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Great land-slides on the Canadian Pacific Railway in British Columbia Stanton, Robert B. 1898

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Array 7
^AH*^
STANTON
ON
LAND-SLIDES IN BRITISH COLUMBIA.
I  '■ THE
GREAT LAND-SLIDES
ON THE
CANADIAN   PACIFIC   RAILWAY
IN
BRITISH COLUMBIA.
BY
ROBERT BREWSTER STANTON, M.A., M. Inst. CE.
WITH AN ABSTRACT OF THE DISCUSSION  UPON THE PAPER.
EDITED  BY
J. H. T. TUDSBERY, D.Sc, M. Inst. C.E.,
SECRETARY.
By permission of the Council.
Excerpt Minutes of Proceedings of The Institution of Civil Engineers.
Vol. cxxxii.    Session 1897-98.    Part ii.
LONDON:
PubligfjeB 65 tfje Institution,
GREAT GEORGE STREET, WESTMINSTER, S.W.
[Tbxegeams, " Institution, London."   Telephone, " Westminstkb, 51."
1898.
[The right of Publication and of Translation is reserved^ ADVERTISEMENT.
The Institution as a body is not responsible either for the statements
made, or for the opinions expressed, in the following pages.
LONDON: FAINTED BY WM.  CLOWES AND SONS,  LIMITED, STAMFORD STREET  AND CHARING  CROSS. TILE INSTITUTION  OF  CIVIL ENGINEERS.
Sect. I.—MINUTES OF PROCEEDINGS.
14 December, 1897.
Sir JOHN WOLFE  BARRY, K.C.B., LL.D., F.R.S., President,
in the Chair.
(Papet No. 3074.)
I The Great Land-Slides on the Canadian Pacific Railway in
British Columbia."
By Robert Brewster Stanton, M.A., M. Inst. C.E.*
The great land-slides which have caused so much trouble and
expense in working the Canadian Pacific Railway, since the
opening of the line in 1885, occur on the banks of the Thompson
River, about 41 miles above its junction with the Fraser River and
197 miles east from Vancouver, the western terminus of the
railway, Fig. 1, Plate 1. Within a distance of somewhat over
5 miles there are seven large land-slides, all of the same nature,
and six crossing the railway line, as well as smaller slips, Fig. 2.
About 20 miles further down the river, at a point opposite Spence's
Bridge, there is a similar large slide.
The railway occupies the east and south-east bank of the river,
at an elevation between 50 feet and 80 feet above low-water level,
and follows closely, with one exception, the contour of the river
bank. At about 200 miles from Vancouver the railway passes
through the Black Canon Tunnel. This portion of the Thompson
River, for a distance of about 20 miles, traverses a gorge about
5 miles wide at the top, and about 2,000 feet deep, with hills and
higher ranges rising back on each side to elevations between
5,000 feet and 7,000 feet. In the middle of this gorge the river
runs in an inner gorge, with-sides between 50 feet and 150 feet
above low-water level and close to the river. There is but
little bottom land near the river. The surface rises from the
water's edge in benches and terraces, varying in height between
30 feet and 200 feet, and extending to a general altitude of
about 1,800 feet, or about 1,000 feet above the river.    The land
b 2
J 4       STANTON ON  LAND-SLIDES IN BRITISH  COLUMBIA.      [Minutes of
rises beyond in broken slopes interspersed with hills and peaks.
The terraces and slopes are cut by many dry gulches and small'
streams draining into the river. The largest of the benches, and
the lowest flat in this neighbourhood is occupied by the town of
Ashcroft.
The terraces of these greater valleys are cultivated for raising
winter food for the cattle that range the higher valleys and the
open timber of the lower hills, and for raising garden, field and
orchard crops for local consumption.1 Artificial irrigation is
necessary, there being no natural growth in the lower valleys,
such as those of the Thompson and the Fraser Rivers, except
bunch grass (agropyrum tenerum) and a few single trees, with
stunted bushes. Systems of irrigation have therefore been established by bringing the small mountain streams on to these tracts.
The water of the river, running at the bottom of a somewhat
deep gorge, is not available; so the smaller streams and lakes of
the adjacent mountains are used as the sources of supply, in some
instances supplemented by artificial lakes or storage reservoirs.
The water is carried on to the land by small rudely-constructed
ditches, built almost entirely by the farmers who occupy the land.
