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University of Wisconsin

LIBRARY

University of Wisconsin

LIBRARY

Class
Book

HHMM ' ttf • "^ «»

THE

BENJAMIN BAKER, M. Inst. C.E
\ »

From Exoerpt Minutes of the Proceedings of the
Institution of Ciyil Engineers.

RSPBINTBD FBOM YAK NOSTRAND'S MAGAZmS.

NEW YORK:
D. VAN N08TRAND, PUBLISHER,

28 MURBAT AND 27 WaBRKN StRBKT.

1881.

SIYB
^Bi7 •

PREFACE.

The much discussed subject of the
pressure of earthwork is in this essay so
exhaustively treated that nothing is left
to be desired. The engineer may find
here a satisfactory explanation of the
causes of the discrepancies between
theory and practice, and of the differ-
ences between different authorities.

It was originally presented as a paper
to the Institution of Civil Engineers,
from the published minutes of which the
essay as here presented was published in
Van Nostrand's Magazine. Abstracts
only of the discussion are presented
here.

The Actual Lateral Pressure
of Earth^vork.

The fact that a mass of earthwork
tends to assume a definite slope, and that
if this tendency be resisted by a wall or
any other retaining structure, a lateral
pressure of notable severity will be ex-
erted by the earthwork on that structure,
must have enforced itself upon the at-
tention of constructors in the earliest
ages. Many of the rudest fortresses
doubtless had revetments, and of the
hundreds of topes, or sacred mounds,
raised in India and Afghanistan two
thousand years ago, not a few afford ex-
amples of surcharged retaining walls on
as, large a scale as those occurring in
modem railway practice. Nevertheless,
long as the subject has occupied the at-
tention of constructors, there is proba-
bly none other regarding which there ex-
ists the same lack of exact experimental

data, and the same apparent indifference
as to supplying this want Thousands
of pieces of wood have been broken in
all parts of the world to determine the
transverse strength of timber, whilst the
experiments that have been undertaken
to ascertain the actual lateral pressure of
Earthwork are hardly worth enumerating*
One authority after another has simply
evaded the task of experimental investi-
gation, by assuming that some of the
elements affecting the stability of earth-
work are so uncertain in their operation
as to justify their rejection, and have so
relieved themselves from further trouble.
It would hardly be less logical to assume
that because timber is liable to become
rotten and possesses no strength at all, it
was therefore unnecessary to conduct
experiments in that case also. As a mat-
ter of fact, although these uncertain ele-
ments are neglected in investigations,
engineers in designing, and still more
contractors in executing, works, do not
neglect them, nor could they do so with-
out leading to a blameworthy waste of

money in some instances, and to a dis-
creditable failure in others. The result
of the present want of experimental data
is then simply that individual judgment
has to be exercised in each instance, with-
out that aid from careful experimental
investigation which in these times is en-
joyed in almost every other branch of
engineering.

The mass of existent literature on the
subject is both misleading and disap-
pointing, for with little exception the
bulk of it consists merely of arithmeti-
cal changes rung upon a century-old
theory, which even at the time of its in-
ception was put forward but as a provi-
sional approximation of the truth, pend-
ing the acquirement of the necessary
data. Writing some fifty years ago Pro-
fessor Barlow excused his " very imper-
fect sketch of the theory of revetments,
at least as relates to its practical applica-
tion," on the ground that there was a
" want of the proper experimental data ;"
and but comparatively the other day Pro-
fessor Bankine had to write in almost

identical terms : " There is a mathematical
theory of the combined action of fric-
tion and adhesion in earth ; but for wont
of precipe experimental data its practi-
cal utility is doubtful." It is not, there-
fore, for want of asking that the missing
data are not forthcoming. Indeed, the
present desiderata could not have been
more clearly formulated than they were
half a century ago by Professor Barlow
in the following words : "To render the
theory complete, with respect to its
practical application, it is necessary to
institute a course of experiments upon a
large scale ; upon the force with which
different soils tend to slide down when
erected into the form of banks. A
well-conducted set of experiments of
this kind would blend into one what
many writers have divided into sev-
eral distinct data. Thus some authors
have considered first, what they call
the natural slope of different soils, by
which they mean the slope that the
surface will assume when thrown loosely
in a heap ; very different, as they sup-

pose, from the slope that a bank will as-
smue that has been supported, but of
which that support has been removed or
overthrown. This, therefore, leads to
the consideration of the friction and
cdhesion of soils, and what is denomina-
ted the slope of maximum thrust ; but,
however well this may answer the pur-
pose of making a display of analytical
transformations, I cannot think it is at
all calculated to obtain any useful prac-
tical results. I should conceive that a
set of experiments, made upon the abso-
lute thrust of different soils, which would
include or blend all these data in one
general result, would be much more use-
ful, as furnishing less causes of error,
and rendering the dependent computa-
tions much more simple and intelligible
to those who are commonly interested in
such deductions."

A. knowledge, however imperfect, of
the actual lateral pressure of earthwork,
as distinguished from what may be
termed the "text-book" pressures,
which, with hardly an exception known

to the author, are based upon calcula-
tions that disregard the most vital ele-
ments existent in fact, is of the utmost
importance to the engineer and con-
tractor. It affects not merely the stabil-
ity of retaining walls, but the strength
of tunnel linings, the timbering of shafts,
headings, tunnels, deep trenches for re-
taining walls, and many other works of
every-day practice. The vast divergence
between fact and theory has perhaps im-
pressed itself with peculiar force upon
the author, because, having had the
privilege of being associated with Mr.
Fowler, Past President of the Inst. C.E.,
during the whole period of the construc-
tion of the " underground " system of
railways, he has had the advantage of
the experience gained in constructing
about 9 miles of retaining walls, and, in
relation to the subject of the present
paper, tihe still more valuable experience
of 34 miles of deep-timbered trenches
for retaining walls, sewers, covered ways,
and other structures. A timber waling
is a sort of spring, rough it may be, but

still the deflections when taken over a
sufGiciently large number of walings
afford an approximate indication of the
pressure sustained — an advantage which
a retaining wall does not possess. Again,
though numberless retaining walls have
failed, in ninety-nine cases out of hun-
dred the failures have been due to faulty
foundations, and, consequently, experi-
ences of this sort seldom afford any di-
rect evidence as to the actual lateral
pressure of earthwork. In timbered
trenches, on the other hand, the element
of sinking and sliding foundations does
not so frequently arise to complicate the
investigation.

All kinds of earth were traversed by
the above 34 miles of trenches, from
light vegetable refuse to the semi-fluid
yellow clay, which at different times has
crushed in so many tunnel linings in the
northern districts of the metropolis.
The heights of the retaining walls ranged
up to 45 feet, the depths of the timbered
trenches to 54 feet, and the ground at
the back of the former was in many

cases loaded with buildings ranging up
to 80 feet in height. Possibly some of
the author's observations and conclusions
in connection with these and other works
of a similar character may be of interest
to engineers, though the information he
is able to contribute, having been ob-
tained chiefly in the ordinary routine of
his practice and not in specially devised
investigations, must necessarily form but
a very imperfect contribution to the data
which have been asked for so long.

The theory underlying all the multi-
tudinous published tables of required
thickness for retaining walls is, that the
lateral pressure exerted by a bank of
earth with a horizontal top is simply that
due to the wedge-shaped mass, included
between the vertical back of the wall and
a line bisecting the angle between the
vertical and the slope of repose of the
material. If this were true in practice,
all such problems could be solved by
merely drawing a line on the annexed
diagram, in which a b d ci8 b. square, a b
g a triangle, having the sides of the

ratio of 1 : a/^, and a h d dk parabolic
curve.*

fl[

'^^^^^'^--r^

^^^

/ \

/
/
/

-10

•90

-SO

^\\

6o\

40
60

70-^

8o\

9()\
c

•70
80
-00
100

♦For earthwork and masonry of the same weight
per cubic foot the equation for stability is :

'y' -g-=-g-tan« \ angle. .

Hence, the required thickness (^i ) in terms of the
height (A) will be t\ — ^% tan ^ angle, which is repre-
sented on the diagram by the line a~g; and the
"equivalent fluid pressure " in terms of that of a cu-
bic foot earthwork will be=tans ^ angle, which is
represented by the parabolic curve ahd.

Thus, if it were required to know the
lateral pressure per square foot of earth-
work, having a slope of repose of 1^ :
1, and the thickness of rectangular verti-
cal wall which, when turning over on its
outside edge would just balance that
pressui'e, it would merely be necessary
to draw the line c f at the given slope of
1^ : 1 and the line c e bisecting the angle
a chy when the line e h would give the
equivalent fluid pressure in terms of that
of a cubic foot of the earth =28. 7 per
cent, and the line e i the thickness of
the rectangular wall in terms of the
height =31 per cent , the weight of ma-
sonry being the same as that of the
earth.

Common stocks in mortar and ballast
backing each weigh about 100 lbs. per
cubic foot, hence, on the preceding hy-
pothesis, the pressure acting on the wall
would be the same as that due to a fluid
weighing 28.7 lbs. per cubic foot. If, as
is usually the case, the masonry be
heavier than the earthwork, the required
thickness of wall would be reduced in

inverse proportion to the square root
of the respective weights, so that should
the masonry weigh 10 per cent more
than the ballast, the thickness would be
about 5 per cent, less than before, or,
say, 29.5 per cent, of the height.

For other slopes of repose the equiva-
lent fluid pressure and thickness of wall
for materials of equal weight would be
as follows :

Ratio of horiz- ) -
ontal to vertical J *

1.0

Fluid pressure. ..5.6
Thickness |.136

7.7
.160

10
.182

12.4
.203

14.8
.222

17.2
.239

Ratio of horiz- )
ontal to vertical j

1.1|1.2

1.811.4

1.5

1.6

Fluid pressure...

19.6 22

24.326.5

28.7

30.7

Thickness

.256 .27

.2841.297

.81;

8?i

Ratio of horiz- )
ontal to vertical )

1.7

1.8

3 4

Fluid pressure...

32.8
.33

34.6
.34

38.2
.357

52 61

100

Thickness

.416.451

.678

In the thickness tabulated above no
allowance has been made for the crash-
ing action on the outer edge ; in practice
the batter usually given to the face of
the wall more than compensates for this
action if the mean thickness be that
given in the table. No factor of safety
is included, but according to theory the
wall in each case would be just on the
balance. Any one accustomed to deal
with works of this class will, however,
know that in practice walls so propor-
tioned would in the majority of cases
possess a large factor of safety.

Doubtless many engineers will, with
the author, have noticed that laborers
and others not infrequently carry out
unconsciously a number of valuable and

suggestive experiments on The Actual
Lateral Pressure of Earthwork. In
stacking materials, rough-and-ready re-
taining walls, made of loose blocks of
the same material, are often run up, and
as it is generally of little moment
whether a slip occurs or not, the work-
men do not trouble about factors of
safety, but expend the least amount of
labor that their every-day experience will
justify, and so a tolerably close measure
is obtained of the average actual press-
ure of material retained. When the
wood paving was recently laid in Eegent
Street, the space being limited, the
stacked wooden blocks in many cases
had to do duty as retaining walls to hold
up the broken stone ballast required for
the concrete substructure. In one in-
stance (Ex. 1) the author noted that a
wall of pitch-pine blocks, 4 feet high and
1 foot thick, sustained the vertical face
of a bank of old macadam materials which
had been broken up, screened, and tossed
against this wall until the bank had at-
tained a height of 3 feet 9 inches, a

width at the top of about 5 feet, and
slopes on the farther sides deviating
httle from 1.2 to 1. Now, referring to
the diagram and table of thickness, it
will be seen that according to the ordi-
nary theory the thickness of wall which
would just balance the thrust of a bank
3 feet 9 inches high of material having a
slope of repose of 1.2 to 1 would be
3.75 X. 27= 1.01, or, say, 1 foot, which is
the actual thickness of the given wall.
But in the table the specific weight of
the material in the wall and backing is
assumed to be the same, whereas in the
present case the weight of the pitch-pine
block wall, allowing for the height being
greater than that of the bank, would

only be, say, 46 lbs. X ^ „^ ^ — ^=49 lbs.
o.7o feet

per cubic foot, whilst that of the broken

granite bank would be, say, 168 lbs. less

40 per cent, ior interstices = 101 lbs. per

cubic foot. It follows, since the wooden

wall stood, that if it had been made of

materials having the same weight per

cubic foot as the bank, the retaining wall

would not have been on the point of
toppling over, as the ordinary theory
would indicate, but have possessed a

factor of safety of at least .^ „ — - , or,

4U lbs.

say, 2 to 1. The effective lateral press-
ure of the earthwork in this instance
consequently could not have exceeded a

a .J ^22 1bs.x49 ^^„ ,,
fluid pressure of -— = 10.7 lbs.

per cubic foot, instead of the 22 lbs.,
which theoretically corresponds to the
given slope of 1.2 to 1.

Taking another case, in which the
wall, instead of being lighter than the
bank, was much heavier, the same con-
clusion still holds good. In this instance
(Ex. 2) the author found a wall of slag
blocks having a batter of \ of the height,
and an effective thickness of 1 foot sus-
tained a bank of broken slag 10 feet
high, with a surcharge of some 5 feet
more. The battering wall, with a thick-
ness of ^ of the height, would have the
same stabiHty as a vertical wall 0.173
thick, and the lateral pressure of the sur-

charged bank with the battering face
would be practically the same as that of
a horizontal-topped bank with a vertical
face; hence, since the relatively closely-
packed slag blocks constituting the wall
would weigh about 40 per cent, more
than the broken slag of the bank, the
thickness of a vertical wall built of ma-
terials of the same weight as the bank,
and having the same stability as the wall
under consideration would be = vTTi X
0.173=0.205 of the height. Eeferring
to the table, the figure 0.205 will be
found to apply to a slope of repose of
0.8 to 1, whereas the actual slope in the
instance of this slag was 1.38 to 1. For
the latter slope the thickness theoret-
ically should have been 0.29, and since
the stability varies as the square of the
thickness, it follows that with the thick-
ness indicated by theory, the wall, in-
stead of being just on the balance, would
have possessed a factor of safety of at

29''
least rr^jrpr^, or 2 to 1, as in the last

example.

Other instances of these unintentional
experiments on the lateral pressure of
earthwork will be found in the stacking
of coal in station yards, in the rubbish
banks at quarries, and in many other
instances which have been investigated
by the author, with the invariable result
of finding that walls which, according to
current theory, would be on the point of
failure, really possess a considerable
factor of safety.

Turning now from indirect to direct
experiments, specially arranged with a
view to determine the lateral pressure of
earthwork, those carried out at Chatham
nearly forty years ago by Lieutenant
Hope, E.E., may be referred to. His
intention was to experiment first with
fine dry sand, as free as possible from
the complications introduced by cohesion,
irregularities of mass and other practical
conditions, and then to extend the in-
vestigation to ordinary shingle, and to
clay and other soils possessed of great
tenacity. Sand and shingle were, how-
ever, alone experimented with.

The direct lateral thrust of sand
weighing 91 lbs. per cubic foot when
lightly thrown together, and 98 J lbs.
when well shaken, was measured by
balancing the pressure exerted on a
board 1 foot square. The mean results
of seven experiments (Ex. 3) was 9 lbs.
7 oz., which is that due to a fluid weigh-
ing nearly 19 lbs. per cubic foot. As
the slope of repose of the sand employed
was 1.42 to 1, the theoretical fluid press-
ure due to the weight of 98^ lbs. per
cubic foot would be 26.2 lbs., or about
40 per cent, more than the observed 19
lbs. per cubic foot.

With gravel (Ex. 4) weighing 95^ lbs.
per cubic foot, and having a slope of re-
pose of IJ to 1, about the same lateral
pressure was found to exist. Lieutenant
Hope attempted to reconcile the differ-
ence between theoretical and actual re-
sults by adding to the measured force an
estimated sum for friction against the
sides of the apparatus, but experiments
of the author's to be subsequently refer-
red to, clearly prove that the difference

is not to be so accotmted for. Indeed,
the knowledge of what the pressure theo-
retically should be would appear to have
given Lieutenant Hope an unconscious
bias in the direction of rather exaggerat-
ing the experimental results. This it is
extremely easy to do, as a trifling amount
of vibration will alter the pressure from
10 to 50 per cent., and a comparatively
innocent shake in a small model will cor-
respond in its relative effects with an
earthquake in real life.

Experiments with colored sand in a
vessel with glass sides did not uniformly
confirm the usual theory that the angle
of pressure of maximum thrust is half
that contained between the natural slope
and the back of the wall (Ex. 5). Thus
the line of separation was at an angle of
24° with the vertical instead of 28*".
Again, with a gravel bank (Ex. 6) 10 feet
high the line of separation ranged from
3 feet 8 inches to 5 feet 8 inches from
the back of the wall, whilst as the natural
slope was 1^ to 1, the distance should
have been 5 feet in all instances if Cou-

lomb's theory applied strictly to even
such exceptionally favorable materials as
dry sand and shingle.

