inches deep.

435

96

Sags 18 inches deep.

450

96

Sags 14 inches deep.

466

96

Sags 12 to 18 inches deep.

470

96

Sags 18 inches deep.

585

102

Sags 24 inches deep.

86 SURFACE SUBSIDENCE IN ILLINOIS

operating companies to purchase the surface overlying the coal to be removed. Pillars could then be robbed and 30 per cent more of the coal bed could be recovered.9

As in a number of the other districts few data are available to show the actual difference in elevation of the surface before and after the coal has been removed. In most of the observations recorded in Table 13 the subsidence has been estimated or measured roughly by comparing the elevation of the low point of a sag with the general slope of the ground. In several instances the depth of filling required to bring railroad tracks to grade has been taken as an indication of the amount of subsidence.

In several of the smaller mining towns considerable damage to residences has resulted from surface subsidence, and some litigation has resulted, but most claims have been adjusted out of court. The mining company has made many of the repairs necessary to restore damaged buildings. One mining company has laid draintile to remove water standing in a sag caused by the mining of coal, and thus a valu- able field has been made available for agriculture.

District VIII

As previously noted both coals No. 6 and No. 7 are mined in the Danville district. The beds lie at varying depths, the deepest shaft in the district being 240 feet. The irregularities in the coal bed, roof, and floor have been discussed in Chapter II. As it has not been advisable to drive the rooms a uniform width on account of the irregularities, pillars have been gouged in a number of mines. The percentage of extraction in six cooperative mines ranges from 55 to 82 and aver- ages 71.5.

Adjacent to the outcrop where the coal beds have been mined by the room-and-pillar system large areas of the surface have been dam- aged by the formation of pit holes (see figures 16 and 35). In the deeper mines the squeezes that have frequently resulted from gouging pillars have often been attended with surface subsidence (see figures 30 and 31). The land back from the streams lies flat and is very fertile, and the formation of sags has caused considerable litigation when deed to the coal has not released the mining company from lia- bility for surface damage. Sags have been measured at several points, one of the largest measured having a maximum depth of 4.7 feet. The coal bed mined in this vicinity was 210 feet deep and averaged 6 feet in thickness.

9Andros, S. O., Coal mining practice in District VII: 111. Coal Mining Investigations Bull. 4, p. 16, 1914.

SUBSIDENCE DATA 87

Some damage has resulted to farm buildings and dwellings, largely because the coal was not removed completely from beneath the build- ings. A building standing over a small pillar may be seriously dam- aged by the draw, and if the pillar left is not directly beneath the build- ing, the building will be cracked seriously or broken by the tilting over the shoulder of the pillar (see figure 41).

CHAPTER V— PROTECTION OF SURFACE General Considerations

Though, as has been noted, it may be unwarranted to assume that coal may be removed over large areas without disturbing the surface, it is well known that over small areas such as roadways, rooms, and even small panels of rooms, the coal may be removed, and the roof will arch or, if it falls, break up to such an extent that the broken material may fill the entire volume of the opening. Under such conditions severe subsidence can not result.1

Where coal beds are to be mined, the surface above may be pro- tected from subsidence in several ways. In general, the most common and the most feasible are (1) by the use of pillars or artificial supports and (2) by filling methods.2

Pillars

In considering the service which a pillar may render, and in de- termining the size of the pillar for protecting specific mine openings or objects on the surface, it will be necessary to consider some of the following factors, in some cases all of them:

1. Unit strength of the material forming the pillar.

2. Height of the mine opening.

3. Dip of the mineral deposit.

4. Angle of "draw" or "drag" or "pull" over the pillars as observed in

the district or under similar conditions.

5. Angle of break of the overlying rock.

6. Strength of the overlying rocks.

7. Nature and amount of filling in the mined-out area adjacent.

8. Depth at which mining may be carried on without affecting the surface.

9. Bearing power of the bottom or floor.

10. Weight of overlying materials that must be supported.

Crushing Strength of Coal

Strength tests have been made upon Illinois coal, but the data secured have not been sufficient to warrant the formulation of rules based upon crushing strength alone. Moreover, the advisability of placing much reliance upon data secured from such tests is questioned

^his presumes that subsequently the weight of overlying measures does not compress the broken material to the same volume it occupied when solid.

frequently the total damage to the surface may be minimized by rapid and complete removal of the coal.

(88)

PROTECTION OF SURFACE

89

by many mining engineers on account of the difficulty of securing sam- ple blocks of coal which are really representative of the entire thick- ness of the coal seam. The blocks usually tested represent the strongest layer of coal, the weaker bands frequently being so soft that a repre- sentative sample can not be secured. Where the weight comes on pillars of such banded coal, the softer bands are crushed, and the harder bands must give way until a more or less uniform bed of crushed coal furnishes a support.

The results of the tests upon Illinois coal are as follows :

Table 14. — Compression tests* of Illinois coals

6

Equivalent

Location

section

Height

Maximum load

Top Bottom

Inches

Inches

Inches

Lbs.

Lbs. per sq. in.

12,401

Penwell Coal Co., Pana. . . .

111x12

111x12

124

316,000

2,090

12,402

Empire Coal Co

i5ixl7§

15 xl5i

11.3 540,000

2,170

12,403

W. W. Williams, Litchfield.

13ixl3I

14 xl4

141 186,000

1,000

12,404

Herdien Coal Co., Galva...

1Ux17:1

16 xl3

12 208,000

1,020

12,405

T. H. Watson, Litchfield. . .

131x12

131x12

15 224,000

1,360

12,406

C, W. & V. Coal Co.,

Streator

tlfx 9]

11 xlli

13

140,000

1,280

*Talbot, A. N., Compression tests of Illinois coals: 111. State Geological Survey Bull. 4, p. 199, 1907.

Angle of Break and Angle of Draw

Considerable discussion has been carried on in Illinois in regard to the angle of break and the angle of draw. European engineers who have studied subsidence for a number of years have observed that there is a first break and then a main or after-break. The so-called "first break" corresponds more or less to the break observed in the immediate roof as the longwall face advances.