The Black Canon of the Thompson River is a narrow gorge about
1 mile in length, where the river has cut its way through an
uplifted ridge of black shale, which was raised in the bottom and
parallel with the course of the valley. On its eastern side there
are two hard sandstone points next to the river. The railway
passes through one of these by a short tunnel, and through the
northerly one by an open cutting, exposing the greenish hard sandstone rock and the position and dip of the overlying shale. The
two greatest slides are situated, one north, the other south of these
points, Fig. 3, Plate 1. At both places the country originally
sloped up from the river in a series of benches or terraces to
the first line of hills. The south slide has an extreme length
of 1,880 feet along the railway, and an extreme width back from
the river of 1,575 feet. It is of somewhat irregular fOrm, with a
semi-circular outline at the back, and covers an area of 66 acres.
The north slide has a maximum width at its widest portion of
nearly \ mile, and a length back from the river approaching
| mile, with the same semi-circular back line. It is of irregular
form, and extends over an area of 155 acres. The height of the
first bench next to the river, in both cases, was originally about
1 " British Columbia: its Present Besources and Future Possibilities," published by direction of the Provincial Government at Victoria, B.C. Proceedings.]     STANTON  ON LAND-SLIDES IN BRITISH  COLUMBIA.     5
80 feet above low-water level. The land then rose in successive
levels to a height, on the south slide, of 400 feet to the bench at
the top, or back edge, where the cave-down broke off the solid
ground, and in the case of the north slide it extended to the
third higher bench 500 feet above the river. It is impossible to
ascertain at what depth these enormous masses of earth and loose
rock broke, or in other words, the depth of the plane on which
the mass moved towards the river; but it is estimated that at the
back edge of the south slide the break fell almost vertically for a
distance of over 300 feet, and on the north slide perhaps over
400 feet.
The terraces on each side of the valley of the Thompson River
along this section consist of the soil on the top of each bench of
light sandy loam to the depth of between 1 foot and 8 feet.
Below, in places, is found between 3 feet and 10 feet of clean
coarse river sand. .Next occurs loose and nearly clean stratified'
gravel and boulders, and below this a partially cemented gravel
with larger boulders. The material which holds together the
gravel and stones of this formation is boulder clay, a porous
arenaceous clay silt, through which water passes freely, yet which,
in a dry state will stand in vertical walls to a considerable height.
It extends to a greater depth on the higher terraces; in places
it is perhaps 500 feet deep. The boulder clay is here found in
two forms: in its original form as first laid down, and, especially
upon the lower benches next to the river, in a secondary or
re-arranged form. Under the lower benches, particularly under
the slips, there is a deposit of silt or imperfect clay, which shows
in places to a depth of between 50 feet and 200 feet. It is the
same silt that forms and binds the boulder clay, but is entirely
free from gravel or boulders. These have been named the white
silt deposits. " They are generally fine and uniform in texture,
and are usually well bedded in perfectly horizontal layers from
£ inch to 4 inches in thickness," with occasional sandy seams and
small pockets of coarse sand, formed locally, appearing in places.
By the continued application of large quantities of irrigation
water upon the cultivated fields above, and upon the upper portions
of what are now the slides, almost the entire surplus not absorbed
by the plants or evaporated, sank down freely through the loose
soil, sand and gravel; and while not as readily, yet with considerable ease, through the boulder clay, and reached the underlying
silt. After some years this water saturated the argillaceous silt
and converted it into the form of river mud of about the consistency of thick pea-soup.    Long before the whole mass, or even 6        STANTON  ON LAND-SLIDES IN  BRITISH  COLUMBIA.      [Minutes of
a very large part of it, reached a state of perfect saturation, the
silt would lose its power of sustaining weight.    In the two places
here referred to, on account of the peculiar topographical and
geological contour of the country, the water applied at the back
was concentrated, comparatively speaking, into  one channel of
descent (in each place) to the body of silt below, and thence
it penetrated in every direction.   The process of saturation required
many years to produce any results, for if a considerable quantity
of the silt had become saturated to the point at which it would
lose all cohesion, it would not move, on account of there being so
great a distance to any point of outlet, together with the self-supporting power of the boulder clay in its confined position, which
was nearly all absolutely dry over the slip; hence a large extent
of the underlying silt became more or less saturated before it
could find an outlet in any direction, even with a considerable
weight upon it in its more or less semi-liquid state.    Finally,
when a large body of the silt had become saturated to such an
extent that it could not sustain even its own weight, except in its
confined position, and  the limit  of resistance, possibly in the
form of an arch, of the boulder clay had been reached, the great
mass of earth and boulders above—in the case of the south slide,
estimated as weighing some 32,000,000 tons, and of the north
slide approaching  100,000,000  tons — the whole mass  dropped
almost vertically, while the immense tracts of broken and mixed
material  seeking an outlet force'd  their lower sides out on the
line  of  least resistance and found  their way into the river.