The really valuable portion of Lieu-
tenant Hope's investigation was the
series of experiments on walls built of
bricks laid in wet sand. The first of
these (Ex. 7) was about 20 feet long and
two-and-a-half bricks, or say, 1 foot 11
inches thick. When raised to a height of
8 feet and backed with ballast, it had in-
clined from the vertical abou^t 1^ inch ;
at 9 feet the inclination had increased to
3i inches, and at 10 feet the wall fell for-
ward in one mass. At the instant when
the thrust of the ballast overcame the
stability of the wall, the overhang must
have been 4 inches, and the moment of
stability per lineal foot certainly not
more than 2,000 lbs. X 0.9 foot =1,800

A"
foot-pounds. Hence, dividing ^y— ^»

is obtained 10.8 lbs. per cubic foot as the
weight of the fluid, which would have
exerted a lateral pressure equal to that
of the ballast piled against this 10-f eet

wall. This is hardly more than half the
pressure obtained with the 1-foot square
board, and shows how desirable it is
that even the most faithful experimenter
should not know what to expect if a
mere shake of a table will enable him to
obtain the desired result. The natural
slope of the ballast being 1^ to 1, and
the weight 96^ lbs. per cubic foot, the
pressure theoretically should have been
23.6 lbs. per cubic foot instead of 10.8
lbs ; hence a wall so proportioned as to
be on the point of toppling over, accord-
ing to the ordinary theory, would in this
instance have had a factor of safety of
rather more than 2 to 1.

Another vertical wall (Ex. 8) was con-
structed with the same amount of ma-
terials differently disposed. At 8 feet
high, after heavy rain, the 18-inch thick
panel between the 27-inch deep counter-
forts had bulged IJ inch; at 12 feet 10
inches the bulging had increased to 4^
inches, and the overhang at the top to
7^ inches, when, after some hours' grad-
ual movement, the wall fell. The moment

of stability at the time of failure cotdd

not have exceeded 2,600 lbs. X 1 foot =

2,600 footpounds, which, divided by

-^ , gives 7.4 lbs. per cubic foot, instead

of the theoretical 23.6 lbs., as the weight
of the equivalent fluid. This result is
clearly not evidence that the pressure of
the ballast was less in the counterforted
wall than in the wall of uniform thick-
ness, but that the binding of the ballast
between the counterforts increased the
stability of the wall by practically add-
ing somewhat to its weight.

A wall with a batter of ^ of the height,
and with counterforts of the same thick-
ness as the last (Ex. 9), was next tried,
with noteworthy results. This wall,
only 18 inches thick, with counterforts 3
feet 9 inches deep, measuring from the
face of the wall, and 10 feet apart, was
carried to a height of 21 feet 6 inches
without any indications of movement,
beyond a bulging about halfway up of 2^
inches at the panel, and 1^ inch at the
counterfort; and in Lieutenant Hope's

opinion it would probably have stood for
years without giving way any more, al-
though the mean thickness was less than
^ of the height. The calculated stabil-
ity indicates that a fluid pressure of 8.5
lbs. per cubic foot would have overturned
the wall, and, correcting for the reduced
thrust of the ballast due to the batter of
its face, the equivalent pressure on a
vertical waU would be that of a fluid
weighing 10 lbs. per cubic foot.

Here, again, doubtless the binding of
the gravel between the counterforts con-
tributed to the stability of the wall ; but,
even adopting the extreme and impossi-
ble hypothesis that the ballast was as
good as so much brickwork, or, in other
words, that the wall was a monolithic
structure of the uniform thickness of 3
feet 9 inches, its stability would barely
balance the 23.6 lbs. per cubic foot
fluid pressure theoretically due to the
weight and slope of repose of the back-
ing. Assuming that the binding of the
ballast between the counterforts in-
creased the stability, as in Examples 8 and

9, by about 45 per cent., the fluid resist-
ance would be 14.5 lbs. per cubic foot ;
and, remembering that this wall did not
fall, though the bricks were only laid in
sand, it is reasonable to infer that this
interesting experiment confirms the pre-
vious conclusion that a properly built
wall in mortar or cement, just balancing
the theoretical pressure, would really
have had a factor of safety of 2 to 1.
Other experiments of Lieutentant Hope's
justify this inference, and so do the ex-
periments of General Pasley, also made
at Chatham many years ago.

General Pasley experimented with
loose dry shingle weighing 89 lbs. per
cubic foot, and having a natural slope of
IJ to 1. His model retaining walls (Ex.
10) were 3 feet long, 26 inches high, of
various forms and thickness, and weighed
84 lbs. per cubic foot. The stability of
each wall was tried by pulling it over by
weights before and after backing it up
with shingle, and the difference between
the two pulls of course represented- the
thrust of the shingle. When the thick-

ness of the vertical wall was 8 inches,
the stability, without shingle, was equiv-
alent to a pull of 47 lbs. applied at the
top of the wall, and with shingle, the
pull required to upset it was reduced to
30 lbs. The difference of 17 lbs. repre-
sents the thrust of the shingle, and
throughout the several hundreds of ex-
periments this appears to have been com-
prised within the limits of 16 lbs. and 24
lbs. The center of pressure being at ^
of the height of the wall, the • mean
thrust of 20 lbs. at the top will be equiv-
alent to 60 lbs. at the center of pressure,
and the area being 6.5 square feet, and
the height 26 inches, the actual lateral
pressure of the shingle, as deduced from
General Pasley's experiments, is equiva-
lent to that of a fluid weighing 8.5 lbs.
per cubic foot, instead of 21 lbs. as
theory would indicate.

General Cunningham tested some
model revetments, and his experiments
led him to believe that General Pasley
had overestimated the thickness required
for stability. The models, in this case

about 30 inches in height, were weighted
with earth and musket bullets to the
equivalent of an equal mass of masonry
weighing 129 lbs. per cubic foot. One
of the models (Ex. 11) represented a
wall 30 feet high, 6 feet thick at the base,
vertical at the back, battering 1 in 10 on
the face, with counterforts 4 feet 3 inches
thick, 18 feet from center to center, and
of a depth equal to the thickness of the
wall or, saj, 3 feet at the top and 6 feet
at ih!b base. This was backed up and
surcharged with shingle weighing 104
lbs. per cubic foot, but required a pull
of 111 lbs. to overturn it. Another
model (Ex. 12) representing a wall 18
feet high, 4 feet 4 inches thick at the
base, and 2 feet 8 inches thick at the
coping, without counterforts, when sur-
charged with shingle to a height great-
er than that of the wall, required a
pull of 84 lbs. to upset it. A fluid press-
ure of 19 lbs. per cubic foot would over-
come the stability of such a wall ; hence,
having regard to the surcharge and to
the pull, it will be found that the actual

lateral pressure of the shingle conld not
have exceeded that due to a fluid weigh-
ing 8 lbs. per cubic foot.

General Burgoyne also commenced an
experimental investigation of the ques-
tion of retaining walls, but circumstances
precluded his pursuing the subject.
About half a century ago he built at
Kingstown four experimental walls 20
feet long and 20 feet high, having the
same mean thickness of 3 feet 4 inches,
or^ of the height, but differing otherwise.
One of them (Ex. 13) was of the uniform
thickness of 3 feet 4 inches, and battered
\ of the height ; another (Ex. 14) was 1
foot 4 inches thick at the top, and 5 feet
4 inche9 at the bottom, with a vertical
back ; the third (Ex. ^16, Fig. 1; was of
the same dimensions, with a vertical
front ; and the last (Ex. 16, Fig. 2) was
a plain rectangular vertical wall 3 feet 4
inches thick. The masonry consisted
simply of rough granite blocks laid dry,
and the filling was of loose earth filled
in at random, without ramming or other
precautions, during a very wet winter.

No. 1 wall stood perfectly, as might have
been expected from the behayior of Lien-
tenant Hope's experimental wall of near-
ly the same height and batter. No. 2
wall also stood well, coming over only

Flg.t

Flg.2

about 2 inches at the top. A fluid press-
ure of 22.6 lbs. per cubic foot would be
required to overcome the stability of this
dry masonry wall weighing 142 lbs. per
cubic foot. Earthwork of the class de-

scribed, consolidated during continuous
rain, would not weigh less than 112 lbs.
per cubic foot, nor have a slope of re-
pose less than 1^ to 1. Referring to the
table, the theoretical pressure of such
earthwork would be 28.67 X 112=32 lbs.
per cubic foot, or nearly one-half greater
than the wall could resist.

No. 3 and No. 4 walls both fell when
the filling had attained a height of 17
feet. The former came over 10 inches at
the top, was greatly convex on the face,
overhanging 5 inches in the first 5 feet of
its height and rending it in every direc-
tion, when finally it burst out at 6 feet 6
inches from the base, and about two-
thirds of the upper portion of the wall
descended vertically until it reached and
crushed into the ground (Fig. 1). The
vertical wall tilted over gradually to 18
inches and then broke across, as it were,
at about ^ of its height and fell forward
(Fig. 2). So long as the wall remained
vertical the calculated stability would in-
dicate it to be equal to sustain the press-
ure of a fluid weigliing 20.4 lbs. per

cubic foot, but the overhang of 18 inches
and the bulging which occurred would
reduce the stability exactly one-half, so
that a fluid pressure of 10.2 lbs. would
really have sufficed to effect the final
overthrow. The character of the failure
both of No. 3 and No. 4 walls clearly in-
dicates that if the walls had been in
mortar or cement, as usual, the overhang
would not have been a fraction of that
occurring with the dry stone waUing,
and the failure would not have taken
place. Since, as already stated, the theo-
retical thrust of the earthwork would be
321b8* per cubic foot, it is hardly unfair
to conclude that a wall in mortar and pro-
portional to that pressure would not have
come over and would have enjoyed a fac-
tor of safety of at least 2 to 1.

Colonel Michon carried out in 1863 an
interesting experiment (Ex. 17) on a 40
feet high retaining wall of a peculiar type
(Figs. 3 and 4), which, perhaps, may be
best described as a very thin wall with
numerous battering buttresses turned
upside down. The face wall, battering

1 in 20, was only 1 foot 8 inches thick,
and the buttresses, spaced about 5 feet
apart from center to center, were also 1

Fig^

Flg^

foot 8 inches thick bj 2 feet 4 inches
deep at the base and 9 feet 2 inches at
the top. The work was hurriedly con-

structed during continnouB rains with
any stones that came to hand, and with
very bad lime. When the filling had
attained a height of 29 feet the wall
bulged a trifle, but no further movement
was noticed, though the filling, when
carried up to the top of the coping,
was allowed some weeks to settle in the
rain. Earth was then piled above the
level of the coping to a height of be-
tween 3 and 4 feet, when the wall fell.
The fall was preceded by a general dis-
location of the masonry at the base, a
bulging at about one-third of the height,
and a slight movement of the top to-
wards the bank. The lower portion of
the wall fell outwards, the upper part
dropped vertically (as in General Bur-
goyne's wall, Fig. 1), and a considerable
number of the counterforts went for-
ward with the slip and even maintained
their vertical position.

This failure arose from a flexure of
the thin wall at the center of pressure of
the earthwork, and would not have oc-
curred had the masonry been in cement

instead of in weak unset lime. No direct
data therefor are afforded for an ex£tct
estimate of the actual lateral thrust of
the heavy wet filling on this lofty wall.
Nevertheless, as the weight of the ma-
sonry was only 18,000 lbs. per lineal
foot, and the center of gravity of the
same from the toe but 6 feet 6 inches, it
follows that the wall, even if monolithic,
would be overturned with the pressure of
a fluid weighing 11 lbs. per cubic foot.
How far the sodden earthwork between
the counterforts contributed to the sta-
bility of the wall is open to question,
but it could hardly account for the differ-
ence between the 11 lbs. or less stabihty
and the 32 lbs. due, according to the or-
dinary theory, to the weight and slope of
the backing. If dirt were as good as
masonry. General Burgoyne's wall with
the battering back (Fig. 1) would have
been more stable than the vertical wall
(Fig. 2) in the ratio of the squares of
their respective bases, or, say, as 2 i to 1,
whereas, these walls proved to be of
equal stability, both falling with 17 feet

of filling. Colonel Michon, by asBiiming
dirt to be ae good as masonry, and a wall
40 feet high and 1 foot 8 inches thick of
iinselected stones and unset mortar to be
as good as a monohth, succeeds in recon-
ciling the behavior of his wall with the
ordinary theory of the stability of earth-
work ; but in the author's experience the
conditions assumed are not approached
in practice. The stability of this lofty
wall battering only ^ of the height on
the face, and averaging hardly more than
^2 oi the height in thickness, is, never-
theless, one of the most remarkable and
interesting facts connected with the sub-
ject of the present paper.

To show how invariably an experi-
mentalist is driven to the same conclu-
sion as to the excess in the theoretical
estimate of the pressure of earthwork,
the " toy " experiments of Mr. Casimer
Constable with little wooden bricks and
peas for filling may be usually referred
to. The peas took a slope of 1.9 to 1,
and weighed twice as much per cubic
foot as the wooden retaining wall. By

the table the thickness of wall, which
would just balance the lateral pressure
would be .36V2=.49 height. By ex-
periment, a wall ( Ex. 18) having a thick-
ness of .40 height moved over slightly,
but took some amount of jarring to bring
it down. Since the stability varies as
the square of the thickness, the calcula-
ted wall would be 50 per cent, more sta-
ble than the actual wall, without consid-
ering the question of jarring. If the
slope of the peas had been measured also,
after jarring, it would probably have been
found to be nearer 2.9 fco 1 than 1.9 to 1,
and the calculated required thickness
would have been correspondingly in-
creased.

The influence of even a slight amount
of vibration is well illustrated by the
difference between the co-efficient of
friction of stones on one another in mo-
tion and repose. Granite blocks, which
will start on nothing flatter than 1.4 to 1,
will continue in motion on spa. incline of
2.2. to 1, and, for similar reasons, earth-
work will assume a flatter slope and ex-

ert a greater lateral pressure under vi-
bration than when at rest. This fact has
long received practical recognition from
engineers; indeed, attention was called
to it by Mr. Charles Hutton Gregory, C.
M.G., Past-President Inst. C.E., in a Pa-
per on slips in earthwork, read before
the Institution in 1844, when the Presi-
dent and others gave instances of slips
in railway cuttings caused by vibration;
The general results of the preceding
and other independent experiments on
retaining walls tending to throw a doubt
on the accuracy of Lieutenant Hope's
measurement of the direct lateral thrust
of ballast and sand on a board 1 foot
square,- the author considered it advisa-
ble to repeat those experiments. Care
was taken to eliminate all disturbing
causes tending to vitiate the results. The
pressure board was held by a string at
its center of pressure, and was perfectly
free to move in every direction, which, of
course, a retaining wall having a greater
hold on the ground than stability to re^
sist overturning has not. In every in-

stance the filling was poured into the bo
and allowed to assume its natural slope
towards the pressure board, and the lat-
ter was rotated and thumped to keep the
ballast alive before the reaction was meas-
ured. In order to avoid all chance of the
bias which the knowledge what to expect
might have given him, as it did Lieuten
ant Hope, the author had the experiment^
made by others who were ignorant even
of the object of them, whilst he himself
purposely experimented with an appara-
tus the dimensions of which he did not
know, and consequently could form no
estimate of the weight which would be
required in the scale

With clean dry ballast having a natu-
ral slope of 1 J to 1, it will be remembered
Lieutenant Hope obtained a lateral press-
ure on 1 square foot of 9 lbs. 7 oz. With
well -washed wet ballast of the same kind
the author found the natural slope to be
1^ to 1, and he decided therefore to use
the ballast wet, because, possessing
greater fluidity, it would give more uni-
form results than dry ballast, and also

impose greater lateral pressure. In a
large number of independent experiments
the resnlts were uniformly as follows
(Ex. 19) : With 6 lbs. in the scale the
board moyed forward about ^ inch, but
continued to retain the ballast; with 7
lbs. very slight movement occurred ; with
8 Ibs.^ no movement at all ; and with 10
lbs., under extreme vibration, the board
moved forward about as much as it did
with 6 lbs. without vibration. The gen-
eral opinion of the different experiment
alists was, that the fair value of the lat-
eral pressure of this wet ballast was 7
lbs., because when that weight was in the
scale pad a slight jolt was sufficient to
let the ballast down by the run to a slope
of 1^ to 1. The board being 1 foot square,
this of course is equivalent to the press-
ure of a fluid weighing 14 lbs. per cubic
foot, instead of the 19 lbs. obtained by
Lieutenant Hope, and the 26 lbs. indica-
ted by theory.

With the same ballast unwashed and
mixed with slightly loamy pit sand (Ex,
20) the natural slope was 1 to 1, without

vibration, and IJ to 1 with a moderate
amount of vibration. A weight of 3 lbs.
was as effective in retaining this ballast
as 6 lbs. in the former instance ; 4 lbs.
held it under a moderate amount of vi-
bration, but 3^ lbs. failed to hold the
board under very little. Practically
speaking, the lateral thrust was about
half that with the clean wet ballast, and
considerably less than half that theoretic
cally due to the slope of repose of the
loamy ballast.

In harbor works both walls and back-
ing are frequently completely immersed,
and, so far as gravity is concerned, stone
blocks and rubble become then trans-
formed into coal. The author, there-
fore, experimented (Ex. 21) with some
coal having a peculiarly "greasy" sur-
face, and ojffering the advantage of ex-
ceptional fluidity. With 3 lbs. the board
moved forward about 1 inch, but no more
until a slight jar was applied, when it
fell; with 4 lbs. a moderate amount of
vibration also generally caused failure ;
with 5 lbs. the board usually moved for-

ward gradually without making a rush so
long as a tolerably considerable amount
of vibration was maintained. When a
slip occurred the slope was invariably If
to 1. The coal proved to be more sensi-
tive to vibration than the wet ballast, and
still more so than the unwashed ballast.
Aweight of 4 lbs. with the coal appeared
to be equivalent to 7 lbs. with the washed
and 3i lbs. with the unwashed ballast-
The weight of the coal being one-half
that of the wet ballast, and the respective
slopes of repose being If and 1 J to 1, the
lateral thrusts would theoretically be as
16.8 : 28.7, which is practically the exper
imental result of 4 to 7.