Mr. S. O. Andros in discussing mining practice and conditions in the longwall field of Illinois states that subsequent to the first break at the shaft pillar and face, if the gob area has been properly filled so that the roof weight rides on the face of the coal, other roof breaks occur every 2 inches to 6 feet parallel to the coal face extending upward away from the face and toward the gob as the face advances. The distance between breaks depends principally upon the character of the roof and the packing of the gob. At the face of solid coal the cracks in the roof are difficult to see ; and they do not become easily visible (fig. 50) until the face has advanced 4 to 5 feet. The distance to

90

SURFACE SUBSIDENCE IN ILLINOIS

which these mining breaks extend into the roof depends upon the roof material, but they rarely extend more than 15 feet above the coal. The angle made by these breaks varies from 50 to 90 degrees from the horizontal, depending upon the roof material and the rate of settling. In summer when the face progresses slowly the cracks are more nearly vertical.3

In a number of places it has been possible to observe the effect of subsidence upon the strata 10 to 20 feet above longwall workings. As reported by Mr. Andros, very few cracks have been in evidence in the overlying shales and the strata have apparently settled gradually

Fig. 50.— Cracks in the immediate roof of a longwall mine in the La Salle area. (Photo by S. O. Andros.)

and with but little fracturing, and such cracks as have resulted have evidently closed after the face has advanced.

Though the observations in Illinois supported the general state- ment that the breaks extend back over the gob and are usually not evident at a height of more than 20 feet above the coal bed, practically no available data support or oppose the theory and observations of European engineers — namely, that the angle of break in the immediate

3Preliminary report on organization and method : Bull. 1, p. 20, 1913.

111. Coal Mining Investigations

PROTECTION OF SURFACE

91

roof is practically unimportant and that the important angle is the angle of draw (this of course refers to stratified rocks).

In discussing these angles the German engineer, Wachsmann, illus- trated his observations by figure 5. The beds directly over the worked- out portion of the coal are broken by subsidence depending largely upon the amount and the nature of the packing. This zone of breaking is limited by the plane through BE. However the measures beyond BE are disturbed by the sinking of the beds immediately over the mine workings and the draw may extend to and is limited by the plane through CE. It is then the angle CED which is important rather than the angle of the local breaks in the immediate roof.

Fig. 51. — Stress diagram of an ideal homogeneous cantilever (after Hal- baum).

Similarly British engineers have noted the angle of the local breaks in the immediate roof, but have given attention to the angle of draw as the more important and controlling factor in determining the size of shaft pillars and in indicating the probable limits of the general sub- sidence over extensive mine workings.

Probably this phase of subsidence has been discussed in more detail in a paper by Mr. H. W. G. Halbaum than by any other British en- gineer. Owing to the importance of a correct understanding of this phenomenon, as it applies to longwall mining in Illinois, the theory4 of Mr. Halbaum will be presented in some detail.

"It may be conceived that the measures overlying the coal bed act as a cantilever where the coal is undermined. The cantilever rests upon

'Halbaum, II. W. G., The great planes of strain in the absolute roof of mines: Trans. Inst. Min. Engrs. vol. 30, p. 175, 1905-1906.

92

SURFACE SUBSIDENCE IN ILLINOIS

the solid unworked coal, the load consisting primarily of the weights of the measures themselves. The cantilever will be like other canti- levers in certain particulars. It possesses a neutral surface. Above this surface all the stresses in the beam of strata are of the tensile order. Below it all are of the compressive order. The uppermost ten- sile stress and the lowermost compressive stress are the maxima of their respective orders; and both orders of stress regularly diminish

^^^mmm^^^^^^^^fi^^^^^^^^

Fig. 52.— Stress diagram of a low-down neutral surface (after Halbaum).

as the planes on which they act approach nearer to the neutral surface, at which plane both kinds of stress are reduced to zero. Figure 51 shows the stress diagram of an ideal homogeneous cantilever, whereas figure 52 shows the stress diagram of a cantilever with a low-down neutral surface. In figure 51 the neutral surface is at NS, at half the depth of the beam. The stresses in the upper section, a, are ten- sional, and the stresses in the lower section, b, are compressive. ABC is the absolute line, and AC is the mean line of elementary strain ; in this case, vertical. In figure 51 the neutral surface lies in the plane

PROTECTION OF SURFACE 93

NS. AS is the tensile component; SC the compressive component; and the broken line, CA, represents the mean elementary line of strain projecting over and toward the solid."

The idea that the main angle extends toward and over the solid coal is generally accepted by British engineers. According to this, subsidence will begin before the coal directly below (in the case of horizontal beds) has been mined. The size of this main angle is vari- able and depends upon the nature and dip of the strata and on the amount and character of the filling.

In Illinois a number of subsidences have been reported to have occurred in advance of the longwall face. Where pillars have been left to protect definite areas or buildings, it has been found that there is some subsidence over the edge of the pillar, but it is generally believed that this movement will occur only after the working face has been stopped for some time. Few data are available in Illinois either to prove or disprove the proposition that the wave of subsidence extends ahead of the longwall face as it advances. At one of the shallow mines in the longwall district a borehole was put down in advance of the coal, and when the longwall face reached the borehole an examination of the surface was made. It was found that subsidence had extended to the collar of the borehole and did not lag behind the longwall face.

Safe Depth theory as previously advanced

It has been argued by various engineers and mine managers that there is a depth below which it is feasible to remove all the coal without disturbing the surface and without introducing filling or making any special provision for filling. This theory is founded upon the assump- tion that after the coal has been mined the overlying beds break and fall and fill up the cavity produced by the removal of the coal. There is little evidence available in Illinois on which to base this theory, and a number of American engineers who have studied the problem agree that the "safe depth" theory is not logical.

INCREASE IN VOLUME OF ROCK BY BREAKING

In order that the effect of caving and crushing of overlying beds may be understood, it is important that the volume occupied by broken material be compared with the volume occupied by a unit of the same material when unbroken "in the solid." Extensive tests have been made by engineers to determine to what extent the volume of rock may be increased by crushing to various sizes. The French engineer

94

SURFACE SUBSIDENCE IN ILLINOIS

Fayol made elaborate tests, the results of which are shown in con- densed form in the accompanying table.

