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“MASS SOIL MOVEMENTS “IN THE HJ. ANDREWS
EXPERIMENTAL FOREST -} A C. 1. DYRNESS |
U.S. DEPT OF AGRICULTURE MATIONAL RG RICUETURAL LIBR ARY
OCT 5- 6/
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CURRENT SERIAL RECORDS
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PACIFIC NORTHWEST we FOREST. AND=RAMGE EXPERIMENT STATION. + 7/5
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U.S. FOREST SERVICE Research Paper PNW— 42),
5967, I
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CONTENTS Page LINE BUG) DNOKO BPI KOWN ae eh a A SNS a) le 1 STUDY AREA. : . . ° . : . . . ° ° 1 ALISO D) SyIOURIWAL Gis : : : Sumner. Go hoy) Moe a 1 METHODS . . . . . . : . . : : . . 2, RESULTS . 4 . . . 5 . . . . . . ° 2 Mass movements by morphological class . 2 Mass movement by mode of action Or GistuT Da NGem eas) Fe whee iene ae inter tues 4 Relationship between mass movements and certain site characteristics. . .- 9 DISCUSSION AND CONCLUSIONS . . - - « 12
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INTRODUCTION
Each winter a number of mass soil movements, such as earthflows and slumps, occur in the Coast Ranges and Cascades of western Oregon. These movements, generally occurring during periods of heavy and prolonged rainfall, often disrupt roads and dam- age other improvements. Some, how- ever, remain undiscovered in remote, undisturbed areas.
During the winter of 1964-65, there was an unusually large number of mass soil movements in western Oregon. <A high proportion of these movements occurred during the severe storm that struck northern California and Oregonthe week before Christmas. Soon after this storm, it was found that the H. J. Andrews Experimental Forest was in one of the hardest hit areas and was in an excellent location for an intensive survey of stormdam- age with emphasis on the origin of large mass soil movements. Accord- ingly, asurvey of allmass movements in the Experimental Forest was initia-
ted April 1 and concluded July 1,1965.
The study had several aims:
(1) to completely describe and photo- graph all major movement areas in the Andrews so that changes in the future may be followed and documented, (2) to identify the types of movement which had occurred and estimate the amount of damage to the site and/or improvements, (3) to attempt to de- fine the relationship between the mass soil movement and (a) amount of man's disturbance and (b) certain site factors such as geology, soil, elevation, and aspect.
STUDY AREA
Wine ele aig GMalchaxe ws IDagoKsigi mental Forest comprises the entire 15,000-acre drainage of Lookout Creek, located approximately 40 miles east of Eugene, Oreg., and part of the Willamette National Forest. The area lies within the Western * Cascades physiographic province and is pre- dominantly mature topography with sharp ridges and steep slopes. Bed- rock in the area is made up of pyro- clastics (tuffs and breccias) and basic igneous rocks (basalt and andesite). Forest cover is comprised of old- growth Douglas-fir (Pseudotsuga menztesitt) , with varying amounts of western hemlock (Tfsuga heterophylla) and western redcedar (Thuja plicata).
THE STORM
Fredriksen has described the characteristics of the 1964 Christmas storm in the H. J. Andrews area in some detail. < At lower elevations, more than 13 inches of rain in 4 days was supplemented by an undetermined amount of snowmelt. This storm has generally been classified as an event with a 50-year return period. How- ever, Fredriksen pointed out that in
ay Berntsien, (‘Carl Min 7 and Rothacher, Jiacks A guide sto: the Hi: J. Andrews Experimental Forest. Pacific Northwest Forest & .Range IDe§5 Sus}. Glas 4 aUllwiso aOao.
gy Fredriksen, R. L. Christ- mas storm damage on the H. J. Andrews Experimental Forest. U.S. Forest Serv. Res. Note PNW-29, 11 Dido wikis, MYOS.
only 12 years of record at the Experi- mental Forest, this was the second storm to total more than 13 inches of precipitation within a 4-day period. He concluded, "we believe 4-day storms which deliver 13 inches are probably not unusual.'' The fact re- mains, however, that this storm caused the most extensive damage re- corded establishment of the Experimental Forest in 1948.
since
METHODS
Information recorded at each soil movement site included type and specifics characteristics of @ soil) tox debris movement, general character- istics of the area, and assessment of factors influencing the soil or debris movement. Photographs were also taken at each site and a sketch made showing the outstanding features. An attempt was made to find and describe all events where 100 cubic yards or
more of soil or debris were involved.