The author having occasion to design
a solid pier 42 feet in height from the
bottom of the harbor to the surface of
the quay, where a soft bottom of great
thickness and small consistency precluded
the use of concrete block or other retain-
ing wall, adopted an arrangement in
which an iron grid of rolled joist, with
a backing of large blocks of rubble, was
substituted for a wall. It was necessary,

therefore, to know the lateral thrust of
large blocks of stone in such a structure,
and mistrusting theoretical deductions,
the author made direct experiments on a
model to a scale of 1 inch to the foot.

In this instance the individual stones
were intended to be fairly uniform in
size, and of lateral dimensions not less
than ^ of the height of the wall, so that
the conditions differed considerably from
those assumed in theoretical investiga-
tions. A number of billiard balls exactly
superimposed in a tightly fitting box
would exert no thrust though their slope
of repose might be as flat as 3 to 1 ; and
it is not quite clear how nearly or re-
motely large boulders in an iron cage
approximate to that condition. The
stones used in the experiment were
waterworn pieces of schistose rock hav-
ing a "greasy" surface and a slope of
repose of 1^ to 1. This inclination was
found by the author to obtain in natural
slopes of all heights, from the pile of
metalling by the roadside to the hills
themselves. He ascended one slope of

1^ to 1, over 500 feet in height, and
found the balance was so nearly main-
tained that a footstep at times would set
many tons of stones in motion; and a
few winters ago a couple of stones of the
respective weights of 18 tons and 22
tons, descended this slope and acquired
sufficient momentum to carry them across
a road at the foot of the slope and on to
the middle of the lawn in front of an ad-
joining shooting lodge.

The conditions of the material were
thus favorable, as in the instance of the
coal, for obtaining uniform results and a
maximum lateral thrust. In order to
exclude all possible influence from side
friction, the length of the box was made
four times the height of the wall. A^
the result of ten experiments (Ex. 22) it
was found that a weight in the scale cor-
responding to the pressure of a fluid
weighing 10.2 lbs. per cubic foot sufficed
to retain the rubble, though the face
planking moved forward slightly as the
last few shovelfuls were thrown against
it. The weight of the stone filling was

•98 lbs. per cubic foot, or practically the
same as that of the wet ballast last
referred to; so the 10.2 lb«., or say
under slight vibration 11 lbs., in the
present instance compares with the 14
lbs. of the previous instance, sand the dif
ierence is a measure of the influence of
large-sized, smooth-faced boulders as
compared with ordinary ballast

With coal of the same size (Ex. 23)
the equivalent weight of fluid was 6 lbs.
per cubic foot, which confirms the pre-
ceding result when regard is had to the
respective weights and slopes of repose
of the two materials. The experiments
collectively proved that a wall, which
according to the ordinary theory would
be on the point of being overturned
by the thrust of a bank of big bould-
ers, would in fact have a factor of safety
of nearly 2^ to 1.

Having thus briefly reviewed some
direct experiments on the actual lateral
thrust of earthwork, the author proposes
to revert to the consideration of indirect
experiments, dealing first with a few of

those arising on tlie 84 miles of deep-
timbered trenches and other works of
the "^ underground "^^ railway.

In tunneling very valuable evidence
was afforded of the direction of the line
of least resistance in a mass of earth-
work. From the coming down of the
crown barsy the changing of props, the
crushing of timber^ the compression of
green brickwork and other causes, a
settlement of from 6 to 8 inches usually
occurred overhead, with a general draw
of the ground towards the working end
of the tunnel and the formation of fis-
sureSy attaining a maximum size where the
line of least resistance cut the surface.
Even when the settlement was slight^
fissures were invariably observed in ad-
vance of the working end, and in con-
tinuous lines running parallel witli the
tunneL The slope of these fissures was
so uniformly at the angle of ^ to 1, meas-
uring from the bottom of the excavation
(Ex. 24, Fig.. 5), that the resident engi-^
neer professed to be able to foretell with
certainty where a building or fence wall^

standing over the tunnel, would crack
most. Assuming this ^ to 1 to represent
Coulomb's line of least resistance, then
the corresponding natural slope of repose

FIg.5

-00^^^-^' "

of the material would appear to be 1^ to
1, which is considerably steeper than
what it was in fact.

There is nevertheless a closer accord
than usual between theory and fact in

the instance of the Beveral miles of
fissures, which occurred during the con-
struction of the tunnels of the Metro-
politan railway. In other instances, such
as the failure of ill-devised timbering, or
the pushing forward of a retaining wall,
by heavy clay pressing against its lower
half, this accord was not always exhib-
ited. In some cases no previous fissures
have occurred, but a wedge of 1 to 1 has
at once broken off and gone down with
the timbering, whilst in others the
fissure has appeared immediately at the
back of the wall; indeed in one instance,
for several consecutive weeks, the author
was able to pass a rod, 15 feet long, be-
tween the wall and the apparently unsup-
ported vertical face of the ground behind
it. The corresponding theoretical slope
of repose would thus appear to be hori-
zontal in one case and vertical in the
other, which is sufficient evidence of the
necessity of giving but a qualified assent
to any theoretical deduction affecting the
line of least resistance in earthwork.
A very fair notion of the relative in-

tensity of lateral and vertical pressure in
earthwork is often obtained in carrying
out headings. The heading for the Camp-
den Hill tunnel of the MetropoHtan rail-
way is a case in point (Ex. 25). The
ground consisted of sand and ballast,
heavily charged with water, overlying
the clay through which the heading was
driven, at a depth of 44 feet from the
surface. After the heading had been
completed some months, the clay became
softened to the consistency of putty by
the water which filtered through the
numerous fissures, and the full weight
of the groumi took effect upon the set-
tings. Both caps and side trees showed
signs of severe stress throughout the en-
tire length of the heading, and the occa-
sional fractures in the roof and sides
iudicated that the timbers were propor-
tionately of about the same strength, or
rather weakness. The caps were of
14-inch square balks, with a clear span
of 8 feet, and the sides of 10-inch square
timber, with a clear span of 9 feet. Their
respective powers of resistance per square

foot of poling boards, supported, would

therefore be as -^ : -^ =3^ : 1.

Now if one thing is settled by experi-
ence beyond all question, it is that the
supei-ficial beds of London Clay, sodden,
as in the present case, with water, will
not take a less slope of repose than 3 to
1. The average weight of the wet ground
over the heading being about 1 cwt. per
cubic foot, the theoretical lateral press-
ure on the side trees, at a mean depth of
48 feet from the surface, would be (see
table) = 48x 0.52x1 cwt. — 25 cwt per
square foot, and upon the caps=44xl
cwt. =44 cwt. per square foot, or 1.76
time greater. But the side trees, as has

been seen, had only -^-= of the strength

o.O

of the caps, so the irresistible conclusion
is that the actual lateral pressure of the
earthwork in this instance did not exceed
one-half of that indicated by theory.*
It is readily shown that the full weight

♦ See also " Zxxr Theorie des Erddrucks," Weyranch
Zeitschrift fUr Baokunde, yol. i., p. 192,

of the groand came upon the settings.

Thns, assuming it to do so, the weight

upon the caps would be =44 cwt.xSfeet

clear span X 3.5 feet distance apart of the

settings = 1,232 cwt, and taking the

effective span at 9 feet, the breaking

weight, upon the basis of Mr. Lyster's

experiments on balks of similar size and

,.^ ,, , 2 X 14"' X 2.03 cwt.
quauty, would be ^7 =

1,240 cwt; hence the occasional fractures
of the balks are fully accounted for. In-
deed, the heading would have entirely
collapsed, in the course of time, had not
the roof been supported by intermedi-
ate props practically quadrupling its
strength.

In the early days of the construction
of the Metropolitan railway, a definite
type of timbering had not been arrived
at, and some remarkably light systems
were tried at times. The lightest the
author remembers was the timbering of
the 14-feet wide gullet at Baker Street
station (Ex. 26). Here the soil cut
through was made up of about 8 feet of

yellow clay and gravel, 7 feet of loamy
sand, 7 feet of sharp sand and gravel,
full of water, and 4 feet of London clay
at the bottom of the gullet. The timber-
ing of the lower half consisted of 9-inch
by 3-inch walings, 3 feet apart from
center to center, in 12feet lengths, with
f-inch poling boards at the back. With
* one-half the distributed breaking load,
the deflection of this 3-inch deep beam
at the span of 12 feet would be at least
4 inches, whilst the ultimate deflection
would be measured by feet. As the
walings did not bend nearly as much as
4 inches, it will be a liberal estimate to
assume that the actual lateral pressure of
the earthwork was equal to half the dis-
tributed breaking weight of the wal-
ing. Having reference to the quality of
the timber, this may be estimated at

y^^X9"x2.6cwt. ,„. , , .

, / 7 =17.6 cwt. ; and smce

li2 U

the area of the poling boards supported

by each waling was 36 square feet, it

follows that the lateral pressure of the

earthwork could not have exceeded 55

lbs. per square foot. But the depth of
the bottom waling below the sarface
was 23 feet, or, neglecting the clay, and
taking only the sharp sand and ballast
charged with water, the depth would
still be 20 feet, and the weight of fluid
corresponding to the 55 lbs. per square
foot pressure no more than 2.75 lbs. per
cubic foot.

It will be remembered that the natural
slope of the sand and ballast in Lieuten -
ant Hope's retaining-wall experiments
was about 1^ to 1, and tliat the actual
and theoretical corresponding fluid
pressures were respectively 10.3 lbs. and
23.6 lbs. per cubic foot. In the case of
the gullet, the natural slope of the ballast
and sand would similarly be not less than
IJ to 1, and yet the fluid pressure could
not have exceeded 2.75 lbs. This one
fact, therefore, is sufficient to prove that
the universal assumption of the pressure
of earthwork being analogous to thL^t of
fluid, and proportional to the depth, is
one of convenience rather than truth.
The explanation of the singularly small

lateral thrust of the ballast in the present
case is to be found in the fact that the
ballast was lying between, and partially
held back by, the two relatiyely tenacious
layers of loamy sand and clay. As an
extreme example of the same kind of
action (Ex. 27), the author may state
that he once applied to a wooden box
full of sand a pressure equivalent to a
column of that material 1,400 feet high
before the box burst. On the fluid
hypothesis, the lateral pressure would
have been 1,400 feet X 23.6 lbs., or
about 15 tons per square foot; but of
course a few lbs. would have burst the
box, and the sand was retained by being
jammed between the bottom and lid
of the little deal box — the equivalents of
the tenacious strata in the gullet.

In shafts the stress on the timbering
is far less than in a continuous trench or
heading, by reason of the frictional ad-
hesion and tenacity of the adjoining
earth. Thus (Ex. 28) at a depth of be-
tween 40 and 50 feet, 10-inch square
timbers, 4 feet 6 inches apart, proved of

ample strength to support the sides of a
12-feet square shaft, though the same
sized timbers at the reduced distance of
3 feet 6 inches apart, failed, as ha& been
seen, to support the sides of a 9-feet
square heading.

After the experience of several rather
troublesome slips, light timbering was
abandoned, and a type which proved to
be of ample strength to meet all the con-
tingencies of heavy ground, vibration
from road traffic, and the surcharge of
lofty buildings, was adopted. In this
type (Ex. 29) the 14-feet walings in-
creased in scantling to 12 inches by 7
inches, were spaced 7 feet apart, and
strutted at each end and at the center.
At one-half the breaking weight the sup-
porting power of the walings would be
, . 7"xl2x3cwt. X 112 -_. „

about 6T'"x7 ~

per square foot, and as the depth of the
excavation was in some instances as
much as 36 feet, this would correspond
to a fluid pressure of 18.6 lbs per cubic
foot. With ground weighing 112 lbs.

per cubic foot, and a slope of repose of
li to 1, the theoretical lateral pressure
would be 32 lbs. per cubic foot; and
when it is remembered that this does
not include any allowance for the sur-
charge due to contiguous buildings, and
that the stress on the timber is taken at
fully half the breaking weight, it is clear
that the average actual lateral pressure
of the earthwork must have been less
than half that indicated by theory.

On the extensions of the Metropolitan
railway, the same type of timbering was
adopted, but the walings were generally
9 inches by 4 inches, and spaced 3 feet
apart. The supporting power, upon the
same basis as in the last instance, would
be about 430 lbs. per square foot. In
most cases this strength proved to be
sufficient, but in a few instances the
walings broke, or showed such signs of
distress that additional support had to
be given. This was the case in some of the
deep trenches along the Thames Em-
bankment, where heavy wet silt was
traversed. Near Whitehall Stairs (Ex.

30) the trenches were 40 feet deep, so
the elastic strength of the timbering was
only adequate to the support of a fluid
pressure of 10.6 lbs. per cubic foot, or
probably but one fourth of that theo-
retically due to the material ; it is there-
fore no matter for surprise that the wal-
ings proved unequal to their work. The
stability of the timbering in more moder-
ate depths was, on the other hand, con-
firmatory of the general deductions
drawn from previous examples as to the
wide divergence between the actual and
calculated thrust of earthwork.

Turning now from the consideration
of the temporary works of timbering to
the finished and permanent structures on
the underground railway, a similar varia-
tion in strength wiU be found to obtain.
The lightest retaining wall on the line is
that at the Edgware Eoad station yard
(Ex. 31, Figs. 6 and 7). This wall is 23
feet in height from the top of the footing
to the ground level, and has a maximum
thickness of but 6 feet 3 inches at the
base, out of which has to be deducted a

panel 2 feet 6 inches deep. Calculating
the moment of stability at the leyel of
the footings and round a point 3 inches
back from the face of the pier — which is
a sufficient allowance for the crushing
action on the brickwork — M=4.4 feet,X
8,800 lbs. = 38,720 foot pounds per

FIg:.6

Flg.7

lineal foot of wall. Dividing by — , then

19;lbs. per cubic foot is the weight of the
fluidjwhich would overturn this retaining
wall. The ground supported is light
dry sand, having a slope of repose of
about IJ to 1, and consequently exert-
ing a theoretical lateral thrust equivalent

to a 24-lb. fluid. There is practically no
tenacity in the soil, as the author re-
members seeing demonstrated on one
occasion when a horse and cart, ap-
proaching too near the top edge of the
slope, broke it away and rolled together
to the bottom of the 23 -feet cutting.
Although theoretically deficient in stabil-
ity, and subject to heavy vibration from
the two minutes train service, the wall
has stood, perfectly without exhibiting
the slightest movement. Upon the basis
of the results of actual experiments, and
having reference to the character of the
soil and other conditions, the factor of
safety would appear to be about 2 to 1.

A far lower factor sufficed to secure
the temporary stability of the dry areas
at the station buildings previous to the
erection of the arched roofs (Ex. 32).
The arrangement at Sloane Square sta-
tion is shown in Figs. 8 and 9. The
joint stability of the front and back
walls is the same 'as that of a solid
rectangular wall having a thickness equal
to (2.4 X 2.5 4- 3.8 X 5.2)* = 5.1 feet, or

say ^ of the height. A fluid pressure of
about 9 lbs. per cubic foot would upset
such a wall, so the factor of safety, until

2) tv'nr

the arched roof abutted against the dry
areas, was only that due to the few
14 inch brick arches which tied the walls
together with a certain amount of rigid

ity. This result would perhaps have
surprised the author more had he not
previously investigated many cases of
old timber wharves, in which the piles
and planking had lost more than | of
their original strength from decay, and

Flg-9

Section on line A B.

yet held on against a theoretically over-
powering thrust of earthwork.

The relatively strongest wall on the
Metropolitan railway system is at the
St. John's Wood Eoad station (Ex. 33),
and that has given considerable trouble.
Though 8 feet 6 inches thick at the base;

and backed up to a height of 16 feet
only out of the total height of 21 feet 6
inches, and supported at the top by the
thrust of the arched roof of the station,
this wall moved over and forward to an
extent which necessitated the immediate
adoption of remedial measures.

The moment of stability per lineal
foot M= 73,000 foot pounds, conse-

quently dividing by -^ the fluid resist-
ance is 107 lbs. per cubic foot, or allow-
ing for the thrust of the arched roof of
the station, considerably greater than
th'it of a perfect fluid having the same
density as the ground supported. It is
n(it contended that such a pressure ever
occurred upon the wall, although the
giound is heavy yellow clay. The fail-
ure arose from causes which will be re-
ferred to more generally hereafter, and
the case is only mentioned as a signal
instance of the futility of hoping to re-
duce the engineering of retaining walls
to the form of a mathematical equa-
tion.

It is a suggesiiye fact that, out of the
9 miles of retaining wall on the under-
ground railway, the exceptionally weak
wall should show no moTement either
during or after construction, whilst the
exceptionally strong wall, though having
six times the stability of the former,
should fail. If an engineer has not had
some failures with retaining walls, it is
merely evidence that his practice has not
been sufficiently extensive ; for the at-
tempt to guard against every contin-
gency in all instances would lead to ruin-
ous and unjustifiable extravagance, and
be indeed as ridiculous a proceding as
the making every soft clay cutting at a
slope of 10 to 1, because in a few places
such cuttings happen to slip down to
that slope.

In two instances comparatively heavy
retaining walls have failed on the Metro-
politan railway. During the construc-
tion of the line, the wall on the west
side of the Farringdon street station,
(Ex. 34), failed bodily by slipping out at
the toe and falling backwards on to the

slope of the earthwork (Fig. 10). This
wall (Figs. 11 and 12) was 29 feet 3 in-
ches high above the footings, and 8 feet

^jg ^

6 inches thick. The ground consisted
of aboat 17 feet of made ground, 3 feet
of loamy gravel, and 9 feet of day. At
A distance of 15 feet from the back of

the wall, and at a depth of 16 feet from
the surface of the road, was the Fleet
Sewer — a badly constructed and much
broken brick barrel, 10 feet 6 inches di-
ameter and 3 rings thick. It was be-
lieved that the leakage from the sewer
induced the failure of the wall, but in
reconstruction both wall and sewer were
strengthened. The latter was made 4
rings thick in cement, and the former
(Ex. 35) was increased in thickness to 12
feet 9 inches (Figs. 13 and 14). Origi-
nally the stability was equal to the resist-
ance of a fluid pressure of 24 lbs. per
cubic foot, and, as reconstructed, to 54
lbs.