Table 15. — Volumes of different materials after crushing as compared with

volume "in the solid"

Nature of rock

Relative volumes

1%

™E

> £° y

.5 £^f, n! ^ y

.■S;§»a

« *° y

~-0 ^

K g »

4) rt 3

2 C

Clay .... Shale .... Sandstone Coal

100 100 100 100

196 213 219 207

209 210 214 224

226 221 211 199

225

-216

224

229

310

214

223

202

COMPRESSIBILITY OF BROKEN ROCK

Experiments have been made upon crushed material to determine to what extent it may be compressed. In general it is known to what extent rock in place, when crushed to a specified size, may occupy an increased volume when the crushed material is subjected to pressure. Fayol's results of compression tests upon crushed material are given in Table 16.

Table 16. — Compressibility of different materials after having been crushed

c

Rocks having been previously crushed or broken occupy space indicated, under pressure*

to

o o

Space occu- pied before being broke

I

Pressure

1,422 1b. per sq. in.

II

Pressure

2,844 lb. per sq. in.

Ill

Pressure 7,110 1b. per sq. in.

IV

Pressure 14,220 1b. per sq. in.

Clay

Shale

Sandstone .... Coal

100

100 100 100

100 90 128 116 136 125 130 125

75 110 120 118

70

97

105

109

The following conclusion was drawn by Fayol : "The material which ordinarily fills the graves of mines always occupies a larger space than it did ordinarily. After an expansion of about 60 per cent, it appears to undergo in workings of from 300 to 900 feet in depth, a compression of about 30 per cent, which leaves a volume of about 12 per cent larger than the volume of the unbroken rock."5

* Pressure I corresponds to a depth of strata of 1,638 feet; II, 3,276 feet; III, 8,190 feet; and IV, 16,380 feet.

BColliery Engineer, vol. 33, p. 548, 1913.

PROTECTION OF SURFACE 95

The United States Bureau of Mines0 had made tests upon crushed material from the Pennsylvania anthracite districts (Table 17) to de- termine its compressibility (1) when confined in steel cylinders, (2) when built into cogs, and (3) when piled in heaps.

Table 17 .—Compressibility tests upon crushed material made by the United

States Bureau of Mines

Lbs- Per Corresponding depth, Compression

SQ- £t- 140 lbs. per cu. ft. (Reduction in height)

Feet Per cent

A. Broken mine rock and breaker refuse compressed in a steel cylinder 16%

inches in diameter and 25^ inches high :

20,000 143 11.4

30,000 215 15.5

90,000 645 24.0

120,000 860 26.2

B. Mine rock passing 1^-inch ring, lying loosely in a conical pile and free

to flow :

23,824 170 60.0

40,256 290 62.3

92,445 660 65.0

165,000 1,180 67.0

C. Rock cog, built of mine rock and shoveled material. Pyramidal form; base

5 by 5 feet ; top, 3 by 3 feet ; height, 1 foot 1 1 inches.

22,000 167 22.0

30,000 215 27.0

90,000 645 36.0

120,000 860 37.2

From these data it may be stated that for the conditions of long- wall mining where the gob is well filled and the walls well built, the compression of the gob will be only 33 ]/$ per cent more for a mine 645 feet deep than for a mine 215 feet deep. Data are not available to show how much the broken shale, commonly forming the gob in the longwall district, will be compacted under pressure. The weight of 100 feet of overburden is sufficient to make it deform and flow.

Where the mine roof falls into a free space it may be more or less shattered, and as overlying beds fall successively, the worked-out volume may be filled eventually. The material which can not fall sinks upon the fallen material which may have been already compressed to such an extent that it may be able to check further subsidence.

In longwall mining the argument has been that, as the beds over- lying the gob subside, they compress the gob to a fraction of the thick-

"Unpublished data.

96 SURFACE SUBSIDENCE IN ILLINOIS

ness of the coal, and that as they sink they increase in volume suffi- ciently to prevent the movement extending to any great height above the coal horizon. As previously noted, observations in the Longwall District of northern Illinois tend to prove that as the roof along the roadways sinks, it breaks, and that cracks are formed extending parallel to the working face. These may be from 2 inches to 6 feet apart and usually extend up into the roof shale for a distance not exceeding 15 to 20 feet. Above this height no cracks are in evidence as the con- fined rock flows or is deformed.

On the basis of these observations there is little justification for presumptions or theories that the increase in volume of the beds sub- siding over longwall workings is sufficient to compensate for the total height of material mined. Apparently the increase in volume is limited to the strata, 10 to 20 feet thick, immediately overlying the coal bed.

Center line of road

Center li

O

Fig. 53. — a, Diagrammatic illustration through the gob and parallel to the face of a longwall mine ; b, plan showing pack walls and loose gob between road- ways and cross entries in the same mine.

The overlying beds are traversed by breaks or cracks "every 2 inches to 6 feet," and it is evident that the greatest increase in volume that can arise will result from the visible breaks or cracks; the increase in volume resulting from invisible cracks may probably be ignored without serious error. If the visible cracks occur every 2 inches to 6 feet, the material, along the roads at least, is obviously broken into slabs 2 inches to 6 feet long, and these slabs settle in fairly orderly fashion upon the filling. Under such conditions any considerable in- crease in volume of the overlying beds is not conceivable. If, however, the strata higher up do not sink gradually but remain undisturbed for a time, while the beds immediately underlying subside several feet, in time such resistant beds may fail, and large falls may occur with a considerable increase in volume of the broken material. In northern Illinois the average distance between room centers is 42 feet, the road-

PROTECTION OF SURFACE 97

ways are not less than 8 feet wide, and the pack walls are 4 yards wide on each side of the road. The distance between the walls should there- fore average about 10 feet. This space, called the "gob," would be filled more or less completely with waste material thrown back as the face is advanced. A cross-section along the face would then show the roof supported as indicated in figure 53 a.

There is little probability that the vertical amount of settling over the gob, if not more than 10 feet wide, will be materially greater than the settling over the roads which are 8 to 10 feet wide. Moreover an examination of old roads protected by pack walls or buildings shows that as the roof settles the pack walls are compressed and they tend to bulge and to spread horizontally, as is evidenced by the reduction in the width of roadways. It may logically be inferred that some spreading of the pack walls occurs on the side toward the gob. In time the gob would offer more resistance to settling than do the roadways although these are probably better protected at first, inasmuch as the pack walls are built more substantially on the roadside than on the gob side.