RESULTS
Descriptions of 47 mass move- ment events were made in the Andrews INope@mis (blero 1h))s Estimated ranged from 100 to 75, 000 cubic yards, and the total quantity of material moved was about 347, 800 cubic yards.
sizes
Mass movements
by morphological class
The movements have been grouped into 10 classes on the basis of their morphological characteristics (table 1). Earthflows occur when the soil becomes saturated, and pore- water pressure builds up to such an extent that the soil mass flows out in a generally tongue- shaped form. Slumps, generally occur in saturated soils; however, they ane characterized by rotational movement
and spoon - shaped
also,
failure planes.
Table 1.--Mass movements occurring on the H. J. Andrews Experimental Forest
during the winter of 1964-65, by morphological class
Morphological class
Earthflow
Slump with earthflow
Slump
Earthflow causing channel scour Slump causing channel scour
Debris slide causing channel scour Channel scour
Roadfill washout
Avalanche with rilling
Debris avalanche with earthflow
Total
Events Material moved
Cubic yards
104,600 14,525 4,050 82,350 106,300 7,500 26,300 1,050 100 1,000
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347,775
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Channel scouring is generally caused by massive and sudden failure of the entire unconsolidated mantle, most often in areas of the oversteepened drainage heads. Debris avalanches involve the sudden downslope move- ment of largely rock material in steep terrain. The earthflow, slump, and avalanche classes are defined by Eckel. 2/
Earthflows were the most com- mon class of soil movement with 18 out of the total 47 events falling within this class (table 1). Channel scouring events ranked second with a total of 14. Only three of these events were confined entirelyto the drainage chan- nel; the remainder were triggered by a separate mass movement which fre- quently began at a road. The third most commonly occurring movement class was the slump with some earth- flow characteristics. Soil material which has slumped down, when satu-
rated with water, will frequently flow out at the toe and may travel a great distance downslope. Only three slump events were completely devoid of flow characteristics. These were backslope slumps where the material came to rest on the road surface.
A consideration of amounts of material moved emphasizes the im- portance of earthflow and channel scouring events (table 1). The re- maining forms of movement contrib- uted very little to the total amount of material moved.
Mass movement by mode of
action or disturbance
Mass movements on the Andrews were also classified accord- ing to type of disturbance or mode of action (table 2), with attention focused on the source area of the event. For
Table 2.--Mass movements occurring on the H. J. Andrews Experimental Forest
during the winter of 1964-65, by mode of action or disturbance
Type of disturbance
Roadfill failure
Road backslope failure
Road backslope and fill failure Events caused by road drainage water Road removed by stream
Events in logged areas
Events in undisturbed areas
Total
3/
— Eckel, Edwin B., ed. Landslides and engineering practice. High- way Res. Board Spec. Rep. 29, 232 pp., tlws.. Washington) Dr Grasse
Events Material moved Number Cubic yards 1b) 40,900 5 14,400 6 38,325 8 86,450 3 4,350 8 227, le)
5 141,200 47 347,775
when a channel scouring
example, event originated in an untouched stand of timber, itwas classifiedas a move- ment occurring in an undisturbed area even though at adownstream location,
roads may have been severely damaged.
Although only five events oc- curred in undisturbed areas, this type contained the largest volume of mate- rial. Events caused by road drainage water ranked second, with the other five types far behind. Each class is
considered separately below:
1. Roadfill failure.
The most common single type of mass movement on the Andrews was the roadfill failure (12 events) (fig. 2). Thisis apparently a rather simple type of movement caused
by the saturation of the roadfill embankment. Most of the fill ma- terials, especially those derived
from pyroclastics, appear to flow readily when saturated. As nearly Asp mCouldsuber wdetermuned,) sthese failures were not due to improper rather, were large
road drainage but, caused by’ the unusually amounts of rainfall or perhaps by
improper fill compaction.
2. Road backslope failure.
This isa common event, which frequently results in road blockage Gaige'S) kn inhie ss tanlunensmost |oLten takes the form ofa slump, but the slump material may have earthflow characteristics near the
some
toe. Themost failure-prone back- slopes were those constructed in areas of deep, well-weathered tuffs and breccias. These materials fail easily when exces- If failures did occur inareas of andesite or basalt parent material, invariably the local bed- rock was highly fractured. Like the majority of other mass move-
saprolitic
sively moist.
ment events, backslope siumapis occur only when the mantle is satu- rated and are also more common inareas where mass movement has
occurred repeatedly in the past.