On the opposite side of the same sta-
tion yard the ground was retained by a
line of vaults (Ex. 36, Figs. 15 and 16),
29 feet high above the footings, and 17
feet deep — or double the original thick-
ness of the wfi^ll last referred to. Al-
though the resistance to overturning was
greater in the proportion of 62 lbs. to 24
lbs. per cubic foot, the vaults some years
after construction came over 15 inches

at the top, and slid forward considerably
more. The movement when once fairly

to '^'

•t -0-81— -

^«;--">5>^^> ^ ' :;'^J^^ f^

2 i ^^

commenced was rapid and alarming, as a
mass of densely inhabited houses was

within 20 feet of the back of the vaults.
Steps were promptly taken to strengthen
the work, by building intermediate piers
and doubling the thickness at the back
(Ex. 37, Fig. 17). This arrested the
movement for a few months, when the
vaults, whose stability had been thus in-
creased to 93 lbs. per cubic foot, again
began to go over and slide forward. It
was clear that mere weight would not
insure stability, so 3-feet square brick
struts were carried at intervals from the
toe of the piers across and under the
railway to the retaining wall of the low-
level line traversing the station yard, at
a distance of about 34 feet from, and 8
feet below, the level of the footings of
the vaults.

The soil in the preceding instance con-
sisted of about 12 feet of made ground
overlying the clay, and, as in the former
case, a sewer was to be foxmd rather
close to the back of the work. West-
ward of the vaults, the clay encountered
in the construction of the line was hard
blue clay, requiring the use of a pick.

and portions of the temporary cuttings
in the station yard, on the site where
the vaults were subsequently built, stood
fairly for many months at a slope of 1^ to
1. At one point, however, troublesome
slips occurred, and even a 2 to 1 slope
had to be piled at the toe to prevent for-
ward movement. It was at this point
that the vaults were subsequently found
to be most dislocated.

In neither of the above cases was fail-
ure due to a deficient moment of stabili-
ty in the wall, and therefore the fact of
their failure does not in any way conflict
with the results of the experiments pre-
viously set forth. In each case water at
the back of the wall was as usual the
active agent of mischief — not in thrust-
ing the wall forward by hydrostatic press-
ure, but in softening the clay and aflfbrd-
ing a lubricant, so that the resistance
was reduced to a sufficient extent to en-
able the otherwise innocuous lateral
pressure of the earthwork to tilt and
thrust forward the wall.

A costly, but conclusive, experience of

this softening action was obtained in the
instance of the central pier to the double
covered way on the District railway near
Gloucester Road station (Ex. 38). The
weight per lineal foot was 21 tons per
foot run of pier, and 4 feet 9 inches was
spread out by footings and concrete to a
base of 10 feet ; hence the pressure on
the ground was 2.1 tons per square foot.
In a similar construction near Alders -
gate the load was 25 tons, and the width
of base 8 feet 3 inches, giving the in-
creased load of 2.9 tons upon the founda-
tions. At the Smithfield market the
author did not hesitate to place a column
carrying 436 tons upon a 12-feet-square
base, which is equivalent to a load of 3
tons per square foot ; and in the.Euston
Boad, the side wall of the covered way
has a load of 16 tons per lineal foot on a
4 -feet-wide base, which is at the rate of
3i tons per square foot. In all in-
stances thQ foundation was clay, of ap-
parently equal solidity, and in every in-
stance but the :drst no settlement at all
occurrisd. For some years no settlement

was observable ia that case either, but
ultimately, after an acci dental flooding
of the line, and permanent accumulation
of water near the foundations, owing to
the line being below the limits of nat-
ural drainage, and the pumping being
neglected, cracks were observed in the
arches, and on examination the concrete

Flg.l8

j^.io_#jp7Tini[j.^.ti ^

and footings of the central pier were
found to be fractured, as shown in Fig,
18. The load of 21 tons per lineal foot
was thus imposed upon a base only 4
feet wide, and the softened clay proved
unable to sustain the pressure of up-
wards of 6 tons per square foot. Con-
siderable difficulty was experienced in

checking the movement when once es-
tabhshed. The center pier was under-
pinned with brickwork in cement, but the
footings, though of exceptional strength,
were again sheared off, and it was found
necessary to use 6-inch York landings.

This failure shows the advisability of
making concrete foundations of sufficient
transverse strength to distribute the
weight uniformly over the ground. As
the result of experiment, the author is
of opinion that the ultimate tensile re-
sistance in a beam of good cement or
Has lime concrete, is about 100 lbs. per
square inch, and in a beam of good
brickwork in cement as much sometimes
as 350 lbs. per square inch.* Taking
the former value (Ex. 39), a 12-inch thick
concrete foundation, projecting 12 inches
from the face of a wall, would break

12' X 100
with a distributed load=— - — — - =4,800

D X 6

lbs.; or, say, 2 tons per square foot.

With a pressure upon the foundation of,

♦ Vide "The Strength of Brickwork." By B. Baker.
'Bngineering," vol. xiv.

say, 3 tons per square foot, and a factor
of safety of 2, the thickness of a con-
crete foundation would therefore be

=1.73 time the amount of

2 tons

its projection beyond the face of the
pier or wall, and the author would not
advise a less thickness being used when
the foundation rests on plastic clay.

Water naturally gravitates to the
foundation of a retaining wall, and a
softening occurs. Owing to the lateral
thrust of the earthwork, the pressure on
the foundations is not uniform, and in-
stead of settling uniformly, the outer
edge descends fastest and the top of the
wall is thrown outwards. The same
softening reduces the clay to a condition
in which it is easily ploughed up by the
advancing wall, and the water acts as an
admirable lubricant in diminishing the
friction between the bottom of the wall
and the clay on which it rests. These
elements are exceedingly variable in their
nature, and it is practically impossible to

foretell the extent of their influence in
each individual case.

In tunneling, clay may be the best or
the worst of materials — almost self-sup-
porting, or pressing with irresistible
force on the crushing timbers and brick-
work. It may be taken for granted that
in good ground bad work will occasion-
ally creep into tunneling, however close
the inspection. How good a material
clay can be was enforced upon the
author's attention once in renewing a
short length of defective tunnel lining,
when on cutting down the work it was
found that for some 50 feet the side wall,
instead of being 2 feet 6 inches thick as
intended, consisted merely of a skin of
brickwork 9 in inches thick on the face,
with a number of dry bats thrown in
loosely behind this thin face wall to fill
up the space excavated. This tunnel
(Ex. 40) was loaded with a weight of 46
feet of clay over the crown, but no meas-
urable settlement had taken place ten
years after completion, and it was rather
by sounding the side wall, than by the

observance of cracks, that a suspicion
was raised as to its solidity. If the full
weight of the ground had come upon the
tunnel as it did upon the heading (Ex.
26), the pressure upon the side wall
would have been 45 tons per lineal foot,
or practically double the strength of the
9-inch work as determined by experi-
ment

Of course the clay in this case was
hard blue clay, which had not been af-
fected by the action of air and moisture.
As explained by the Rev. J. 0. Clutter-
buck, many years ago, the superficial lay-
ers of the London clay are yellow, because
the protoxide of iron is changed into a
peroxide by the action of air and moist-
ure in the disintegrated mass, and it is
the yellow clay, therefore, which is the
dread of the engineer. As good an ex-
ample as any of the difference between
the two materials was afforded forty
years ago, in the well known slip which
occurred in the 76-feet-deep cutting at
New Cross, when nearly 100,000 tons of
yellow clay slipped forward on the hard

smooth surface of the shale— like under-
lying blue clay, and buried the entire
line for a length of more than a hundred
yards to a depth of 12 feet.*

Owing to a misunderstanding, a sec-
tion of concrete wall designed by the
author to form one side of a running
shed, and to retain the earthwork in a
13-feet cutting through light-made
ground, was adopted also in a similar
case, but where the ground was heavy
wet clay, and the cutting 30 feet deep
(Ex. 41). A wall 13 feet in height from
formation to coping, and only 3 feet 3
inches thick at the base, had thus to sus-
tain a surcharge of 17 feet. As the
slope of repose was at least 1^ to 1, the
lateral thrust was theoretically equiva-
lent to a fluid pressure of about .70 lbs.
per cubic foot, whereas a pressure of less
than one- third that intensity would have
overturned the wall. The latter, never-
theless, held up the ground fairly for
some months, though the nature of the

* Vide Minutes of Proceedings Inst. C.E.. voL lii., p.
189.

soil was such that it ultimately became
necessary to add strong counterforts to
the wall, and to reduce the slope of the
cuttings generally to 2 : 1.

On the Thames Embankment heavy
clay filling was in places cut through by
the District railway, and in several in-
stances the light side walls of the cover-
ed way were thrust over a few inches at
the top before the girders were bedded
(Ex. 42). The side walls were eighteen
feet in height from the invert to the
ground level, and 5 feet 6 inches thick,
with panels 6 feet 6 inches wide,by about 2
feet 9 inches deep,and piers 2 feet 6 inches
wide. A fluid pressure of 16 lbs. would
overcome the stability of these walls, but,
though subject to the pressure of the
heavy clay filling, none of them failed.
The existence of an undue pressure was,
however, manifested by the thnisting for-
ward of the green brickwork during the
few weeks that the walls were left unsup-
ported by the girders.

The retaining walls, at the approach
to Euston Station, afford a good iUustra-

tion of the impossibility of maidiig any
reasonable approximate estimate of the
possible lateral thrust of yellow clay, or
of stating positively that no movement
will ever occur. These walls, soon after
construction, were forced out in an ir-
regular way at the top, bottom, or
middle, but on pulling them down, the
clay behind appeared to be free from fis-
sures and to stand vertical. Cast-iron
struts were subsequently put in between
the opposite retaining walls; and al-
though General Burgoyne, who had
given much attention to the subject of
revetments, prophesied at the time that
they would be removed in a few years,
" when the ground had become consoli-
dated," the struts still remain, and the
walls still give signs of severe and in-
creasing stress.

It is not only London clay that proves
so embarrassing to engineers. In a re-
cent Paper* particular attention was
called to the treacherous nature of some

*nde "Earthwork Slips," Minutes of Proceedings
Inst. C. E„ vol. Ixlli,, p. 280 et seg.

boulder clay which, " although bo tough
and tenacious as to give the utmost dif-
ficulty in excavation, after a short ex-
posure became soft and pasty in the
winter, often jolting down the slurry."
Examples were given of formidable slips
in this material, in contrast with which
the author would point to the comparati-
vely slow wasting of the huge boulder clay
cliffs near the mouth of the Tyne, a mat-
ter which he had occasion to investigate
very closely in connection with the Duke
of Northumberland's Jands in that dis-
trict. From a comparison of surveys
extending over a period of one hundred
and fifty years, it appeared that the
wasting of the cliff was very slow, and
due solely to the wash of the waves at its
base. At no time was the slope of repose
of this 105-feet-high cliff more than 1 to
1, and in places it stood for years at an
average slope of less than f to 1. With
his experience of North London clay, the
author was startled to find people con-
tentedly living in houses partially over-
hanging the brow of this steep and

ragged cliff, but the stability of the chy
was so great, and the waiting so uniform,
that the fact of the outhouses being at
the bottom of the 100 feet slope, and the
TnniTi building at the top, did not appear
in any way to disturb the equanimity of
the householders.

The failures of dock walls, though nu-
merous and instructive, afford no direct
evidence as to the actual lateral pressure
of earthwork, because in practically every
instance the failure is traceable to defec-
tive foundations. The author cannot re-
call any case in which a dock or quay
wall founded on rock has overturned or
moved forward, though on other founda-
tions a movemejit to a greater or lesser
extent is so much the rule that Voisin
Bey, the distinguished engineer-in-chief
of the Suez Canal, once stated to the
author that he could name no exception
to it, since he had failed to find any long
line oi quay wall, which on close inspec-
tion proved to be perfectly straight in
line and free from indications of move-
ment. A brief examination of some in-

stances of the failures of dock walls will
show how powerfully unknown practical
elements affect theoretical deductions in
such cases.

Fig. 19

ijlft^O^

■c

3— ' '

M^-

Si— ^ -

MeJ

^ p

1 ^

y-- -^

fi^^^^^ —

.*.*;

A well-known and often cited case is
that of the original Southampton dock
wall, constructed now some forty years
ago (Ex. 43, Fig. 19). This wall, 38 feet
in height from the foundation to the
coping, was built on a platform of 6-inch
planks, resting on a sandy and loamy
bottom. Before the water had been let

into the dock, or the backing carried to
the full height, the wall moved forward
in some places as much as three feet, but
came over hardly anything at the top.
When the water was let into the dock,
the filling behind becoming saturated,
the pressure on a receding tide exag-
gerated, and to secure stability it was
found necessary to discontinue the fiUing
at some distance below the full height
of the wall, and to substitute a timber
platform.

The thickness of this wall at the base
is 32 per cent, of the height between the
buttresses, 45 per cent, at the buttresses,
and a rectangular wall containing the
same quantity of material would have a
thickness equal to 26 per cent, of the
height. Though the base is wide, the
weight is light as compared with most
other dock walls, and the tendency to
slide forward is therefore greater. If
founded on a rock bottom, a fluid press-
ure of about 40 lbs. per cubic foot would
have been required to overturn the wall,
but of course a fraction of this pressure

would suffice to make it move forward on
the actual bottom.

The conclusion drawn by Mr. Giles,
M. Inst. C. E., the engineer of the docks,
from this and other failures is, that the
quality of a dock wall is of little conse-

quence compared with the quantity, and
that it ought to be sufficiently strong not
only to hold any amount of any kind of
backing put against it, but to carry a

head of water equal to its height if it
were left dry on the other side.*

These principles have been adhered to
in the recent extension of the South-
ampton docks (Ex. 44, Fig. 20). Here
the wall is founded on a mass of con-
crete 21 feet wide; the effective thickness
at base is about 45 per cent, and the
mean thickness 41 per cent, of the height.
A fluid pressure of from 60 to 70 lbs.
would be required to overturn this wall
if on a hard foundation, and probably as
much to make it move forward, unless
the bottom were of clay or of other un-
favorable material. Mr. Giles has found
even a heavier wall slide, when founded
on a thin layer of gravel overlying clay.
In the earlier wall, if the co-efficient of
friction of the base on the ground were
less than f , the wall would slide rather
than overturn ; but in the latter wall,
without buttresses, any co-efficient ex-
ceeding ^ would be sufficient to prevent
sliding.

*Vlde Minutes of Proceedings Inst. C. E.,vol. Iv.,
p. 6S.

For comparison with the above, the
section of the east quay wall of the
Whitehaven dock may be next referred
to (Ex. 45, Fig. 21). Having the same
height as the Southampton dock wall,
the thickness at the base is but 37 per

cent of the height, the mean thickness 31
per cent., and the concrete foundation 16
feet 6 inches, instead of 21 feet wide.
This wall has stood perfectly, though it
would fail to resist the head of water
mentioned by Mr. Giles, but would be

overturned by a fluid weighing from 45
to 50 lbs. per cubic foot. During con-
struction, weep holes were, however, left
in the walls to relieve them of hydro
static pressure.

Fig.22

Another dock wall of the same height
as the preceding ones, is that of the
Avonmouth dock (Ex. 46, Fig. 22). In
this instance the thickness is 42 per cent,
of the height, and the concrete base 22
feet 6 inches wide, dimensions which,

with a good foundation, would enable the
wall to stand a full hydrostatic pressure
at the back. Owing to the treacherous
nature of the bottom, a long length of
this wall nevertheless slipped forward at
one point as much as 12 feet 6 inches,
and sunk 4 feet 6 inches without the
latter being affected, whilst at another
point, where thete was no forward move-
ment, the wall came over about 1 foot 8
inches. When the failure occurred, the
foundation rested on apparently stiff blue
clay, but in subsequent portions the
concrete was carried down through the
clay to the sand.* On the east side of
the dock, though the walls were founded
at an average depth of no less than 9
feet below the bottom of the dock, they
still moved forward in the mass some 15
feet 6 inches, and sunk 7 feet 6 inches,
The filling was carefully punned in
layers, with material which seems to have
stood fairly at a slope of 1^ or 2 to 1, so
that the wall theoretically possessed an

♦ Vide Minutes of Proceedings Inst. C. B., vol. Iv.,
p. 16.

excess of strength, and yet, owing to the
existence of conditions which it was im-
possible for the engineer to foresee,
failures occurred as described.

A somewhat similar case of sliding for-
ward occurred at the New South Dock,

Fig.23

West India Docks (Ex. 47, Fig. 23). The
wall is 35 feel 9 inches high from the top
of the footings to the coping, and 13
feet, or 36 per cent of the height, thick
at the base. The concrete foundation is
17 feet wide, and 6 feet deep below the

bottom of the dock, and the fluid press-
ure required for overturning would be
about 45 lbs. per cubic foot. A coefficient
of friction of less than i would be suffi-
cient to guard against sliding under this
pressure, but owing to the existence of a
thin seam of soft greasy silt between the
hard strata of blue clay upon which the
foundations rested, several portions of
the wall slid forward. The original ground
level was about 15 feet below the top
of the dock wall, and the excavation stood
fairly as a slope of 1 to 1. Favorable
material for backing did not appear to be
available.