Where roads have been driven through abandoned longwall work- ings in which the old roads have been closed for years, it has been found on brushing to the necessary height for the new road, that there is but little undulation in the previously horizontal roof shales on ac- count of the settling over the pack walls. If such undulation exists it is undoubtedly much more marked in the strata immediately overlying the coal than in those at a greater height above the coal horizon. From the observations that have been made in the deeper longwall mines in Illinois, where the measures immediately overlying the coal are thick beds of shale, it is apparent that minor irregularities in filling and pack walls have but little effect upon the general subsidence movement; in fact, so little that only the most precise observations may indicate that such irregularities have influenced the general movement. When it is so impracticable, as is usually the case, to secure complete data on the continuity and uniformity of the overlying beds, it seems unwise to attempt to discuss the effect of minor irregularities in the filling and pack walls.

The conditions in the average longwall mine may be, for the pur- pose of discussion, considered to be as shown in figure 53,/;. As the longwall face is advanced new cross entries are started and the old ones are abandoned. Where the cross entries are turned at an angle of 45 degrees to the main entry, the distance between cross entries may be from 225 to 300 feet.7

Preliminary report on organization and method: 111. Coal Mining Investigations Bull. 1, p. 18, 1913.

98

SURFACE SUBSIDENCE IN ILLINOIS

From these cross entries are turned the roadways to the rooms or working places. These working places will average 42 feet in length along the working face, and the distance from center to center of roadway will be 42 feet. As shown in figure 53, the worked-out area is laid out more or less regularly in a series of parallelograms approximately 225 by 32 to 34 feet. Around the four sides of each parallelogram is a wall of mine rock built 9 to 12 feet thick. Within the four walls the space is at first partly filled with shoveled material,

A[

Cross entry

Cut for fire wall

JB

Main entry

Plan Fig. 54.— Diagrammatic illustration showing the flow of roof shale under pressure in a mine near Peoria.

and later the unfilled space, if any, may be filled by falls of roof rock. After the working face has been advanced a short distance, the roof settles upon the pack walls, and in time as these are pushed down into the underclay the roof within the pack walls may sag and bear upon the gob (fig. 54). When this state is reached, the parallelogram bounded by four pack walls, becomes in reality a long cog with walls built of mine rock, the center being filled with shoveled material.

PROTECTION OF SURFACE 99

These cogs tend to prevent surface subsidence. Their success in accomplishing this will depend upon the extent to which the clay bottom heaves in the roads, the amount the pack walls bulge, and the power of the pack walls and the gob to resist compression. As usually built these walls are not rigid.

Normally, when the main entries have been advanced 225 to 300 feet beyond a cross entry, a new cross entry is started, and in time the old cross entries are abandoned. Until they are abandoned suffi- cient height is maintained to permit the passage of mules. The roads are brushed and the bottom is lifted as long as it is necessary to keep the road open. Though the amount of material thus removed is occa- sionally large, relatively it represents but a small volume of the material within the parallelograms of rock filling on each side of the road. The bulging of the packs and the flow of the bottom may eventually cease where the material within the cog has been compressed to such an extent that it does not flow easily. After the roads have been aban- doned, the amount of flow is limited to the volume of the roadway itself.

For the purpose of this discussion it may be assumed that two longwall mines are operated under identical conditions except that the coal seam in one lies at a depth of 200 feet and the other at 600 feet. The plan of mining, dimensions of rooms, and other factors are the same, and the same amount of material is built into walls and stowed in the gob.

Considering only the matter of compressibility of pack walls and gob, it should be noted that the maximum amount of subsidence which can result in the mine 200 feet deep, is determined by the amount the gob and walls are compressed under the load of 200 feet of overlying strata. If the load were less, the distance through which the roof would sink would be less ; and if the load were increased, the distance through which the roof would sink would be increased up to the limit of compressibility of the material in the gob.

As the weight of the overburden increases directly with the depth, therefore the compression of the gob is greater for deep mines than for shallow mines ; it is greater for a mine 600 feet deep than for a mine 200 feet deep, other conditions being the same, providing of course that the limit of compressibility of the material has not been reached. It follows logically that so far as the factor of compressibility of filling controls — and in a majority of cases it is the controlling factor — the total amount of vertical movement tends to increase rather than to de- crease with the depth of mining.

To this general statement there may be exceptions, and there may be depths beyond which this would not apply; but it is the opinion and

100

SURFACE SUBSIDENCE IN ILLINOIS

the experience of British mining engineers that subsidence will follow longwall mining irrespective of depth. It is generally supposed that there is more or less flow in the rock beds overlying the coal after these beds have stood for a time depending for support upon the pillars or filling. However, it is difficult to find evidences of such flowage on a large scale.

In a mine near Peoria it became necessary to open a section of the mine that had been abandoned and build a fire wall extending up to an overlying limestone. Immediately overlying the 5-foot coal bed is a bed of shale approximately 10 feet thick, and resting on the shale is a stratum of limestone. The roadway from 12 to 15 feet wide, was

Scale of Feet o joo eoo

a i i—

Fig. 55.— Diagrammatic illustration showing to scale the size of shaft pil- lars for given depths as recommended by various mining engineers.

filled tightly with shale from the overlying bed. A cut through the shale to the limestone showed that the shale had under pressure prac- tically flowed to fill the opening, whereas the overlying limestone showed no breaks or cracks where it was possible to make an examina- tion. The conditions are illustrated by figure 54.

Shaft Pillars

Commonly in determining the size of pillars necessary to protect mine openings of a given width it is customary to assume a span of roof and overlying rock to be supported, to estimate the total weight of such

PROTECTION OF SURFACE 101

a block for the depth of the workings, and then with the known or assumed unit crushing strength of the material to be left in the pillar, the cross-section may be calculated readily. The dimensions may then be proportioned in order to secure the most economical and safest working conditions ; on the same general plan shaft pillars or other important pillars may be determined.

Numerous rules have been formulated for the calculation of shaft pillars in flat seams ; a great diversity of opinion prevails among en- gineers as to the required dimensions at various depths and with dif- ferent thicknesses of coal seams. This diversity of opinion is well shown8 graphically by figure 55.

In determining the size of pillar necessary to protect objects upon the surface, as has been noted previously, the ability of the pillar to carry the load is not the only uestion to be considered. Among the most important of the other problems is that of draw or pull over the pillar and the ability of the underlying bed to sustain the load concentrated upon it by the pillar. Quite frequently the underlying bed is less stable and has less crushing strength than the pillar. It seems logical then to proceed as follows in determining the size of pillar necessary to protect an object upon the surface:

1. Determine the lateral extent of pillar necessary in order to prevent dam-

age by draw.