Figure 3.--Backslope failure in an area of soil derived from andesite
colluvium.
Figure 2.--Roadfill failure in the H. J. Andrews Experimental Forest.
3
Road backslope slump causing fill failure.
Six events in the Andrews fit within this type (fig. 4). In effect, this is a ''chain reaction''! with the follows: A portion of the backslope becomes saturated and slumps down onto the road surface, thus blocking the in- side drainage ditch.
general sequence as
The drainage water is then diverted across the road surface and down onto the fill slope from the outside portion of the road. The fill slope becomes saturated with water and an earth- flow type of movement follows, en- croaching to a varying extent onto the road surface. In some cases, the ''chain''is further lengthened by the fact that the earthflow on the fill slope may trigger channel scour- ing in a drainage below.
Mass movement caused by concen- tration of road drainage water.
Thereare twoclasses ofevents under this heading: (a) those due to the concentration of road drainage water, even though the road drain- age system was apparently function- ing properly (fig. 5), and (b) those
caused by failure of some portion-
of the drainage system--for exam- ple, a clogged, culvertion inside ditch.
Events caused by concentration ole road) jidrainagemmavatek. our
events in the Andrews fall. within this category. In ~all™ cases, ‘the event was associated with stream channel scouring. The typical se- quence was as follows: Extremely large quantities of water flowing from aculvert completely saturated the soil mantle below the road. In
mostcases, this saturatedarea was
Figure 4.--View of typical backslope slump which has also caused roadfill! failure.
near the head of a tributary drain- age. When the soil became satu- rated, the entire mantle either slumped or flowed downslope and temporarily blocked the drainage channel. When a sufficient head of water was built up, this temporary dam breached, and a wall of water and debris rushed down the channel removing everything in its path. The endresult was adrainage chan- nel from which virtually all uncon- solidated material has been remov- ed exposing bare rock.
All four events of this type occurred in areas of hard, frac- tured, greenish breccias and tuffs.
Figure 5.--Soil movement at the head of a drainage caused by high flows through the culvert and inadequate energy dissipation at the outlet end.
Events caused by failures in the road drainage system. --Of four
of these events in the Andrews, three were 200 to 500 cubic yards in size, while the fourth involved approximately 75,000 cubic yards of material. In every case, these failures occurred in areas of deeply weathered tuffs and breccias. All were flow-or slump-type movements of roadfill or sidecast material, none of which resulted in channel scouring. In these cases, the con- centration of water resulting in the necessary saturation came from the clogging of aculvert or inside ditch (fig. 6). These blockages had grave consequences because of the tremen- dous volumes of water diverted onto the roadfill or sidecast materials.
Road removed or damaged by stream.
In three cases, culverts carry- WAS jo wmEiboMENL Org shiny S i iaalaliy is Oooh streams failed either because of inadequate size or, more probably, because they became blocked by debris (fig. 7). Asa result, roads were completely washed out or were severely damaged.
Figure 7.--Debris which blocked the culvert and
caused several feet of material to be deposited on the road surface. (Road, in foreground, has been repaired.)
Figure 6.--Massive roadfill failure caused by blockage of road drainage ditch.
SM SNP ON
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6.
Mass movement events in logged areas.
The eight mass movement events in logged-over areas were principally earthflows (fig. 8), al- though two also had some slump characteristics. They ranged in size from 150 to 9,500 cubic yards of material. Four ofthe movements occurred inareas which gave ample evidence of considerable mass soil movement in the past (i.e., very uneven, hummocky relief). The remaining four were on smooth, steep slopes. In two cases, where the movements occurred at the heads of drainages, there was ex- tensive channel scouring.
Althoughthese movements took placein disturbed, logged areas, in every case it is impossible to say whether the logging per se caused the instability. It is probably safe to assume that at least one or two of them would have occurred even if the area had not been logged.
Mass movement events in undis- turbed areas.