The fact that the stability of a dock
wall depends far more upon the founda-
tion than upon the thickness or mass of
the wall itself, is well illustrated by the
quay wall at Carlingford (Ex. 48, Fig.
24). With a height of no less than 47
feet 6 inches, the thickness of wall and
width of foundation at the base are each
but 15 feet, or less than 32 per cent, of
the height, and the mean thickness is
but 24 per cent. A lateral pressure of

half that due to a hydrostatic pressure
would probably suffice to overturn this
structure.

In contrast with the preceding wall
may be cited that of the dock basin at
Marseilles (Ex. 49, Fig. 25). In both in-
stances the foundation was good, and
the wall rested immediately upon it with-
out the interposition of any broad mass
of concrete; but the French engineer,
though the wall was but 32 feet high,
made the thickness at the base no less
than 16 feet 9 inches, or 52 per cent of
the height — an unusually large propor-
tion, which he was led to adopt in conse-
quence of the stratification of the
ground inclining towards the yralL

Perhaps one of the boldest and most
successful examples of a lightly-propor-
tioned wharf wall is that built by Colo-
nel Michon in 1857 on the Moselle at
Tour(Ex. 50, Figs. 26 and 27). With a
height of 26 feet, and a batter of 1 in
20, the thickness of the wall through the
counterforts is but 3 feet 7 inches at the
base, and though the filling is ordinary

material, haTing a slope of repose of 1-J-
to 1, and the floods rise within 6 feet of
the top of the coping, no movement
whatever has occurred since the wall was
built.

As striking a contrast as could be
wished to the above light construction is
found in Sir John Macneill's quay wall
at Grangemouth harbor (Ex. 51, Fig. 28).
Both walls are of about the same height,
but whilst the mean thickness of the
first is only 3.7 feet, or \ of the height,
that of the second, inclusive of the mass
of concrete backing, is no less than 23
feet, or, pay, ^ of the height.

One of the most troublesome cases of
dock-wall failures was that at the Belfast
harbor* (Ex. 52, Fig. 29). This wall
was founded upon round larch piles 15
feet long, 10 inches in diameter at the
top, and 4 feet 6 inches apart from cen-
ter to center. Symptoms of settlement
became apparent soon after the filling
was commenced, and some remedial

* VkU Minutes of Proceedings Inst. C.E., vol. ly., p.
81.

measures were attempted. The gronnd,
however, was hopelessly bad, the slope
of repose ranging from 3 to 1 to 6 to 1,
and the backing material being equally
bad, the light piling was inadequate to
resist the thrust. Two years after erec-
tion a length of about 70 lineal yards of
wall was overtunied and carried forward
into the middle of the dock entrance, the
piles being sheared off about 6 feet be-
low the bottom of the wall. The height
from the top of the pile to the coping is
31 feet 6 inches, and the thickness at the
base 16 feet, or half the height. On
good ground, therefore, the wall would
have had an ample margin for stability.

A somewhat similar failure occurred in
the instance of the original side walls of
the lock chamber of the Victoria docks*
(Ex. 53, Fig. 30). These docks were built
at a time when little confidence was placed
in concrete as a durable material for
dock work, and consequently the walls
were faced with cast-iron piling and
plates, as in previous instances at Black-

♦iWtf.,vol.xvlil.,p.468.

wall and elsewhere. The foundations
were on a layer of gravel overlying the
clay, but the face piling had little hold
in the gravel, and the base of the wall
itself was only some 30 per cent, of the
height, hence, when the water was let
into the dock, the hydrostatic pressure
at the back of the lock wall forced it
bodily forward into the lock, ploughing
up the puddle in front of it, and break-
ing tie bolts and tie piles as it advanced.
In reconstruction a solid concrete wall 20
feet thick, and having nearly treble the
stability, was carried through the gravel
down to the clay.

The wall of the Victoria Dock Exten-
sion Works, by Mr. A. M. Rendel, M.
Inst. C.E. (Ex. 54, Fig. 31), has a thick-
ness of about 50 per cent, of the height
at the point where the 18-feet wide
foundation meets what may be termed
the body of the wall, and the wharf
wall of Mr. Fowler's Millwall dock (Ex.
55, Fig. 32) has a maximum thickness
of 13 feet 6 inches for a height of 28
from bottom of dock to coping, of

practically the same ratio. Either or
these walls would be capable of resist
ing the full hydrostatic pressure.

An early example of a successful wall
on a very bad foundation is afforded by

Fig.3l

Fig. 32

;^ — -18,0 — ■>*

Sir John Kennie's Sheemess wall (Ex.
56, Fig. 33) The subsoil consisted of
loose running silt for a depth of about 50
feet, covered with soft alluvial mud, and
the depth at low water was at some
points as much as 30 feet. A piled plat-
form about 42 feet in width, with sheet-

ing piles on the river face, and 12-inch
piles pitched from 3 to 4 feet apart over
the whole area, and driven until a 15-cwt.
monkey falling 25 feet did not move

Fig.33

them more than ^ inch at a blow, was
prepared, and upon this the wall, no less
than 50 feet in extreme height and 32
feet in effective thickness at the base,
was raised. In no case has any yielding

100

or unequal settlement taken place, ex-
cept in the instance of the basin wall,
the cracks in which Sir John Eennie at-
tributed to other causes than a failure in
the foundation. Although the voids in
the masonry were designedly filled in
with grouted chalk and other light mate-
rial, the Sheemess river wall has per-
haps a greater moment of stability than
any other wall in the world.

Another exceptionally heavy wall, more
than a half century younger than the
preceding, is that of the Chatham Dock-
yard Extension (Ex. 57, Fig. 34). The
height from the bottom of the dock to
the coping is 39 feet, and the founda-
tions are carried down to the loam gravel
or chalk at a depth of 4 feet 6 inches
below the bottom of the dock. The
thickness of the wall is 21 feet at the
base, or, say, J^ of the extreme height.
On a hard chalk bottom it would resist a
fluid pressure of about 80 lbs per cubic
foot.

Two examples of Liverpool dock
walls, namely, that at the Oanada half-

101

tide basin, and that at the Herculanean

docks, are given in Figs. 35 and 36. The

102

former (Ex. 58) is 43 feet in extreme
height, and 19 feet, or 44 per cent, wide
at the base. The latter (Ex. 59) is 39
feet high, and 18 feet, or 46 per cent,
wide at the footings, which rest on a
marl bottom. A dock wall at Spezzia
(Ex. 60) of somewhat similar propor-
tions, the height being 41 feet, and the
width at the bottom of foundations 23
feet, or 56 per cent, of the height is
shown on Fig. 37.

Walls made of large concrete blocks,
resting upon a mound of rubble, have
been constructed in many of the Medi-
terranean ports, generally with success,
but occasionally with failure, as at Smyr-
na, where, owing to the great settlement,
six and seven tiers of blocks had to be
superimposed instead of four, as in-
tended, and the quay wall had after all
to be supported by a slope of rock in
front extending up to vnthin 7 feet of
mean sea level, and seriously interfering
with the use of the quays. The propor-
tions arrived at by experience are a width
of 9 meters at the top, and a thickness

103

«:>

104

of not less than 2 meters for the rubble
mound ; a depth of 7 meters below the
water line, and a thickness of 4 meters
for the concrete block wall resting on
the mound ; and a minimum thickness of
2.5 meters, and a height of 2.4 meters
■for the masonry wall coping the concrete
blocks.

At Marseilles (Ex. 61, Fig. 38), the
top of the rubble mound is only 6
meters below the water-line, so vessels
occasionally bump; and the concrete
block wall 3.4 meters, or 40 per cent, of
the height, in thickness has proved
rather less stable under the contingencies
of working and the surcharge of build-
ings and goods than is considered desir-
able.

Examples are not wanting, however,
of walls founded on rubble mounds
where the thickness holds a smaller ratio
to the height than the 42 per cent., con-
sidered necessary by the French engi-
neers. Mr. Fowler has made concrete
block walls in the Eosslare Harbor (Ex.
62) 42 per cent, of the height on the sea

105

face, and but 28 per cent, on the harbor
side, but cross walls at 50-feet intervals
considerably strengthen the work. The
inner wharf wall of the Holyhead new
harbor, again (Ex. 63, Fig. 39), is 27 feet
high and 8 feet thick, a ratio of under 30
per cent., but though stable, the line of
coping is somewhat wavy on plan. The
original wall of the West Pier at White-
haven (Ex. 64, Fig. 40), is 42 feet 6 inches
high, with a thickness of 8 feet 6 inches
between the buttresses, which latter are
6 feet deep by about 4 feet wide and 15
feet apart ; but the lightest of all, per-
haps, is the dry masonry outer wall of the
St. Katherine's breakwater, Jersey, (Ex,
65, Fig. 41), which is only 14 feet wide
at the base for a total height of 50 feet,
or a ratio of 28 per cent.

It must not be forgotten, of course,
that the three latter walls have to sup-
port rubble hearting only, instead of
sand and other material, having a much
flatter slope of repose. Occasionally, as
has been stated (Ex. 22), rubble will not
stand at less than 1^ to 1 ; but at Holy-

106

head and Aldemey the slope of the rab-
ble mound on the harbor side is only
about li to 1. At Cherbourg it is 1 to
1, and at Leghorn the large concrete
blocks are found to be stable at a slope

Fig.40

Fig.4l

of f to 1. By a very little care in selec-
tion, the thrust of a rubble filling may
be reduced to a fraction of that arising
from bad material, and indeed in the
ordinary run of fishing piers in the North

.107

of Scotland, however great the height,
the face wall of the rubble-hearted pier
consists simply of stones from 3 to 4
feet in depth, laid dry to a batter of about
1 in 5. The north-east pier at Seham,
again, has an inner wall 25 feet high>
battering 1^ inch to the foot, and only 5
feet thick, and many similar examples
are to be found at other points of the
coast.

The most cursory examination of cases
of failure cited above will serve to justify
the statement that the numerous dock-
wall failures do not afford any direct
evidence as to the actual lateral pressure
of earthwork. Thus, remembering Gene-
ral Burgoyne's battering wall, only 17
per cent, of the height in thickness,
supported the -heavy sodden filling at its
back, no calculation is required to show
that the 32 and 45 per cent. Southamp-
ton Dock counterforted wall, the 42 per
cent. Avonmouth Dock wall, the 36 per
cent. West India Dock wall, the 50 per
cent. Belfast Harbor wall, and the 30 per
cent. Victoria Dock wall, would all have

io:s

stood perfectly had the foundation been
rock, as in the instances of General Bur-
goyne's experimental walls, instead of
the mud, clay, and silt which it actually
was.

Not only the strength, but the type of
cross-section, is singularly indicative of
the small influence which theory and ex-
periment have exercised upon the design
of dock walls. If the early theorists and
experimentalists were in accord upon one
point, it was upon the immense advant-
age afforded by a counterforted wall.
Lieutenant Hope was led by his experi-
ments to conclude that if good counter-
forts were introduced, the merest skin of
face wall would suffice for the portion
between them, and theorists of course
arrived at the same conclusion, from a
comparison of the moments of stability
of rectangular blocks of masonry edge-
wise and flatwise. Nevertheless, in only
one of the preceding dock walls, and that
one forty years old, are counterforts in-
troduced. In practice it was found that
counterforts frequently separated from

109

the body of the wall, and they were con-
sequently regarded as untrustworthy.
It is open to question whether this con-
clusion does not require reconsideration
in these days of cheap, strong, and
easily-moulded Portland cement concrete.
Nothing but blasting would separate the
counterforts from a good concrete wall.
The author has used concrete in many
varieties of structures, and as long back
as fifteen years built a four-story ware-
house, walls and floors, entirely of con-
crete, without the introduction of any
iron girders. He is bound to admit,
however, that by far the boldest and
most thorough adaptation of the ma-
terial to multifarious uses met with by
him was in the instance of some farm
buildings in an out-of-the-way district in
Co. Kerry, Ireland. The small tenant-
farmer and his laborers — none of whom
were receiving over lis. a week — with-
out skilled assistance of any kind, had
constructed dwelling-house, cattle- sheds,
and hay- bam wholly of concrete. The
cattle-shed was roofed with concrete

110

arches of 15 feet span, 1 foot rise^ and
4 inches thick, springing from octagonal
concrete pillars 8 inches in diameter,
spaced 15 feet apart from center to center.
A layer of concrete constituted the pav-
ing, concrete slabs divided the stalls, the
cattle fed and drank out of concrete
troughs, the windows were glazed in
concrete mullions, the gates hung on con-
crete posts, and the farmer seemed to
regret somewhat that he had not adopted
concrete doors and concrete five-bar
gates.

Portland cement concrete being thus
possessed of such great tenacity, there is
no risk of counterforts separating from
the body of a wall, but it by no means
follows that there would be any advant-
age in using them in other than excep-
tional cases. In practice, as failures have
shown, it is weight, with the consequent
grip on the ground, rather than a high
moment of stability, that is required in a
dock wall. It may be asked, with reason
why a bad bottom should affect the
thickness of a retaining wall, or, in other

Ill

words, why the foundation should not
first be made good, and then a wall of
ordinary thickness be built upon it. The
answer, of course, is that if weight is re-
quired to prevent sliding, it is just as
economical to distribute the material
over the general body of the wall as to
confine it to the foundations. It follows,
therefore, that under the stated condi-
tions the adoption of a counterforted
wall would lead to no economy in ma-
terial, whilst it would involve additional
labor in construction.

A dock wall is subject to far larger
contingencies than an ordinary retaining
wall, and the required strength will be
included only within correspondingly
large limits. Hydrostatic pressure alone
may more than double or halve the
factor of safety in a given wall. Thus,
vnth a well-puddled dock bottom, the
subsoil water in the ground at the back
of the walls will frequently stand far
below the level of the water in the dock,
and the hydrostatic pressure may thus
wholly neutralize the lateral thrust of the

112 ,

earth, or even reverse it, as in the case of
the inner retaining walls on the Soon-
kesala canal, some of which, though 35
feet in height, are only 2 feet thick at the
top and 7 feet 6 inches at the base. On
the other hand, with a porous subsoil at
a lock entrance, the back of the walls
may be subject, on a receding tide, to the
full hydrostatic pressure due to the
range of that tide plus the lateral press-
ure of the filling. Again, the water may
stand at the same level an both sides of
the wall, but may or may not get under-
neath it. If the wall is founded on a
rock or good clay, there is no more
reason why the water should get under
the wall than that it should creep through
any stratum of a well-constructed ma-
sonry or puddle dam, and under those
circumstances the presence of the water
will increase the stability by diminishing
tW, lateral thrust of the filling. With
nibble filling, assuming the weight of
the solid stone to be 155 lbs. per cubic
fcot, and the voids to be 35 per cent.,
the weight of the filling would be 100 lbs.

113

per cubip. foot in air, and 59 lbs in water,
and the lateral thrust will be that due to
the latter weight.

If, however, as is perhaps more fre-
quently the case, the wall is founded on
a porous stratum, the full hydrostatic
pressure will act on the base of the wall,
and reduce its stabiHty in practical cases
by about one-half. Thus, the 30-ton
concrete block walls on rubble mounds,
at Marseilles and elsewhere, have the
stability due to a weight of, say, 130 lbs.
per cubic foot in the air, and QG lbs. per
cubic foot in sea water : but the rubble
filling at the back of the wall, being simi-
larly immersed, is also reduced in weight,
and consequently thrust to a correspond-
ing extent, so the factor of safety is un-
affected.

In walls with offsets at the back, as in
Figs. 25 and 36, and water on both sides,
the stability will be much increased by
the hydrostatic pressure on the top of
the offsets, should the wall rest on an
impermeable foundation. It is generally

114

aesumed, in theoretical investigations,*
that the weight of earthwork super-im-
posed vertically over the offsets should
be included in the weight of the wall in
estimating the moment of stability ; but
the author has found no justification in
practice for this assumption. He has in-
variably observed that when a retaining
wall moves by settlement or otherwise,
it drops away from the filling, and cavi-
ties are formed. A settlement of but -^
of an inch, after the backing had become
thoroughly consolidated, would suflSce to
relieve the offsets of all vertical pressure
from the superimposed earth, and the
latter cannot therefore be properly con-
sidered as contributing to the moment of
stability.

A wall with deep offsets at the back is
not a desirable form where the foundation
is bad, and where, consequently, the
pressure over the foundation should be
as uniform as possible, so that a settle-
ment may take the form of a uniform

♦ Vide " A Manual of Civil Engineering." By W. J.
M. Rankine, p. 402.

116

sinking, and not a tilting forward of the
coping by reason of the toe sinking faster
than the back of the wall. A paneled
wall, such as that shown on Figs. 11 to
14, though liot admissible in dockwork,
is on bad ground far less liable to come
over than a wall with offsets at the back,
and with a consequent concentration of
weight at the front, where the conditions
of a lateral thrust especially require that
it should not be.

The latter conditions also indicate the
expediency, of adopting raking piles, as in
Fig. 33, rather than vertical piles, as in
Fig. 29, where a piled foundation is un-
avoidable. Thus, taking an ordinary
case of dock wall, in which the factor of
safety, as regards overturning, is 3, and
the ratio of weight of wall to the lateral
pressure of earthwork required to over-
turn it is If to 1, it follows that if the
foundation piles are driven at the
rate of 1 to 3 -f- If = 1 : 6 there will be
no transverse strain tending to break
them off, as in the case illustrated by Fig.
29, and no tendency to plough up the

116

soft ground in front of the toe of the
wall.