2. Determine whether the pillar thus outlined is sufficiently large to

support the burden of the overlying beds without crushing.

3. Determine whether the load upon the pillar will cause the pillar to

be forced down into the underlying bed, or cause a flow of the under- lying material.

Room Pillars

At various points in Illinois it has been necessary for the coal mining companies to assume responsibility for any surface damage which may result from coal mining and it has been deemed advisable to leave in the ground a sufficient amount of the coal to prevent move- ment, at least to prevent the movement from extending to the surface.

Where the roof is strong it may stand for some time without breaking, but eventually in parts of the mine at least, movements of the top or bottom may cause a considerable area to be affected. The general theory of the arching of strata has been presented by a French engineer, Fayol.

8Knox, G., Mining subsidence: Proc. International Geological Congress, vol. 12, p. 798, 1913.

102 SURFACE SUBSIDENCE IN ILLINOIS

In his discussion of methods of protecting the surface, Fayol re- ferred to the use of pillars between the working places. "The meshes of the network consisting of pillars with working places between them should be made smaller as the workings are shallower. As the depth becomes greater the size of the meshes can be enlarged, and the dimen- sions of the areas worked can be increased relatively to the sizes of the pillars that are abandoned, regard being had to the height and width of the zones of subsidence, so that the various zones may be kept dis- tinct from each other. This general rule is susceptible of many com- binations according to the thickness, the inclination, the number, and the depth of the seams worked. If the excavation is of small dimen- sions the subsidences which take place above them are restricted in size and become enlarged both in width and height as the excavation increases in area. If the pillars at 1, 3, 5, and 7 be taken out (fig. 56), zones of subsidence similar in Zx, Z3, Z5, and Z7 would be produced ; but when pillar 2 is taken out the line of roof subsides on to the floor, and the zone of subsidence rises in Z2. The same thing happens when

&*~M?''m. zZk\ Vsr^rferSfer

t-\ ; — r — r-< — •) ^~N\

J. i» 1 i L

12 3 4-5 6, 7 8 3 JO II /Z f3 14 J5

Fig. 56.— Diagrammatic illustration showing the extent of subsidence result- ing from the removal of adjacent pillars (after Fayol).

No. 6 pillar is taken out, and if No. 4 pillar is taken out, the space comprised between the zones Z2 and Z6 is set in motion and deter- mines the formation of the zone Z4."9

It follows then from this statement of Fayol, that if the room pillars are properly proportioned and properly spaced, the disturbance of the strata may be limited to the volume within the zones. The material outside these zones throws no weight upon the material within the zone. Necessarily then any vertical pressure must fall upon the unmined material forming the pillars, and the pillars must be large enough to withstand this pressure. In stratified beds the problem is more complicated, owing to the fact that the beds act as single members and frequently sag under their own weight.

9Proc. South Wales Inst. Eng, vol. 20, p. 340, 1897. It should be noted that these zones outline the dome through which the movement extends, and not the limit of the "falling zone" as described by the Austrian engineer, Rziha.

PROTECTION OF SURFACE

103

In a paper before the Pennsylvania State Anthracite Mine Cave Commission, 1913, Mr. Douglas Bunting said, "The application of a formula for determining the safe size of coal pillars for various thick- nesses of veins and depths can be considered practical for depths greater than 500 feet, but it is doubtful if the same formula would be of any practical value for application to veins at less depth, and cer- tainly of diminishing practical value with reduction in depth and thick- ness of veins for the reasons that the variable conditions of vein, top, bottom, and other factors are of more consequence with small pillars than with large pillars."10 The average dimensions of pillars and rooms in ordinary room-and-pillar mining in Illinois are shown11 in Table 16.

Table 16. — Dimensions of rooms and pillars in Illinois coal mines

District

Average depth

Room width

Pillar width

Feet

Feet

Feet

II

140

26

19

Ill

90

22

18

IV

201

25

9

V

243

26

16

VI

270

22

18

VII

227

31

30

VIII

174

27

8

Average for State

208

26

19

Filling Methods

In a number of important mining districts in America and in Europe, filling methods have been used extensively both to increase the percentage of coal recovery and to reduce the surface movement. These methods are adapted particularly to dipping seams.

GOBBING

Waste material resulting from regular mining operations or broken for this particular purpose may be stowed or packed into the excava- tion. If sufficient or suitable material is not available underground, it may be lowered or dropped from the surface and stowed where needed.

10Bunting, D., Pillar and artificial support in coal mining with particular reference to adequate surface protection: Pennsylvania Legislative Journal, vol. 5, Appendix, p. 5988, 1913.

11Andros, S. O., Coal mining in Illinois: 111. Coal Mining Investigations Bull. 13, p. 76, 1915.

104 SURFACE SUBSIDENCE IN ILLINOIS

In the longwall field the waste produced at the working face is stowed in the gob, but usually a large amount of rock resulting from falls and brushing in the roads is hoisted and piled on the surface. Owing to the nature of longwall work it is claimed that it is imprac- ticable to stow much of this material in the gob. It might be done at an increased total cost per ton of coal mined.

In room-and-pillar mining the waste material can be stored under- ground much more readily than in longwall mining, and in many Illi- nois room-and-pillar mines practically no rock is hoisted.

The coal beds over the greater part of Illinois lie practically flat and the stowing of material in both room-and-pillar and longwall mining would be expensive on account of the necessity for hauling the filling long distances and because a large amount of shoveling would be required underground.

HYDRAULIC FILLING

The stowing of material by means of water12 has been employed extensively in a number of mining districts, but is not being employed at any Illinois coal mines. Hydraulic filling seems to be impracticable at present on account of the flatness of the beds, the clay floor, and the difficulty in securing suitable filling material in the coal district. As previously noted, little stone, sand, or gravel is available, and the surface is so valuable for agricultural purposes that the cost of material secured from surface pits would be prohibitive in most places. Deep pits would be impracticable generally, as considerable pumping would be necessary during a number of months in order to keep the pits unwatered. In parts of the State there might be a shortage of water during several months of the year.