Of five mass movement events in completely undisturbed areas, all were connected with channel scouring (fig. 9), but three alsohad earthflow characteristics at the source area. Size of these move- ments ranged from 200 to 75,000 cubic yards--four were among the largest slides that occurred on the Andrews. The average for all five events was well over 28,000 cubic yards.
Figure 9.--Channel scour event in
undisturbed timber.
Figure 8.--Massive earthflow in recently logged area.
With one exception, these events constituted failures at the heads of drainages. These areas are often very steep and unstable due to the headward erosion of the stream channel. When these areas become completely saturated, the weight of water adds to the driving force and at the same time de- creases the strength of the soil. Sudden, massive failures frequently Result bhwts es) uncdoubtedly,) thie normal course of geologic erosion in this area, and there is little that man can do to forestall events of this type.
Relationship between mass movements
and certain site characteristics
The influence of roads on mass movements inthe Andrews is clearly indicated in table 3. Although over 72 percent of the mass movements occurred in connection with roads, only 1.8 percent of the total area of the forest is in road rights-of-way. On the other hand, only about 11 per- cent of the events occurred in undis- turbed areas. Since 84.6 percent of the total area is undisturbed, this is far less than would be expected if the events occurred in a random manner.
In the western Cascades, it has been observed that mass soil move- ments occur much more frequently in areas of pyroclastic rocks (tuffs and breccias) than in areas wherethe bed- TOE 1S, COladyoirigsecl VOR) lyeiseule 9 ee andesite. It has also beennoted else-- where that greenish tuffs andbreccias are more unstable than are their red- dishcounterparts. Theserelationships are borne out by the data presented in table 3. About 94 percent of the events occurred in areas with a tuff
and/or breccia substratum, even though only 37 percent of the total area is made up of these rocks. Moreover, 64 percent of the mass movements were on greenish tuffs and breccias which make up only 8 percent of the total area--clearly indicating the un- stable nature of these materials. Al- though about 63 percent of the area is underlain by basalt and andesite mate- rials, only 6.4 percent of the mass movement events occurred there.
When soil series in the move- ment area are considered, the rela- tionship described above still holds but to a lesser extent. Eighty-seven percent of the events occurred in areas of soilfromtuffs and breccias-- the breakdown being almost 49 percent on soils from reddish tuffs and brec- cias and 38 percent ‘on ‘soul's from greenish tuffs and breccias. The apparent discrepancy between these figures and those for the substratum reported above may be attributed to the fact that many soils have been formed in transported parent mate- rials and oftenthere is little relation- ship between the soil profile and the substratum. It was noted in several cases that the discontinuity between the soil profile and the underlying bedrock constituted the failure plane, probably due to inherent weakness and water flowing through the zone.
Elevational range in the An- drews Experimental Forest is approx- imately 1, 400-5, 300 feet. All but two of the mass movements occurred at elevations below 2,900 feet (fig. 10). The great bulk of the events (over 72 percent) occurred in the elevational range from 2,000 to 2,600 feet (table 3). There are two possible reasons for the lack of mass movements at high elevations: (1) Most of the de- posits of tuffs and breccias occur at
pp eS eee
Table 3.--Relationship between occurrence of mass movement events and certain site factors
in the H. J. Andrews Experimental Forest during winter of 1964-65
Site factors
Disturbance: Undisturbed Logging Road construction
Substratum in movement area: Greenish tuffs and breccias Reddish tuffs and breccias Andesite colluvium Basalt and andesite residuum
Soil series:
Soils from reddish tuffs and breccias
Frissel Series McKenzie River Series
Soils from greenish tuffs and breccias
Limberlost Series Budworm Series Slipout Series Soil from andesite colluvium Carpenter Series Rock land
Elevation (feet): 1,400-1,700 1,700-2 ,000 2,000-2, 300 2 ,300-2 ,600 2,600-2,900 2,900-3,200 Over 3,200
Slope (percent):
0-15
15-30
30-45
45-60
60-75
75-90
Over 90
Aspect: North Northeast East Southeast South Southwest West Northwest Level
10
Mass movement events
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Events per 1,000 acres
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1.8 N29) 8.0 25.0 29.2 342 42.7 3 20.1 3 22.8 14.2 C58} 9.3 355) 4.9 9.1 BoP Goel 19.5 3.0 4.4 2.0 1343 34.9 9 4.1 1.6 Pest 7255) 6.2 4.3 8.8 11.4 10.9 11.6 ibsyoal 3.0 10.9 6 47.4 1 5.9 ieee 25.2 0) 34.6 133} 22.8 5.0 8.4 11.9 Dies) 18.7 -6 0 16.8 6 5.0 9.3 5)q8) 4.5 11.4 Arey; 11.9 6) 16.8 8 9.9 Grell! 19.3 4.1 3.0 DS
lower elevations; (2) temperatures prevailing above 3, 000 feet at the time of the December storm may have been low enough to maintain some snow cover.