If an engineer could tell by inspection
the supporting power and frictional ad-
hesion of every bit of soil laid bare, or
see through 5 or 10 feet of earth into a
"pot hole," or layer of shmy silt, he
might avoid many failures, and even hope
to frame some useful equations for ob-
taining the required thickness of a dock
wall. Taking things as they are, how-
ever, it is hardly worth while to use even
a scale and compass in such work, for
being in possession of all the informa-
tion obtainable about the foundation and
backing, an engineer may at once sketch
as suitable a cross- section for the parti-
cular case as he could hope to arrive at
after any amount of mathematical inves-
tigation. Something must be assumed
in any event, and it is far more simple
and direct to assume at once the thick-
ness of the wall than to derive the latter
from equations based upon a number of
uncertain assumptions as to the bearing
power of the foundations, tlie resistance

117

to gliding, and other elements. This
being so, it has often struck the author
that the numerous published tables
giving the calculated required thicknesses
of retaining walls to three places of deci-
mals, stand really on exactly the same
scientific basis, and have the same prac-
tical value, as the weather forecasts for
the year in Qld Moore's Almanack. In
both cases a pretence is made of foretell-
ing what experience has shown can often
not be known until after the event. One
well-known authority gives young en-
gineers the choice of five hundred and
forty-four different thicknesses for a
simple vertical rectangular retaining
wall, so that an unfortunate neophyte
might not unreasonably conclude that
the task before him was not to decide
whether, say, a 32 -feet wall should be 20
feet thick, as in Example 60, or 9 feet, as
in Example 62, but whether it should be
14 feet 6 inches or 14 feet 5J inches
thick.

Although dock wall failures do not
afford any data as to the actual lateral

118

pressure of earthwork, a knowledge of
the latter will enable much valuable in-
formation to be deduced as to the bearing
power of soil and other matters from
such failures, and the data so obtained
will be applicable to other stnictures
beside retaining walls. Knowing the
actual lateral thrust, the coefficient of
friction of the base of a wall which has
been pushed forward on the ground can
be at once deduced, but if the theoretical
as distinguished from the actual thrust
were introduced into the equation, the
result would be valueless.

The aim of the author in the present
paper has been to set forth as briefly as
possible what he knows regarding the
actual lateral thrust of different kinds of
soil, in the hope that other engineers
would do the same, and that the infor-
mation asked for by Professor Barlow
more than half a century ago may be at
last obtained. Although the acquirement
of the missing data would probably lead
to no modification in the general propor-
tions of retaining structures, since these

119

are based upon dearly -bought experience,
it is none the less desirable that it should
be obtained ; for an engineer should be
able to show why he believes that a given
wall will stand or fall. To assume upon
theoretical grounds a lateral thrust,
vsrhich experiments prove to be excessive,
and to compensate for this by giving no
factor of safety to the wall, is not a scien-
tific mode of procedure.

Experience ,has shown that a wall ^ of
the height in thickness, and battering 1
inch or 2 inches per foot on the face,
possesses sufficient stability when the
backing and foundation are both favor-
able. The author, however, would not
seek to justify this proportion by assum-
ing the slope of repose to be about 1 to
1, when it is perhaps more nearly 1 J to 1,
and a factor of safety to be unnecessary,
but would rather say that experiment
has shown the actual lateral thrust of
good filling to be equivalent to that of a
fluid weighing about 10 lbs. per cubic
foot, and allowing for variations in the
ground, vibration, and contingencies, a

120

factor of safety of 2, the wall should be
able to sustain at least 20 lbs. fluid press-
ure, which will be the case if J of the
height in thickness.

It has been similarly proved by expe-
rience that under no ordinary conditions
of surcharge or heavy backing is it ne-
cessary to make a retaining wall on a
solid foundation more than double the
above, or ^ of the height in thickness.
Within these limits the engineer must
vary the strength in accordance with the
conditions affecting the particular case.
Outside these limits the structure ceases
to be a retaining wall in the ordinary ac-
ceptation of the term. A 9 -inch brick
facing might secure the face of a friable
chalk cutting which, if suffered to re-
main exposed to the action of the weather,
would crumble down to a slope of 1 to 1,
and a massive bridge pier, with an "ice-
breaker " cutwater, might stand firm
against an avalanche, but in neither case
could the structure be fairly stated to be
a retaining wall.

Hundreds of revetments have been

121

built by Royal Engineer officers in ac-
cordance with General Fanshawe's rule
of some fifty years ago, which was to
make the thickness of a rectangular brick
wall, retaining ordinary material, 24 per
cent, of the height for a batter of -^,
25 per cent for ^, 26 per cent, for J, 27
per cent, for -^^ 28 per cent, for -j^, 30
per cent, for t^, and 32 per cent, for a
vertical wall.

As a result of his own experience the
author makes the thickness of retaining
walls in ground of an average character
equal to J of the height from the top of
the footings, and if any material is taken
out to form a face panel, three-fourths of
it are put back in the form of a pilaster.
The object of the panel, as of the 1^
inch to the foot batter which he gives to
the wall, is not to save material, for this
involves loss of weight and grip on the
ground, but to effect a better distribu
tion of pressure on the foundation. It
may be mentioned that the whole of the
walls on the District railway were de-
signed on this basis, and that there has

122

not been a single instance of settlement,
or of coming over or sliding forward.
The author has in the present paper
analyzed a few dozen experiments, and
discussed as many more facts; but an en-
gineer's experience is the outcomp not of
a few facts, but of the thousands of in.
cidents which force themselves onliis at-
tention in carrying out work, and it is this
experience, acquired in the construction
of works of a somewhat special character,
which has convinced the author that the
laws governing the lateral pressure of
earthwork are not at present satisfac-
torily formulated.

DISCUSSION.

Mr. B. Baker desired to add, that his
object in bringing forward the paper was
not so much to present certain facts for
criticism as to induce others to give the
results of their experience, and if every
one helped a little he thought a very use-
ful result would be attained.

Mr. W. Airy said he had given consid-

123

ejable thought and attention to the sub-
ject of earthwork, and he considered the
collection of examples in the paper
would make it an extremely useful one
for purposes of reference. The subject
of earthwork was a very difficult one to
deal with, and he wished to point out
briefly in what this difficulty consisted.
A B C D (Fig. 42) might be taken to be

Fis.42

the section of some ground having a
small vertical cliff at B C. There would
be a tendency for the ground to break
•away and come down along some such
line as D B. The whole problem of the
stability of the ground, both as affecting
the slope of the earth and the pressure
against a retaining wall, depended upon
the accurate determination of the line D

124

B. It was not an exceedingly difficult
matter to determine this line, if the con-
stants of cohesion, friction, and weight
of the ground were known ; and he had
himself dealt with the problem in a
paper communicated to the Institution.'
The mechanical conditions of equi-
librium were very simple ; the force tend-
ing to bring the earth down was the
weight of it ; the forces tending to keep
it from coming down were the friction
along the line D B and the cohesion of
the ground along that line. All those
forces acted according to well-under-
stood laws, and therefore if the con-
stants of weight, cohesion, and friction
of any particular ground were known, it
was not difficult to find out the exact
position of the line D B, and therefore
the pressure on the retaining wall, or the
shape of the slope. The question then
arose, what was the real difficulty of con-
structing tables for practical use with re-
gard to earthwork? Simply this, that
the varieties of ground were infinite in
number and very wide in range, and

125

when that was the case it was quite idle
to think of constructing tables for prac-
tical use. A man having a particular
kind of earth to prescribe for, would not
be able to ascertain by inspection what
the constants of that earth were, and
therefore he would not know where-
abouts in a table to look ; he would have
to determine the constants for himself ;
and if he had to do that he had to do
the whole work, and the tables were of
no use to him. He thought the author
had rather overlooked the enormous
number of conditions of earth when he
contrasted the small number of experi-
ments upon earthwork with the large
number of experiments made with tim-
ber. A piece of oak would give very
nearly the same results for strength,
elasticity, and so on, whether it was
grown in Kent or in Yorkshire; and,
therefore, when a few experiments had
been made upon it, it was not necessary
to repeat them over and over again.
That was not the case with earthwork,
because the conditions 'were so exceed-

126

ingly variable. He exhibited a little
rough machine he had used for testing
earthwork and taking the cohesion of
the ground. The block of wood might
be taken to represent a block of raw
clay taken out of a cutting. There was
a common lever balance, and a couple of
movable cheeks were fitted into chases
cut in the sides of the clay block ; and
the clay having been rammed in a box so
that it could not move, weights were put
in the scale until the head was torn off.
After subtracting the weight of the
piece that was torn off, and measuring
the area of the cross section that was
broken, the constant of cohesion was
determined. For the constant of fric-
tion he arranged a certain number of
blocka of the same clay in a tray, and
scraped them off smooth ; then he had
another block of clay with a smooth sur-
face which he put on it, and then tilted the
tray until the loose block sHd ; that gave
the coeflScient of friction. He should
like to refer to the exceedingly wide
range of tenacity shown by different

127

kinds of clay. In one set of experi-
ments with ordinary brick loam, that
clay gave a coefficient of cohesion of 168
lbs. per square foot, and a coefficient of
friction of 1.16. With some shaley clay
out of a cutting in the Midlands, he had
found a coefficient of tenacity of 800
lbs. per square foot, and a coefficient
of friction of 0.36. That was a very
wide range, but it was only a part of
what was actually to be found in practice.
Mr. L. F- Vkrnon Harcoitrt wished to
say a few words on the subject, as the
author had referred to two or three
works with which he had been connected.
The author had pointed out, from the
experiments he had recorded, that the
pressure upon the back of a retaining
wall was a good deal less than it ijvas
theoretically supposed to be — about one-
half — ^but as he allowed a factor of safe-
ty of 2, it apparently came to very much
the same thing. With regard to walls
on a rubble mound, the author remarked
that the base was in many cases small.
That, he thoaght, was owing to two

128

causes ; first, that with a rubble mound
for a base there was no chance of slid-
ing ; and secondly, that in those cases
there was a rubble filling behind, which
he supposed was about as good a mate-
rial for backing as could be got. The
slope of the inner face of the rubble
mound of the breakwater at ^Llderney
harbor had been referred to as 1 J to 1 ;
but it ought to be remembered that in
that case the materials used were very
large blocks of stone, and therefore the
slope would be naturally steeper than
under more ordinary conditions. Ref-
erence had also been made in the paper
to St. Katharine's breakwater, Jersey, as
an example of a wall built with a very
small base. The author took the whole
of the height of that wall as the proper
height ; but it would be observed that
the top of the wail had what used to be
called a promenade along it, and there-
fore the whole of the filling did not ap-
ply to the entire height of the wall.
The author stated that the base was 28
per cent, of the height of the wall, but

12i)

leaving out the promenade it would be
35 per cent. Of course it would be
something intermediate, as there would
only be the small piece of filling under
the promenade to be taken into account
additional, instead of what would be the
filling at the back if it was filled up en-
tirely to the top level. The author had
referred to the West India dock wall,
and stated that several portions of it had
come forward. That, however, was not
quite the case. It was true that two
portions of the south wall came forward
— that two surfaces of clay at some little
depth below the wall slid upon one an-
other. Probably some seam of sand or
,silt was washed out by the water behind
the wall from between two layers of clay,
and in that way the two detached sur-
faces of clay were free to slide upon one
another. He was quite certain of the
exact position of the surfaces of rup-
turis because he saw the two surfaces of
clay after the excavation was made for re-
building the wall, and they were as sruooth
as glabH. The remedy for tli:it a]^)peared

130

to him to be veiy simple, and it was cer-
tainly successful in the case in point.
The wall had failed, as the author had
stated, not from any fault in the thick-
ness or the weight, but simply owing to
the sliding forward ; and instead of add-
ing any further weight to the wall, the
founjlations were carried down to a
greater depth ; but it only required 2 or
3 feet more in depth in the basin wall
foundations that had to be executed af-
terwards under precisely similar condi-
tions. That was quite sufficient to keep
the wall in a perfect state of equilibrium
without the least coming forward ; and
he should imagine that was decidedly
better on the whole than adding to the
weight of the wall. It appeared to him
that practice was rather contrary to
theory in giving too great a thickness to
the top of the wall, and too small a
thickness, comparatively speaking, to the
bottom ; and that it would be better to
have a wall more of the shape of the
Sheemess wall a good deal lessened at
the top, rather than a wall like those

131

generally adopted, which had more par-
allel faces with a little additional thick-
ness from the batter. He thought it
would be better to make a wall narrower
at the top and widening out more to-
wards the bottom, and to bring the
foundations of the wall well down into
the ground so as to prevent any chance
of sliding. In the case of the West
India dock wall, besides the badness of
the backing, there was a large amount of
water that seemed to percolate from the
Millwall docks, which were filled with
water while the wall was being built, the
docks not having been puddled. It was
clearly shown that that had a considera-
ble eflfect, because the north wall, though
it was built in exactly the same manner,
and though the water of the Export
dock was really nearer, stood perfectly,
as there was not the same amount of
water pressure at the back, owing to the
water being unable to penetrate through
the silted-up bottom of the Export dock.
He considered that the Institution was
much indebted to the author for collect-

132

ing and comparing so many valuable
facts, as, whilst descriptions of particu-
lar works were very useful, it was by
taking a general survey, from time to
time, of the existing state of knowledge,
in any special branch, that definite prog-
ress in engineering science was most
likely to be promoted.

Mr. J. Wolfe Bakry believed the state-
ment was true, that the pressure against
retaining walls did not approach to the
theoretical thrust ; at the same time he
was of opinion that large retaining walls
gave the engineer as much anxiety as any
work he ever undertook. It should be re-
membered that, as a rule, the thrust which
the walls had to bear came against them
when the material of which they were
composed was green, and unless con-
tractors and others were very careful in
strutting the new work, and allowing
plenty of time for the material to get,
there would be a condition of affairs in the
early stages of the wall which would
never arise after the materials were thor-
oughly consolidated. He wished to point

133

out that it was for such reasons most en-
gineers were now getting to realize the
extreme desirability of using cement
as much as possible. The early stages
of engineering works were generally
those in which the greatest risks were
run, and if a slow- setting material
were used, the strains would be exerted
against it in its weakest condition, and
disasters would occur such as would not
happen at a later period. He agreed
with the statement of the author with
regard to the failure of retaining walls.
No doubt, in ninety cases out of a hun-
dred, the failure happened from bad
foundations. The remedy in railway
works was in many cases that shown in
Fig. 17, which practically amounted to
strutting the toe of the wall against the
opposite wall, and so preventing it slid-
ing forward. That was a very simple
arrangement, and resembled in its effect
the strutting of timber, which was gen-
erally carried out as a temporary meas-
ure by a contractor, when, an invert was
going to be put in. If the engineer

134 '

thought that a continuous invert could
be dispensed with, a half measure, which
was often perfectly good, was to adopt
some of these struts— which, in fact,
were a discontinuous invert, as shown in
Fig. 17. In railway walls he thought
engineers were a little too apt not to use
struts above the trains, it being consid-
ered in many cases rather infra dig, to
strut a wall. He could not see why it
should be so. The horizontal strains ex-
erted against the retaining wall about 14
feet or 15 feet high above its base were
small ; the struts consequently involved
a very small expense, but they prevented
all possibility of movement, and they
saved a large amount in the cost of the
wall. Having had something to do with
the Metropolitan District railway, he
could thoroughly corroborate the state-
ment of the author, that the walls had
stood remarkably well. As far as he
knew, there was no sign of failure or in-
cipient failure in any of them. There
was, however, one little miitter he had
noticed, viz., that in many of the walla

136

there was a small angular crack across
the external angles of the piers. He did
not know how the cracks had originated,
but he thought they might be due to the
action of frost ; the comers getting sat-
urated, the frost attacked the brickwork
at the angles and broke them off. If so,
it rather pointed out that in such walls
it might be desirable to round the angles,
or have angular bricks and avoid the
sharp corners.

Mr. W. B. Lewis said the experience
gained in the construction of the Under-
ground railway was so large that the
profession naturally looked for the opin-
ions of some of those who were con-
cerned in it ; and they all felt grateful
for the fullness and ability with which
those opinions had been expressed in the
paper. He thought the paper was open
to this reflection ; that, whereas, the
author in the earlier pages discredited
the theoretical views that generally pre-
vailed respecting retaining walls, in the
latter part he stated that his practice
had pretty well accorded with them.