Griffith's method of filling

It has been suggested by Mr. William Griffith that worked-out portions of mines be filled by blasting up the bottom and shooting down the roof. This suggestion was made in connection with a report to the

12The Copper Range Consolidated Copper Company, Painesdale, Michigan, is using compressed air to carry filling material for short distances through iron pipe. The material is the J4 -inch "stamp-sand" or crushed waste rock from the concentrating plants which is dropped from the surface through steeply inclined raises to the levels where filling is being carried on. On these levels the material is dropped into cars and hauled to some convenient point above the stope to be filled. The filling material is dumped from the cars into pockets or chutes and is distributed horizontally in the stopes from the bottom of these chutes. It has been found economical to carry the filling material by compressed air not to exceed 300 feet through the pipes. The pipe is shifted laterally, lengthened, shortened, or raised as may be necessary to distribute the material. There are no bends in the pipe and the material is handled dry.

PROTECTION OF SURFACE 105

Scranton Mine Cave Commission, and Mr. Griffith has secured a patent (U. S. Patent No. 1,004,418) covering this method.13

It has been suggested that by this method an increased percentage of coal could be recovered. So far as known this method has never been applied to beds as flat as those in Illinois. The conditions in Illi- nois do not lend themselves toward making this scheme practicable as the immediate roof is generally not hard enough to make the proper kind of filling and the bottom is underclay which is too soft to serve as filling that will increase in volume when blasted and in that condi- tion support any weight without being reconsolidated to occupy prac- tically the original volume.

Artificial Supports

At only a few places have artificial supports been introduced to protect objects upon the surface. The construction of cogs was neces- sary at one place to prevent the collapse of the beds under a railroad right-of-way. The workings were at shallow depth and complete filling seemed to be impracticable though the workings were still accessible. Two types of cogs were used, the more successful being a timber cog filled with mine rock. Masonry, concrete, reinforced concrete, iron, and steel structures have been employed in American and European districts to protect important structures and roads.

13Messrs. Griffith and Connor say, "It is a well-known fact that loose rock occupies from If to twice the volume of the same weight of rock in place. Your engineers have conceived the idea of taking advantage of this fact, well known to engineers, for the purpose of cheaply producing an adequate support of the rock and surface above certain classes of coal beds under the city of Scranton. So far as we know, this method, in its entirety, has never been used before in any coal-mining district, and the suggestion is here made for the first time.

"The process is applicable to beds less than 6 feet in thickness and consists simply in blowing up the floor and shooting down the roof of the mine, each to a depth equal to the thickness of the coal bed. This produces a total thickness of loose rock equal to three times the thickness of the coal. The rock would be well packed together and have great supporting power, and moreover the desired ends would be attained in a comparatively in- expensive manner."— Griffith, William, and Conner, E. T., Mining conditions under the city of Scranton: U. S. Bureau of Mines Bull. 25, p. 57, 1912.

CHAPTER VI— INVESTIGATIONS OF SUBSIDENCE

European

As previously noted a number of investigations have been made in Europe to determine the amount of subsidence which results from min- ing at various depths and under different geological and mining condi- tions. Some of these investigations have continued over long periods of years, and a few of them have been organized so that additional data will be secured from year to year. It has been possible, in districts where such observations have been made, to adapt the system of mining so that the maximum recovery of mineral may be made with least dam- age to the surface. Knowing what effect mining will have upon the surface, it has been possible for several countries to formulate laws and rules for the protection of the owner of the surface and at the same time to secure for the mine owner a just consideration of his rights. The problem of subsidence in Europe has been discussed by the author and Mr. H. H. Stoek in Bulletin 91 of the Engineering Ex- periment Station, University of Illinois.

United States

In the United States very little work has been done along these lines — practically nothing in the bituminous coal districts. Very few of the data that have been collected are available for the use of the public. The scarcity of data in the bituminous fields may be due largely to the fact that the most extensive mining operations for bituminous coal have been carried on where the surface is mountainous or is of little value for agricultural purposes.

If the percentage of extraction from the flat coal beds of the Middle West is to be increased it seems timely that there should be secured, in more or less typical mining areas, data which will serve to show the amount and extent of subsidence which may be expected to result from mining operations.

Suggested Illinois Investigation considerations It has been suggested that in Illinois investigations be made in- cluding measurements to determine the surface movement under va- rious conditions and also laboratory tests to determine the strength of rocks forming mine roofs and of materials used for supports. The bearing power of the materials forming the mine floor should also be investigated. The nature and amount of surface subsidence should be studied when different methods of mining are used and when various methods of filling are employed.

(106)

INVESTIGATIONS OF SUBSIDENCE 107

TYPICAL MINES

The idea has been advanced by George S. Rice,1 that typical dis- tricts should be selected giving special consideration to the longwall fields. For this work the following may be suggested :

1. Wilmington district where the coal occurs at a depth of 50 to 150 feet.

2. La Salle district where the depth to coal No. 2 is from 300 to 500 feet.

3. Decatur district where coal No. 5 lies at a depth of 500 to 600 feet.

4. Assumption district where coal is mined at a depth of 1,000 feet.

For the investigations in connection with room-and-pillar and panel work the following may be suitable :

1. Springfield district where the coal is mined at a depth of 150 to 200 feet.

2. Williamson County district where mining is at varying depths up to 300

feet.

3. Gillespie-Staunton district with depths of 300 to 400 feet.

4. Franklin County district where the depth of mining is 400 to 700 feet.

MONUMENTS AND SURVEYS

It has been suggested that substantial surface monuments be es- tablished in parallel lines across the working face in longwall mines and in room-and-pillar mines across panels that are to be worked. The monuments should be established over the solid coal before any move- ment has commenced, and the location of these monuments with regard to points underground should be determined accurately. At regular intervals levels should be run, and profiles made along each line of monuments showing the position of the working face, the original position of the surface, and the elevations at the time of the successive surveys.

The monuments should be located where they will be easily acces- sible to the surveyor but where they will not be interfered with, and they should be so constructed that they will not be lifted by frost. In addition to the levels, measurements should be made from time to time to determine the amount of lateral movement or draw.

UNDERGROUND WORK

It would be essential that a careful record be made of underground conditions. In room-and-pillar and panel work the date of opening rooms, the actual dimensions of pillars, and the position of the rooms with regard to surface monuments should be noted. When rooms are finished and the pillars are drawn, particular attention should be given to movements of roof and floor, and if a squeeze follows, a detailed report should be made as a basis for the study of surface movement and of other squeezes.