As would be expected, steep- ness of slope and frequency of slides were apparently positively correlated (table 3). Only about a sixth of the events occurred on slopes with a gra- dient of less than 45 percent (fig. 10).
For some reason, mass movements were almost nonexistent on slopes with a south or southwest aspect-- probably because rock weathering and soil formation proceeds much more slowly on the drier aspects. The re- sulting shallow soils and less deeply weathered rocks may give rise to a greaterdegreeof stability. It appears that mass soil movements occur more readily inareas of deep soilsand well- weathered bedrock.
20 Figure 10.--Relationship between number of mass movement events and
number of events i) w pS ui a | © ive) (=)
N NE E SE
Ss SW WNW
aspect
elevation, aspect, and slope.
ml
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DISCUSSION AND CONCLUSIONS
Mass soil movements are the end result of acombination of causative factors. In our area, these generally include soil materials of low internal strength, sufficient water for satura- tion or near saturation, and at least moderately steep slopes. Man's acti- vities may contribute to movement by changing the slope or other character- istics of the mantle and by influencing the distribution of water. Since a complex of factors interact in the pro- duction of mass often extremely difficult to pinpoint the specific cause of amass movement In a very real sense, differs in respect from any other event, and
movements, it is
event. every
movement event some each represents a particular combina- tion of conditions which culminates in
massive mantle failure.
Perhaps one of the most im- portant problems facingus isto deter- mine the extent that man's activities contribute to the occurrence of mass soil movements. From our experience in the H. J. Andrews Experimental Forest, we are forced to conclude that man-caused disturbance has had an important influence in the occurrence
Although apparently
of mass soil movements. logging disturbance also increased movement frequency, this relationship is especially marked in the case of road construction. How- ever, it should be borne in mind that inanarea such as the Andrews Forest, where slopes are steep anda large portion is underlain by soft, deeply weathered pyroclastic rocks, the stage is set for extensive mass movements during high rainfall periods whether or not disturbanceis afactor. There- fore, it is perhaps often true that man's activities accelerate the occur- rence of mantle failure events in an rather than
already unstable area,
2
contribute in any significant way to this basic instability. In other words, disturbance may cause some small and, by itself, insignificant change whichisnonetheless sufficient toupset the tenuous equilibrium and trigger mass soil movement.
It is perhaps of some signifi- cance that the largest mass movement events encountered in this study were those which occurred in undisturbed areas. All involved extensive stream channel scouring, andthe source areas were oversteepened drainage heads. This general pattern is interpreted as the normal course of geologic erosion. On the other hand, man-caused move- ments are generally smaller, since the entire slope--from drainage head to main stream channel--is generally not involved. disturbed involve only rights-of-way, or perhaps onlya small portion of the slope below the road,
Instead, many events in
areas road
Since roads are so oftenan im- portant factor in causing mass move- ments, the problem is) to ,determunie means of minimizing their effect. Perhaps the most obvious means is to reduce road mileage to an
minimum. In
absolute steep, mountainous terrain, this may be done by the use of skyline and, possibly, balloon log- ging methods. In many areas, it is possible that improvements in road location may appreciably reduce the frequency of mass soil movements. Unstable soils and landforms should be identified, and the route selected should avoid
possible.
these areas wherever In addition, improvements in road design and construction may also contribute substantially to in- creased mantle stability. Modification of waste handling to avoid sidecasting on steep slopes and provision for ade- quate road drainage are two of the more important means to minimize mass movement hazard,
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The FOREST SERVICE of the
U. S. DEPARTMENT OF AGRICULTURE is dedicated to the principle of mul- tiple use management of the Nation’s forest resources for sustained yields of wood, water, forage, wildlife, and recreation, Through forestry research, cooperation with the States and private forest owners, and management of the National Forests and National Grasslands, it strives — as directed by Congress — to provide increasingly greater service to a growing Nation.
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