For instance, he gave the theoretical
thickness for a retaining wall in ground
that naturally stood at a slope of 1^ to
1 as 31 per cent, of the height ; and in
the last paragraph but one he said his
habit had been to make his walls ^, or 33
per cent.; and in the Table with slopes
from 1 to 1 to 4 to 1, which included all
that engineers usually had to deal with,
his theoretical thickness ran from 0.239
to 0.451, while in the concluding parar
graphs of the paper he stated that the
engineer must work between the limits
of J the thickness and i, which seemed
to agree with the theoretical thickness.
The general conclusion that engineers
must work between J and ^ was differ-
ent from the practice in which Mr. Lewis
had been trained, and he had therefore
brought a diagram (Fig. 43) of a retain-
ing wall constructed according to Mr.
Brunei's rules. Of course Mr. Brunei,
who had to carry out very great works,
modified his rules to suit the circum-
stances; but the diagram represented
his standard section of wall such as was

137

constructed at Lord Hill's land in the
early days of the Great Western railway,
and at the Britain Ferry docks two years
before his death. It would be seen that

Bcale,10 feet =1 inch.

the dimensions and peculiarities of that
wall differed very much from those given
in the paper. In the first place the wall
had an average batter of 1 in 5, and at
the top a batter of 1 in 10. Batter

138

was a point on which Mr. Brunei al-
ways insisted, and Mr. Lewis was a little
surprised that the author seemed to treat
it with so much indifference. He was
evidently aware of its value, because in
the early part of the paper he mentioned
a wall with a batter of 1 in 5, and a
thickness of 1 foot, which he said was
equivalent to a vertical wall of 1 foot 9
inches. Now anything that was equiva-
lent to an increase of the original value
of 73 per cent, was well worthy of con-
sideration. Mr. Brunei's custom was \o
curve the face of the wall. The radius
was 150 feet in the case of a 30 feet wall,
or five times the height. The thickness
was ^ to J- the height. The counterforts
were 2 feet 6 inches thick, and placed 10
feet apart from center to center, but
were omitted in good clay cuttings. In
the case of docks sometimes there was
a difficulty, in consequence of the neces-
sity of having the top more upright,
and at Britain Ferry docks the radius
was reduced by nearly one-half. Mr.
Brunei, too, was in ihe habit of building

139

behind what he called sailing courses
and the projections in Fig. 43 were 1
foot 3 inches. In the case of embank-
ments the wall was supported by earth
carefully punned against it and against
the sailing courses, thereby adding con-
siderably to the weight that had to be
overturned when pressure came from be-
hind. Then his rule for thickness was
■J^, which was below the minimum given
by the author. There were a number of
such walls at Paddington, Bath, Ply-
mouth, Briton Ferry, 30 feet high and &
feet thick, and generally of nearly the
same thickness at the top as at the bot-
tom. Another point Mr. Brunei was
particular about was that the footings
yrere made square to the batter, and
when the ground was not good consider-
ably larger footings were introduced.
At Briton Ferry a 2-feet lining of con-
crete was employed at some places for
watertightness. Concrete was not then
in such general use as it was at present.
Of course when exceptional ground was
met with it was dealt with exceptionally.

140

At a tunnel on the Wilts, Somerset, and
Weymouth railway, some heavy ground
had been found ; the tunnel mouth was
in a 60-feet cutting, a retaining wall 30
feet high was built, and the top was
sloped back at f to 1, with a 2-feet cov-
ering of masonry, and the wall was built
precisely of the dimensions represented by
Fig. 43 ; but as fche ground was heavy, the
batter, instead of being 1 in 5, was 1 in
4, and that was the only alteration. That
wall was built in 1854, had never given
any trouble, and was standing at the pres-
ent moment. It seemed to him that Mr.
Brunei, forty years ago, came nearer to
the teaching of the experiments and of
the reasoning in the paper, than the au-
thor had ventured to do in his own prac«
tice.

Mr. J. B. Redman observed that the
author had undoubtedly filled a void id
the literature of engineering; for, not-
withstanding the great experience that
most of the members of the Institution
had of such catastrophes as those which
had been referred to, it was only human

like that they had not been often record-
ed by the designers of the works. Those
who constituted what was now a select
minority of the Institution would re-
member the partial failures of Mr. Rob-
ert Stephenson's retaining walls in the
Euston cutting of what was then the
London and Birmingham, and now the
London and North-Western railway.
Those partial failures were met by over-
head horizontal girder struts supporting
the walls, and it was rather curious that,
notwithstanding all the experience that
had been since gained, in a large number
of instances, in metropolitan railways,
the overhead girder had been, as it were,
the natural sequence of what might be
termed the unretaining wall. There was
one circumstance which very much com-
plicated the question of the direct lateral
thrust of earthwork upon a retaining
wall, and which rather curiously had not
been mentioned by the author. It was
incidentally referred to in the latter part
of the paper where the author said
French engineers, in designing a wall at

142

Marseilles, made the width of the base
58 per cent, of the vertical height, in
consequence of the dip of the strata be-
ing towards the wall. In a large num-
ber of cases of the failures of retaining
walls in open cuttings near London, he
thought it would be found that the fail-
ure was entirely on one side. Where
the dip of the strata was towards the
cutting, and more especially if there
were laminaB of clay, the superimposed
strata often struck near the base of the
wall ; and a retaining wall on that side
not only had to support the normal lat-
eral thrust of the mass of earthwork im-
mediately behind, but it had also a long
wedge-like piece of earth impinging
against the earth at the back of the wall,
so that in many cases the thrust on the
wall at the one side must be something
like double the amount that it was on
the other; because on the other side, the
dip being away from the wall, the wall
was subject only to the lateral thrust of
the earthwork in its rear. The author
had stated that the failures of many

143

dock walls did not illustrate entirely the
ordinary lateral thrust of earthwork; but
Mr. Kedman thought that such cases as
the failure of the walls constructed by
the late Mr. G. P. Bidder, Past-Presi-
dent Inst. C.E., at the Blackwall entrance
to the Victoria docks, the partial failure
of the same engineer's walls in the en-
largement of the Surrey Docks, the simi-
lar catastrophe at the Victoria dock, Hull,
in the work designed by the late Mr. ^
John Hartley, and possibly also a similar
movement in the South West India Dock
wall, were all clearly attributable to lat-
eral thrust. It might be said that the
foundation was not taken down deep
enough, and consequently the wall did
not resist that thrust ; but having had a
somewhat extended and varied experi-
ence for a great number of years, he
certainly was not prepared to indorse the
dogma that a dock wall or a river wall
must necessarily be so strong as to resist
a head of water, or in width at the base
equal to one-half the height. In the
first place, the water ought not to be al-

144

lowed to come behind the wall. There
were exceptional cases, perhaps, where
that could hardly, be avoided; but it
seemed to him that laying down such a
tenet was a premium for loose engineer- '
ing, imperfect supervision, and lavish
expenditure. He had himself, in the
lower reaches of the Thames, erected
some of the heaviest embankment walls
on the river, where the thickness was
only J of the vertical height. It was true
' that the walls were founded on the best
possible foundation — Thames ballast —
and it was done as tide work ; and the
greatest possible care was also taken to
keep the backing up to the same level as
the wall, and indeed rather above the
wall. In fact, the great mistake in re-
taining walls was the imperfect supervi-
sion exercised over the backing. If the
backing were put in with tolerably fair
material in thin horizontal layers and
brought up in that way, the lateral
thrust was reduced to a very small mat-
ter. The author had stated that the de-
cayed timber wharves on the Thames and

145

in other neigbborhoods showed that the
lateral thrust must be over-estimated;
but it should be remarked that the skin
might be stripped off the face of the
earthwork, assuming that no water was
coming against it, and it would stand,
because from the length of time and
consolidation of material, there was no
lateral thrust. The example quoted of
the breakwater at St. Katherine's, Jer-
sey, appeared to be a case in point. He
had nothing to do with the inception or
execution of that work; but he thought
the wall might be taken down and the
lieart of the pier would still stand. He
^would refer to two great Metropolitan
failures which were well known, and
-which might be interesting in illustra-
tion of this subject. One was that of
Greenwich Pier and the other of the
Island Lead Works. The Greenwich
Pier was constructed nearly half a cen-
tury ago from the design of a local archi-
tect, Mr. Martyr. It was one of the
heaviest embankments on the Thames;
it had the greatest depth of water up to

146

it, and it was, being in the hollow of the
reach, subject to every condition of
weather. The base was formed by cast-
iron piling and cast-iron sheeting be-
tween, constituting a half-tide dam, and
concrete was got in behind. Upon the
top of the concrete there were large 6-inch
York landings and a very solid, heavy
brick wall. There were also outer piles,
and the work was constructed in the best
possible way. The case was somewhat
complicated by the fact that a large
amount of land-water came down and a
large amount of spring water. There
was a common sewer running through
the heart of the work, and a large tidal
reservoir for the Ship Hotel.* The whole
of that work, with the exception of the
two returns and quoins and a small por-
tion in front of the Ship Hotel, slipped
into the river during the night some
forty years back. The late Mr. Chad-
wick, who built the Hungerford suspen-
sion bri4ge, entered into a contract to
restore the work on his own plan, acting
as engineer and contractor, and he re-

147

stored the portion that had failed with
timber-bearing piles and a solidly-con-
structed brick wall. Shortly after the
demise of Mr. Chadwick, the restored
portion showed signs of failure, and Mr.
Redman was called in by the Pier Direct-
orate, and the matter resulted in a law-
suit, and a large simi of money was ob-
tained in compensation. All he did was
to bleed the pier by inserting a cast-
iron pipe with a self-acting flap at the
eastern end, and to remove and sub-
stitute with better material some part of
the backing. He proposed driving land-
tie piles at the back and some in front ;
but on consideration with the Director-
ate, it was thought that driving piles
might be a ticklish operation. That- was
twenty years ago, and up to the present
time the work had remained in the same
state. It had settled somewhat at the
eastern end, and there were reopened
fissures in front, so that the movement
had not altogether ceased. The wall of
the Island Lead Works designed by the
late Mr. R. Sibley, M. Inst. C.E., was the

148

pioneer of cast-iron wharfing; and from
the fact of the Limehouse cut having
been deepened too close np to it, the
wall failed. As the author had said, that
case did not illustrate the absolute lat-
eral pressure of earthwork, because this
work, as long as it was not meddled with,
stood satisfactorily. The leaseholders
called in Mr. Eedman on that occasion,
and the freeholder consulted Mr. Bate-
man, Past-President Inst. C.E., and the
late Mr. N. Beardmore, M. Inst. C.E.,
Tery wisely — to avoid a lawsuit — con-
structed a wall deeper down, to their
satisfaction.

Mr. W. Atkinson agreed with Mr.
Lewis's remarks with respect to the
large amount of masonry or brickwork
that the author had introduced in the
cases of the metropolitan railways. He
had been much struck with the propor-
tion of J of the height for the mean
thickness of a wall ; but looking at the
diagrams, and taking into consideration
what he had seen oi the work, there was
a very good explanation. It struck him

149

that on the Metropolitan railway, where
property was so valuable, the batter
which the late Mr. Brunei introduced of
1 in 5 would be extremely inconvenient ;
either the roadway, would have to be
narrowed, or a great deal more property
would have to be taken, than would be
otherwise necessary. No doubt the
author would be able to say whether that
had any influence in the carrying out of
the work. Then with regard to the gen-
eral question of the walls and their fail-
ure due to bad foundations, it struck
him that the two things should be en-
tirely separate; that the foundation
should be treated as a foundation, and
that having been made sufficiently strong,
a properly proportioned wall should be
placed upon it. He rather gathered
from the paper that the two points had
been taken as a whole, and that the au-
thor meant, *'I have a bad foundation,
and I will make the whole to stand." If
that were so, it would have been better
policy to have made a foundation of con-
crete, and then put a wall sufficiently

160

strong. With regard to the question of
theoretical calculation, there was a
French formula which agreed remark-
ably with what might be considered the
ordinary practice. He himself had put
up a good many walls, not perhaps as
distinct retaining walls, but in connec-
tion with bridges on 48 miles of the Mid
Wales railway, and he had found practi-
^ cally that the ^ of the height for the
mean thickness stood perfectly well. In
that case, it was to be borne in mind that
there were two elements in addition to
the theoretical calculation, namely, the
projection of the footings where there
was so much leverage, which was not
taken into account in the calculation,
and the weight of the earth resting on
the projections or stoppings at the back
of the wall, Fig. 44. That, of course,
aided the wall very materially ; in fact, it
might be called so much masonry Wved.
At all events, if merely the theoretical
thickness of the wall was given, then,
with the projections of the footings, and
the weight of earth on the steppings.

161

there was a very good margin of safety ;
and in that way the wall was erected
with ^, or 33 per cent, less than the
dimension advocated by the author, and

Bcale J6 feet = 1 inch.

was a good and sufficient wall. One
point with regard to walls was brought
to his notice when in Canada, namely,
the thickening of the top to resist frost.
In ordinary circumstances the practice

152

would be to put about 2 feet at the top,
and then about 9 feet down a projection
of 9 inches, and so on ; but in Canada,
on account of the penetration of the
frost, it had been found necessary to
make the top of the wall much thicker
tban was the practice in England.

Mr. H. Law desired to add his testi-
mony to the great value of the facts laid
before the Institution. It was upon such
facts, the result of actual experience, that
the most valuable data were formed. In
the early part of the paper the author
had pointed out that the formula usually
►adopted — Coulomb's — did not give the
results which were obtained when loosely
heaped materials were placed at the back
of the wall; but a little consideration
would show that that formula never was
intended to apply to such cases. Cou-
lomb's theorem distinctly took into ac-
count the adhesiveness or coherence of
the ground, and then determined, de-
pending upon the Hne on which the
ground separated, what the amount of
pressure would be; and the value to the

153

engineer was, that it determined what
was the maximum which that pressure
could be. Putting ?o=the weight of a
cubic foot of the soil in lbs., h = the
height of the wall in feet, r = the limit-
ing angle of resistance of the soil, 8 =
the angle between the line at which the
soil separated and the horizontal, and P
= the horizontal pressure in lbs. of the
soil against the wall, then Coulomb's
theorem might be thus expressed:

P=-^r- . cot5. tan(s— r).

Now in the case of a fluid, r, or the
limiting angle of resistance, vanished,
and consequently the result was that the
co-tangent of s into the tangent of 8
became equal to unity, and

2 •

When the ground was sufficiently co-
herent to stand vertically, then the angle
of separation being 90° the co-tangent
of $ became nothing, and the pressure
became nothing. When the line of

154:

separation coincided with the limiting
angle of resistance or r, that was to say,
when there was a mass of earth suffi-
ciently coherent not to break of itself,
and lying upon a bed which happened to
be at the limiting angle of resistance, the
tendency of the earth to slide was exact-
ly overcome by its friction, and r being
equal to «, the tangent vanished, and P
again became nothing. Now, between
those two values there was a certain
angle at which, if the ground separated,
it would produce the maximum pressure,
and that was given by Coulomb's theorem,
which proved that when the line of sep-
aration bisected the angle made by the
limiting angle of resistance with the
vertical, then cot«=tan(«— r), and

P = -jr-.COt«,

and the maximum pressure was obtained.
The great value of the formula was to
show, with a given weight of earth and a
given limiting angle of resistance, what
the maximum pressure was. It could

155

not exceed the value expressed by making
8 half the angle between the limiting
angle of resistance and the verticaJ. «This
>formala could not be applied in the case
of loose materials, as sand and gravel,
because it was impossible for such ma-
terials to stand at any other than than
their limiting angle of resistance; and
under such circumstances there would be
upon the wall only a comparatively small
pressure, due to the unbalanced weight
which remained from the efforts of the
sand and the gravel to roll down upon
itself. He wished to direct attention to
one or two interesting exemplifications of
excessive pressure which were met with
in the works for the Thames tunnel. The
Eotherhithe shaft, 50 feet in diameter,
was built upon the surface and sunk by
excavating beneath. That operation was
successful until a depth of 40 feet was
reached, and then, although the exterior
surface had been made perfectly smooth
by being rendered, it became earth-
bound, and notwithstanding the earth
w as excavated to a depth of 2 feet round

156

the whole margin, and 50,000 bricks were
placed upon the top as a load, making the
total weight 1,100 tons, and water was
allowed to rise inside, the shaft refused
to sink any farther. Now, taking the
weight of the ground at 120 lbs. per
cubic foot, which was about what it was
on the average, and taking the coefficient
of friction at 67, it would be found that
a limiting angle of resistance of about
31° 15', and a line of fracture of about .
27° 30', would show, by Coulomb's
theorem, that the shaft would be bound,
and therefore the practical result was
quite in accordance with the pressure
given by the formula. The author had
mentioned a case of some heavy clay
which had a pressure equivalent to a
fluid pressure of 107 lbs., and if that clay
was taken as having a limiting angle of
resistance of about 5° or 1 in 10, and the
weight was assumed to be 130 lbs. per
cubic foot — which clay of that descrip-
tion might very well have — the formula
would give 107 lbs. for the fluid pressure.
He therefore thought these circumstances

157

fully showed that where ground was co-
herent and adhesive, Coulomb's theorem
applied. In the progress of the Thames
tunnel there had been some remarkable
cases of excessive pressure, where of
course the weight of the water was super-
added to that of the ground. He knew
many instances of poling boards, 3 feet
in length, 6 inches wide, and 3 inches
thick, sui^ported by two poling screws
bearing against cast iron plates, being
split lengthwise by the pressure of the
earth against the outer surface.