^hief Mining Engineer, U. S. Bureau of Mines, one of the representatives of the Bureau in this cooperation.

108 SURFACE SUBSIDENCE IN ILLINOIS

After the movement underground has ceased, whatever observa- tions are possible should be made in the vicinity, noting particularly the angle of break in the roof, the condition of pillars, the amount of open space above the falls, and the condition of the roof material which is standing.

In longwall mines the rate of advance should be noted regularly as well as the stability and width of the pack walls, the amount of material placed in the gob, the angle of fracture of roofs, the height to which breaks extend into the roof, the rate of settling of the roof, and the ratio of compression of pack walls. Observations should be made to discover the amount of sag in a direction parallel to the face and between the pack walls.

If practicable the amount of filling in one section of the mine should be kept constant, but a different amount should be used regularly in another part, in order to discover the effect that filling has upon sub- sidence.2

POSSIBLE BENEFITS

From the data secured it should be feasible to determine the amount of subsidence which may be expected per foot of material ex- cavated at different depths and with different geological sections. It should be possible to predict in longwall mining whether the wave of subsidence will move in advance of the mining face or lag behind it. The amount of draw after the advance has ceased could undoubtedly be predicted with more accuracy than is possible at present. The effect of leaving pillars could probably be shown with greater certainty, and it would undoubtedly be possible to protect such structures as must be left undisturbed with less interference with mining operations.

As previously noted, nearly one-half the coal is being left in the ground and made unavailable for future mining. At present the lost coal in the mines of southern Illinois represents a larger tonnage of coal per acre than there is in the virgin coal seam now being mined in northern Illinois, and each ton of this abandoned coal has a heating value of 7 to 8 per cent greater than the coal mined in northern Illinois.3

2This might be feasible in a mine in which the undercutting is done in part by hand and in part by machines. In one mine the ratio between the volume of material removed in the undercutting by machine is about one-fourth as much as in undercutting by hand.

sThe average analysis of 58 samples of coal No. 6 from east of the Duquoin anticline showed a heating value of 11,825 B. t. u. The extraction of 56 per cent of the seam which averages 9 feet in thickness results in leaving 6,732 tons to the acre. In northern Illinois the average thickness of the coal mined is 3 feet 2 inches, and the average heating value, as indicated by 38 samples, is 10,981 B. t. u. In other words, in northern Illinois the virgin coal seam contains per acre 5.700 tons of coal of 10,981 B. t. u., whereas in District VI the average worked-out mine contains 18.1 per cent more coal of a heating value 7.7 per cent greater than does the unworked tract in northern Illinois.

INVESTIGATIONS OF SUBSIDENCE JQ9

It is hoped that a careful study of the subsidence problem in the various mining districts will lead to a better realization of the necessity to the State for extraction of the coal where the damage to the surface will be reparable. Where the damage to the surface is reparable and in excess of the value of the coal, the facts should be made known and the proper steps taken. Where the damage to the surface is irreparable and in excess of the value of the coal, it seems advisable to stop the mining of coal completely until such time as the increased value of coal may be sufficient to warrant the renewal of mining and the com- plete extraction of the coal possibly under improved methods of min- ing.4

The State should give attention to the problem of conservation in those extensive areas of coal fields where the right to the coal is held by one party and the surface by another, but under the existing deed the owner of the coal is held liable for any surface damage resulting from mining operations. Under such conditions it may be expected that nearly 50 per cent of the coal will be left in the ground. Against this practice a protest may well be raised and the State of Illinois should find or create a remedy.5

t ♦•* tGe7™-S' R^C' ^ a" informal address in New York, in 1913, before the American Institute of Mining Engineers, stated that he had been shown leveling data in Upper Silesia, Germany relating to surface elevations in and about a certain important iron works, which showed that where granulated slag had been hydraulically stowed in coal-mine excavations in a bed 20 feet thick in which there had been practically complete extraction of coal the subsidence had not been appreciable, at the maximum point showing only 2^ inches Also that since the hydraulic stowing system had been inaugurated in Westphalia the German government had allowed mining under important munition manufacturing buildings at the Krupp works in Essen, whereas formerly under the old dry-filling methods there had been serious subsidence in and about parts of Essen resulting in cracked walls and buildings which he had observed in 1911.

"It has been suggested that a tax be levied upon coal left in the ground. This might be done by assessing the land or coal right upon the full tonnage originally in the ground less the amount which has actually been recovered, and continuing this assessment against the land until the unmmed coal is recovered. If this were to become the general practice it would undoubtedly tend to induce the mining companies to remove more of the coal

INDEX

PAGE

Agriculture, effect of subsidence

on 50-61

Angles of break and of draw. 31, 89-93 Assumption, mining at 75

B

Belleville coal, see coal No. 6 "Blue band" coal, see coal No. 6 Bond County, subsidence in.. 73, 85-86 Brown County, subsidence in. 73, 78-79 Buildings, effect of subsidence on. 64-72 Bunting, Douglas, acknowledg- ments to 103

Bureau County, subsidence in. 73, 75-78

Calhoun County, subsidence in...

73,78-79

Canals, effect of subsidence on. ..64-65 Carbondale formation, description

of ...:.. 19

Carlinville limestone, description

of 22

Carterville, subsidence at 37

Cass County, subsidence in. . .73, 78-79

Caves, description of 34-42

Christian County, coal No. 6 in... 25

subsidence in 73, 85-86

Clark County, structure in 20

Cleat, effect of, upon subsidence. .26-27

Clinton County, coal No. 6 in 25

subsidence in 73, 85-86

Coal City, subsidence near 43, 65

Coal field, extent of 15

"Coal Measures", description of. 18-21

Coal No. 1, production from 15

roof and floor of 24

stratigraohic position of 19, 21

Coal No. 2, faults and rolls in 27

production from 15

roof and floor of . ; 24

strata associated with 21

stratigraphic position of 19, 21

Coal No. 5, faults and rolls in... 27-28

production from 15

roof and floor of 25

Coal No. 6, depth of 21

faults and rolls in 28-29

production from 15

roof and floor of 25-26

strata associated _ with 22-23

stratigraphic position of 19, 21

subsidence above 32

Coal No. 7, faults and rolls in. . . . 29

production from 15

roof and floor of„ 26

strata associated with 23

PAGE

stratigraphic position of 19, 21

Coal rights, values of, in Illinois. 55-56

Compressibility of rock 94, 95

Compression tests of coals 89

Conservation of coal 11, 13

Cracks, surface 30-34

Crushing strength of coals 88-89

Cuba, subsidence near 39

Damage caused by mining 30-72

Danville, subsidence near

39,49,59,66,67,86-87

Dewitt County, subsidence in. .73, 79-81

Dewmaine, subsidence at 40

Dikes, subsidence associated with. 38 Duquoin anticline 21

E

Edgar County, subsidence in.. 73, 86-87 Edwards County, structure of.... 19 Europe, study of subsidence in.. 106

Faults, relation of, to subsidence. 27-29

Fayol, acknowledgments to

93-94,101-102

Filling methods 103-105

Franklin County, coal No. 6 in... 25

structure in 20

subsidence in

... .32, 44, 46, 62, 64, 69, 70, 73, 83-85

value of land in 52

Fulton County, subsidence in. 73, 78-81

G

Gallatin County, subsidence in.

73,81-83

Gobbing, relation of, to subsidence

... 103-104

Greene County, subsidence in. 73, 78-79 Griffith, William, acknowledgments

to 103-105

Grundy County, subsidence in. 73, 75-78

H

Halbaum, acknowledgments to... 91-93 Hamilton County, structure of . . . . 19 Hancock County, subsidence in..

. 73,78-79

Harrisburg, subsidence at 37

Henry County, subsidence in. 73, 78-79 Herrin coal, see coal No. 6

Jackson County, subsidence in

J ...73,78,83-85

Vergennes sandstone in 21

Tersey County, subsidence in. .73, 78-79

(110)

INDEX— Continued

111

PAGE

K

Kentucky, value of land in. . .50, 52-54 Knox County, subsidence in. ..73, 78-81

Lands, values of 50-61

La Salle anticline 20

La Salle coal, see coal No. 2

La Salle County, subsidence in...

, • 73,75-78,90

value of land in 52

La Salle _ limestone, stratigraphic

position of 23

Lawrence County, structure in....' 20 Logan County, subsidence in . . 73, 79-81 Longwall District, geology of... 23-24

subsidence in

.... 42-43, 48, 64, 71, 73, 75-78, 95-96 Longwall mining, relation of, to

subsidence 30

M

Macon County, subsidence in. .73, 79-81 Macoupin County, coal No. 6 in . . 25

subsidence in 73^ 85-86

Marion County, coal No. 6 in . . ! . 26

structure of 19

subsidence in '.'.'.73, 85-86

Marshall County, subsidence in...

73 75-78

Mason County, subsidence in.73,' 79-81 McDonough County, subsidence in

7 '3 78-79

McLean County, subsidence in.'

^T _ ••••• 73,79-81

McLeansboro formation, descrip- tion of 19

Madison County, coai No. 6 in... 25

subsidence in 73, 85-86

Menard County, subsidence in.73, 79-81 Mercer County, subsidence in.73, 78-79 Montgomery County, coal No. 6 in 26

subsidence in 49, 73, 85-86

Morgan County, subsidence in.73, 78-79 Moultrie County, coal No. 6, in. . . 26

structure of 19

subsidence in ....73, 85-86

Murphysboro coal, see coal No. 2

N

New Haven limestone, strati- graphic position of 22

Nokomis, subsidence near 49

PAGE

Pillars, relation of, to subsidence.

p. ,• •••••........; 100-103

Pit holes, description of 34-42

Pottsville formation, description

„ ,of ;••••• 18-19

r roduction of coal 15-17

Protection of surface .88-105

Putnam County, subsidence in.73, 75-78

R

Railroads, effect of subsidence on

Randolph County, coal No*. 6 in.'. 26

subsidence in 7^ 85-86

Kice US., acknowledgments to.. 107

Kichland County, structure of 19

j Roads, effect of subsidence on 64

I Rock Island coal, see coal No. 1 Rock Island County, subsidence in

p „ ••••••;..... 73,78-79

Kolls, relation of, to subsidence. .27-29 Room-and-pillar mining, relation of, to subsidence 30

Peoria County, subsidence in. .73, 79-81 Pennsylvania, value of land in.. 50, 54 Pennsylvanian series, description

o£ 18-21

Perry County, coal No. 6 in 26

subsidence in 43, 45, 73, 83-86

Piatt County, structure of 19

"Safe depth" theory 93

Sags, description of '.'.42-50

St. Clair County, coal No. 6 in . 26

subsidence in 73,85-86

Valine County, subsidence in.. 73, 81-83

structure in 20

Sangamon County, coal No. 6 in. . 26

structure of 19

subsidence in '.'.73, 79-81, 85-86

value of land in 52

Schuyler County, subsidence in...

73,78-81

Scott County, subsidence in.. 73, 78-79 Sewers, effect of subsidence on.. 69-71

Shafts, depths of 17 ig

Shawneetown fault ' 20

Shelby County, coal No. 6 in 26

subsidence in 7^ 85-86

Mioal Creek limestone, description

.of 22

Springfield, subsidence in 68

Streams, effect of subsidence on. 64-65 Streator, subsidence near. .41, 60, 66, 68 Streets, effect of subsidence on.. 69-71 Structure of "Coal Measures". .. 19-21 Subsidence, damage caused by... 28-72

data on 73-87

evidences of WW. .30-50

geological conditions affecting.' 18-29

investigations of 106-109

protection against 88-105

T

Tazewell County, subsidence in.

73,79-81

Third Vein coal, see coal No. 2 Transportation, relation of, to sub- sidence 61-65

112

INDEX — Continued

PAGE

U

United States, study of subsidence

in 106

Vermilion County, subsidence in..

39,47,49,59,73,86-87

value of land in 52

Vergennes sandstone, strati-

graphic position of 21

Virginia, value of land in 54

W

Wachsmann, acknowledgments to

31,91

Warren County, subsidence in. 73, 78-79 Washington County, coal No. 6 in 26

subsidence in 73, 85-86

Water supply, effect of subsidence

on . . 71-72

Wayne County, structure of 19

West Virginia, value of land in.. 50, 54

White County, structure of 19

Will County, subsidence in 73, 75-78

Williamson County, coal No. 6 in. 25

subsidence in 63, 73, 83-85

value of land in 52

Woodford County, subsidence in.

73,75-78,79-81