Mr. E. A. Bernays said the inconsis-
tencies alluded to in the paper tended to
make it still more interesting than it
otherwise would have been. There were
few engineers who had carried out
works, but were conscious of inconsist-
encies in their own practice and theories.
The author had quoted M. Voisin Bey,
the distinguished French engineer, as
saying that he had rarely seen a long
wall straight, and Mr. Bernays* expe-
rience fully confirmed that view. When
it was straight the chances were there

158

was a superabundance of material to keep
it so. If it was run fine, as the calcula-
tions advised, the chances were 50 to 1
against having a straight wall. With re-
gard to Mr. Brunei's section of wall, no
doubt if it had a good foundation it was
very strong for the material in it. It
not only had a rising abutment to bring
the pressure down upon the foundation,
but it had counterforts, which added
greatly to the strength of the wall,
although of late they had gone out of
fashion. He considered it was nearer 10
feet at the base than 5 feet, as, if the
counterforts were 10 feet apart, the wall
was, practically, a solid wall. If made of
concrete instead of brickwork, it would
probably be found better to make it solid
at once. The batter added consider-
ably to the strength, but it was not
without practical disadvantages. The
greater the batter the greater the disad-
vantage. The tendency of the batter was
to throw the side of a vessel farther away
from the wall than need be, and to entail
cranes with longer jibs, as well as the use

159

of much larger fenders. Iron ships were
now all covered with anti-fouling com-
position, which might easily be scraped
off. With all its disadvantages he would
rather have a smaller batter for pmctical
purposes when ships were to lie along-
side the wall. He had seen a wall of
this section in Woolwich Dockyard (built,
he believed, by Sir John Rennie, Past-
President Inst. C.E.), partially pulled
down and refaced by the late Mr. James
Walker, Past-President Inst. O.E., for
the purpose of deepening the dock. It was
about 30 feet deep, and was increased to
about 38 feet by putting a thin wall in front
of it. In pulling down such walls he had
always found that the backing in settling
hung upon the set off, and he had seen
holes under the backing large enough for
a man to creep in. He would not say
that they were objectionable in other
respects, but he preferred a battered
back to a retaining wall to square sets
off. The author had alluded to a wall
that he was building, and had character-
ized it as *'exceptionably heavy.*' But

160

for that expression he would have been
quite content to sit still : he hoped to be
able to show that the exceptionable
heaviness was justified by the exceptional
circumstances under which it was being
built. He did not think much of experi-
ments with peas and pea-gravel, and bits
of board a foot square when he had to
deal with big walls. The author stated
(p. 47) that " experience has shown that
a wall ^ of the height in thickness, and
flattering 1 inch or 2 inches per foot on
the face, possesses suflicient stability
when the backing and foundation are both
favorable." Unfortunately for dock en-
gineers it rarely happened that either
the foundation or the backing was
favorable, and it was still rarer to find
both favorable. This fact made the in-
consistencies that really showed the
thoughtful way in which the paper had
been written. There was no attempt to
square theory with practice; but the
author had candidly pointed out where
theory broke down in referring to the
retaining walls of the Metropolitan Rail

161

way, and at the approach to the Euston
Station, and in other instances. He agreed
with the author that the Sheerness river
wall had perhaps a greater moment of sta-
bility than any other wall in the world.
The section assumed that the pile foun-
dation would stand, though he doubted
its stability ; but if it would stand, half
the thickness of the wall would have been
ample. He did not know sufficient of
the nature of the subsoil at Sheerness to
be able to decide the point. He had been
told that in many cases the piles were
40 feet long ; and a few years ago, when
a new caisson was put in the basin at the
yard, there was great fear lest it would
come forward when the water was let
out. That was merely an instance of the
cases where provision must be made for
very different calculations from those
which were set out in any table. He
quite agreed with the author that no
calculations would meet cases where the
work was exceptionally difficult. In most
instances, engineers were called upon to
make docks and other great works in the

162

worst kinds of soils, such as estuaries,
beds of rivers, or in deep alluvial de-
posits. The reason that he strengthened
the wall at Chatham was because the
original design showed symptoms of
weakness, and several of the walls
^ yielded about 10 or 12 inches. He did
not say that that was entirely the fault
of the walls, because the foundation was
far from satisfactory, and there was a
decided forward movement of the piles ;
but it was evident that the wall, for the
greater part of the area, was not at any
rate too otrong for the work. When,
however, he came to the east end of the
works, where he had to build a wall
1,050 feet long without a single break,
and with 35 feet depth of soft mud to ex-
cavate through, it was absolutely neces-
sary to strengthen the wall, and it was
decided to build it entirely of concrete,
in order to be able to give the additional
strength without additional cost. He
was asked some years ago what angle
this mud would assume at rest, and the
answer he gave was that it would not lie

163

flat. The basin at Chatham was being
built in an old arm of the river Medway,
and the basin generally stood in the
middle of the river bed. On each side of
it the mud was 35 feet deep on the aver-
age, and in some cases the distance to be
filled in with backing was 600 feet. The
whole of that backing had to be laid on
this sliding mud, which brought pressure
on the wall in a way far beyond anything
he had ever seen allowed for in any cal-
culation. If he understood correctly,
Mr Giles had thought it necessary to
provide, not only for the backing, but
for a pressure of water nearly as high as
the water in the dock. He did not agree
with Mr. Redman that it was possible to
get the walls built up so as to prevent
water percolating. At Chatham there
was a standing level of the water in the
district, and wherever excavations were
made to that depth water was found.
The bottom of the basin was 20 or 25 feet
below that level, and the water exerted a
pressure just as if there was an ocean of
that depth behind it. Then it was some-

164

times necessary to put heavy baildings,
as at the Victoria Dock Extension; where
large sheds loaded with heavy goods
were placed from 100 to 150 feet from the
wall. No one would say that such sheds
would not exercise a great pressure on the
adjacent wall. He would be happy to
show the author the wall at the Chatham
Dockyard Extension, and abide by that
gentleman's judgment, whether " the ex-
ceptionally heavy wall " was not neces
sary to meet the peculiar conditions of
the case.

Mr. A. Giles, after what Mr. Bemays
had said about the pressure of mud be-
hind dock walls, thought he was quite
justified in adhering to the assertion he
made many years ago, that a dock wall
ought to be strong enough to carry a
head of water behind it equal to its
height. He cordially joined in thanking
the author for the paper, but he consid-
ered it would have been better described
as " On the Stability of Eetaining Walls."
The author had given many examples of
dock and retaining walls, but after

165

throwing over the theoretical calculation
as to the pressure of earth against a wall,
he said that in ninety-nine cases out of a
hundred walls failed from faulty founda-
tions, and not from want of strength in
themselves. The various diagrams af-
forded rather congratulatory evidence of
his own theory, that practically all the
thick walls had stood, and most of the
thin walls had given way. Referring to
the old Southampton dock wall, mention-
ed as having been built 40 years ago, that
had only a thickness of 32 per cent, at
the base, but with the counterforts it was
35 per cent. That wall had been pushed
forward, but it never came down ; but it
was saved by taking out the wet soil at
the top and covering the top by a timber
platform. Another wall which he had
built had been referred to. That had a
thickness of 45 per cent, at the base, and
an average thickness of 41 per cent.
Surely that wall ought to be strong
enough to resist not only the pressure
of water behind it, but even the pressure
of mud that would not stand at a level.

166

It had not stood without moving — ^not
from any want of strength in the wall,
but simply from the want of adhesion in
tiie foundation. At Whitehaven the
thickness of the wall was 37 per cent, at
the base, and the mean thickness was 31
per cent., and it had stood. At Avon-
mouth these values were respectively 59
per cent, and 42 per cent. There was a
very fine example at Carlingf ord of a wall
with the base only 32 per cent, thick, and
a mean thickness of 24 per cent; but
what could be said about a wall at Sheer-
ness with a height of 40 feet and a base
of 43 feet? It was stated that that wall
had not moved, and Mr. Bemays had con-
tended that it ought not to move ; but
he did not think any engineer of the pres-
ent day would dare to design, or con-
template building, a wall of that char-
acter, because a wall of similar height in
ordinary ground could be built for £60 a
yard, while that wall would cost £300 a
yard. In many instances it was not the
inherent weakness of the walls that
caused them to fall, but the slip at the

167

bottom, and that was shown in Fig. 17 by
the necessity which arose for thickening
the wall so as to make the strength as 62
to 24. After all, the wall required still
further strengthening ^by putting but-
tresses in front of it. The conviction
he had arrived at was, that it was not
generally the fault of the wall that caused
the failure ; but tlfe fault of the founda-
tion—not only that the foundation was
not wide enough to give sufficient hold
on the ground, but that there was not
sufficient footing in front of the wall to
enable the soil upon which the wall rested
to sustain the weight. It was the same
as if a cliflf, 30 or 40 feet high, were put
on tender soil. The soil would not be
strong enough to bear it, and conse-
quently the edge of the cliff would settle
into the soil, the soil would burst up in
front, and the pressure from behind would
then make itself felt He had seen that
process take place in a wall which he had
constructed, and it was only saved by
putting, buttresses in front of the foot-
ings. Something had been said in the

168

paper about allowing a margin for con-
tingencies. In that matter every engi-
neer must decide for himself; but he
thought that from i to ^ was rather a
large margin, and he would suggest that
the thickness of a retaining wall ^ of the
height would be, in nine cases cut of ten,
ample to resist the b^kward pressure ;
but he would insist upon haying a large
buttress in front of the foundation, car-
ried down as deep as the lowest founda-
tions of the wall. He was at a loss to
imagine what the extraordinary projec-
tion in front of the wall at Marseilles
(Fig. 25) meant. It might be that it was
intended to hold the bottom down ; and
it was in that direction he would recom-
mend retaining walls should be strength-
ened. A remark had been made about
the necessity of having the upper part of
dock walls nearly perpendicular, because
of the friction of ships rubbing against
them, and the inconvenience of ships
lying at some distance from the quay at
the coping level. That was perfectly
true, and he believed it had been a com>

169

mon practice, in designing walls where
there was a curved batter, to make the
center of the ctirve level with the coping,
by which a certain depth of almost per-
pendicular work was obtained from the
coping level. He believed that was the
correct principle ; but he would urge par-
ticularly that, in making dock walls, the
foundation should be much wider than
they were in general, and that the bulk
of the buttress should be in front of the
face of the wall, and not behind. In all
walls the excavation at the bottom should
be carried down perpendicularly, with as
little disturbance of the soil as possible ;
because in excavating the work, it was
better to fill up the void so made, that there
should be no tendency to slip after the
wall was put in. There was another
point which he thought was not suffi-
ciently considered by engineers in de-
signing dock walls. They were apt, when
the excavation had been carried out, to
think that they had got a good founda-
tion ; but he cordially agreed with the
author when he used the word " lubricat-

170

ing." Notwithstanding what Mr. Red-
man had said, he did not think it was
possible to keep water from getting be-
hind a dock wall : he believed there was
a point at which water would always be
found : it would get up from the bottom
or through the wall somewhere ; and that
being so, he thought that all the soil
upon which the wall stood must be sod-
dened and lubricated to a certain extent.
He knew of instances where walls had
stood for many years ; but all at once the
moment of lubrication had arrived, and
they slipped in. He could only account for
it by supposing that there was a tendency
on the part of walls to get surrounded
with water, by which they became of less
specific gravity, or that the soil got satu-
rated, and therefore less able to bear the
load put upon it. He would therefore
urge upon all his professional brethren
who had the conduct of dock works to
look particularly to the front of the walls
to ensure their stability.

Mr. W. R. BousFiELD desired to make
one or two remarks, from the theoretical

171

standpoint which the author had depre-
cated. He referred to a point in which
theory and practice would agree, viz., as
to the effect of water behind a retaining
wall. If there was an interstice of even
an inch the effect on the wall would be
exactly the same as if the whole ocean
were behind it; therefore, a dock wall
should be made to withstand a pressure
equal to the hydrostatic pressure due to
a head of water of the height of the wall.
He wished to ask if the author could ex-
plain, somewhat more at length, the
effect of lateral pressure in General Bur-
goyne's experiments, for he did not think
the remarks were quite 'sound. If the
lateral pressure of the ground, consist-
ing, say, of loose rubble, was greater
than the hydrostatic pressure, the fact of
water being admitted would not make
the slightest difference, because the
water pressed equally on the earth and
on the wall in opposite directions, so
that the earth would be kept back by the
pressure, and the difference between the
lateral pressure and the hydrostatic

172

pressure would be exerted by the earth
on the wall. The only effect would be
in the distribution of the pressure, which
instead of being taken by the points of
stone alone, would be distributed by the
water over the whole wall. If the lateral
pressure of the soil was less than the
hydrostatic pressure, then of course, if
water was admitted behind, it would ex-
ert upon the soil a force greater than the
pressure of the soil on the wall ; there-
fore, supposing the soil were rigid, the
lateral pressure of the soil would be
kept entirely off the wall. Of course, in
practice, there were many points at which
the pressure was excessive, so that, on
the whole, the maximum pressure to be
provided for would generally be rather
more than the hydrostatic pressure.

Mr. E. Benedict described a retaining
wall (Fig. 45) lately put up at Kyde. The
ground was sidelong and at the foot of a
clay hill, the strata dipping towards the
work, and with a heavy building close to
it. By cutting a trench and filling it as
soon as possible with sohd concrete in '

173

Portland cement carried up to the sur-
face, the clay, which weathered rapidly
when exposed to the air, was covered

without delay, the concrete became ag-
glomerated with the clay at the back,
and did not allow any percolation of
water. The excavation in front of the

174

wall then proceeded without any move-
ment of the ground occurring, and he
thought that none would take place.
Eventually a covered way was formed on
the lower side of the wall, the arch of
which was designed so as to form a con-
tinuous lying buttress.

Mr. B. Baker, in reply, observed that
he agreed, to some extent, with almost
everything that had been said in the dis-
cussion, and he considered that the criti-
cism had been very fair. He was glad
indeed that he had elicited so many valu-
able opinions on the subject. Mr. Airy
spoke about the difference in the cohesion
of different clays. He had noticed the
same thing himself, not merely in differ-
ent clays but in the same clay. A rail-
way cutting often refused to stand at a
less slope than 4 to 1, and yet the same
clay, after being tempered a little, might
be found in an adjoining brick kiln
standing with a vertical face. He had
nothing to say with regard to Mr. Ver-
non Harcourt's comments, except that he
agreed with almost everything that gentle-

175

man had said. Mr. Barry had made some
sensible remarks about the advantage of
using struts to retainiQg walls, and he
thought it would be a good thuig, in
many cases, to imitate the old architects
of cathedrals, and substitute flying
buttresses for a heavy mass of materials
Some time ago he designed some vei^y
cheap sheds upon that principle, in
which the roofs were light concrete
arches supported by flying buttresses.
Mr. Barry had referred to the cracks
at the angles of some of the piers
of the Metropolitan District railway re-
taining walls. He was satisfied that
these did not arise from pressure, but
from chemical action, because they oc-
curred only in the case of certain bricks,
and he knew where the bricks came from,
and had every reason to mistrust them.
He had sometimes found scaling occur all
over the face of a wall, though of course
the angle was always the weakest point,
and nature always tried to round off an
angle, as might be seen in Cleopatra's
Needle, where there was no square angle.

176

Mr. Lewis had described a wall designed
by the late Mr. Brunei, but he did not
approve of it, for reasons that had been
set forth by Mr. Bemays. He himself
had found exactly the same thing, name-
ly, that in pulling down work where
there had been the slightest settlement,
the earth at the back did not rest on the
offsets, indeed, not infrequently, a man
could push in his arm between the oflfset
and the filling. It was therefore idle to
maintain, as Professor Rankine and oth-
ers did, that the earthwork resting on
the offset was as good as so much mason-
ry. There was no economy in putting
in the offsets, and he attributed the sta-
bility of apparently light walls so con-
structed to the pressure of the counter-
forts and the good quality of the back-
ng. Mr. Bedman had directed attention
to the fact that there was an increased
thrust when the ground at the back was
sloping. No doubt that was so ; and a
case of that sort was referred to in the
discussion on Mr. Constable's paper at
the American Society of Engineers,

177

where the ground at the back was slop-
ing rock. When the wall was first put
up and the backing was filled in, the
whole mass came forward in consequence
of the wedge of earth sliding down the
surface of the rock. The masonry was
pulled down, the rock cut in steps, and
the wall rebuilt of the same thickness.
Pig iron to the extent of 55 tons to the
lineal foot was then placed behind the
wall, and it stood perfectly well, though
the thickness was less than 30 per cent,
of the height. That was sufficient evi-
dence of the importance of stepping the
ground at the back. Mr. Atkinson said
he thought it would be better to make
the foundation satifactory first and then
to build a thin wall on the top of it. At
page 43 of the paper that point was al-
luded to and the answer given. Mr
Law had submitted a formula, and drawn
deductions from it, which he could not
follow; but it seemed to him that the
contention was that a loose material ex-
erted less thrust on the wall than a more
compact material, and that Coulomb's

178

theory was not applicable to loose soil.
He did not agree with that view in theory,
and Lieutenant Hope's experiments
ghowed that that was not so in practice,
at least on a small scale. Lieutenant
Hope placed a board behind the pressure
board at such an angle as to include
Coulomb's wedge of maximum thrust
between the two boards, and found that
the lateral pressure was quite as much
when the board was at the slope of re-
pose, 1^ to 1, as when it was at half the
angle. There was hardly any diiference
whether the board was horizontal or at a
slope of i to 1, or at any intermediate
slope. Then it had been remarked with
regard to one of his examples, in which
the stability of the wall was equal to the
fluid pressure of 107 lbs. per cubic foot,
that theory would indicate the pressure
to be about that amount ; but a state-
ment in the paper did not seem to have
been noticed, that the wall never had
that pressure on it, but failed by sliding
forward. Of course it might have slid
forward with a pressure of 40 lbs- per

179

cubic foot, but since the struts had
been put in, there was not the slightest
indication of movement, and therefore
the moment of stability could not have
been deficient. Mr. Bemays seemed to
imagine that the expression " excessively
heavy" reflected on the design of the
Chatham dock wall, but the intention
was the reverse. He entirely approved
of it, and considered that it was a well
designed and creditable engineering
work in every respect. It was one that
he should imitate. His contention
throughout the paper was that formulae
did not apply to such works, and al-
though he began the paper with a dia-
gram he set it up merely in order to
knock it over. Mr. Giles considered
that instead of the limits of i to ^ of
the height for the thickness of a retain-
ing wall, J should be the limit with a
buttress in front of the toe ; but he did
not think that that was the practice
which had been followed in the South-
ampton Dock Extension, where the limit,
he believed, was nearer i than J — 45 per

180

cent. The curious slope projection in
Fig. 25 was really an apron to protect
the foundation, which was of clay. The
clay was very hard when laid bare, and a
sort of shield was put there to prevent
its softening. He believed the same
thing had been recommended by a com-
mittee of engineers in the case of the
Belfast dock, where the wall failed ; but
it was applied too late, or the conditions
were different, because the wall came for-
ward notwithstanding.

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