This action is distinctly shown by the almost vertical walls in
the boulder clay along the outline of these two slides.    While
at  their foot there is  now a talus slope of crumbled material,
these  walls stand vertically to  a height  of between .50 feet
and 200 feet, more clearly shown in the north slide, where the
vertical cliffs of boulder clay, and in places of the silt itself,
extend round the whole slide for a distance of over l£ mile.    It
is also shown by the present position of large sections of the
original surface of the highest bench, which broke off at  the
line of the back wall, and which now stand in the sunken mass
at an angle of about 45°, with their former level surfaces tilted
back away from the river.     The back edge thus dropped first
and lower than the portion some distance in front of it.    In
dropping and pushing out towards the river, the whole tract was
broken  into sections by great cracks, whioh still  exist.     The
larger cracks run parallel with the river and at right-angles to
the line of movement, while other and smaller cracks run in Proceedings.]     STANTON  ON LAND-SLIDES IN  BRITISH  COLUMBIA.     7
every direotion, cutting the whole into blocks of boulder clay and
dry silt.
Both these larger slides, together with some others of the kind,
occurred before the railway was built. Others have also occurred
in entirely new places since that time. On the land above the
south slide (and on most of the others also) irrigation has continued since the railway has been working, and this has kept up a
continual movement of the south slide, with a number of others,
towards the river. This movement is much more marked for
500 feet or 600 feet in the centre of the slide, where the water
seems now to be concentrated. Thus the railway is being continually pushed out into the river. At times the road-bed has sunk
4 feet and has moved out twice that distance in a night, so that
a constant rebuilding of the line has been necessary for the last
10 years. The movement is continuous, though it is greatest in
the months of July, August and September. An extra force
of section men, watchmen, construction trains, &c, are continually
required on this section. At the south slide alone over £10,000
has been spent on such work. This section of 5 miles or 6 miles
of slides has cost the Canadian Pacific Railway Company directly
and indirectly £100,000, in keeping up a safe road-bed, and in
other necessary expenses. At one point a train-load of tea was
thrown into the river, and completely lost, by the sudden movement of a portion of the road-bed, caused by an extra amount
of water being put upon an already saturated field. The most
careful guard by extra watchmen is kept on this part of the road,
and trains run over it at only 6 miles an hour. A few years ago,
after the watchmen had passed over the line, and an east-bound
train had also passed over it in safety, a west-bound train oa^ne
suddenly upon a section of the line sunken out of sight. The
train fell into the river and the engine-driver was killed. This
also was caused by the excessive irrigation of a small field above
the line.
The great north slide happened in October, 1881. Irrigation
had been carried on above it for some years, and some time before
the final catastrophe occurred, a reservoir 2 miles distant in the
hills from which the irrigation water came, broke its dam, and
most of the water liberated spread over the upper benches of this
land, already well soaked. The whole tract of 150 acres sank
vertically in one movement to a depth, at the back edge, of over
400 feet. The lower portion, about 2,000 feet wide, was forced
entirely across the river, a distance of 800 feet to 1,000 feet;
and corning against the steep bluff on the opposite side, it filled 8        STANTON ON LAND-SLIDES IN BRITISH  COLUMBIA.   . [Minutes of
the whole inner gorge of the valley, and formed a dam fully
160 feet high, completely stopping the flow of the river for
several days, so that men walked dry-shod across the river-bed
below the dam. The Thompson River carries in the autumn
15,000 cubic feet to 20,000 cubic feet of water per second. The
dam formed a lake over 12 miles in length, which is roughly
estimated to have contained 7,000 million cubic feet of water,
Fig. 2, Plate 1. As soon as the water rose above this dam of loose
broken material it was swept away, and caused a terrific flood in
the valley below.
All the arable land above the north slide was carried down by
the first break, and hence irrigation was stopped at that point.
After some years the water drained out so completely that the
movement practically ceased and the land became again stable.
The railway crosses this slide 500 feet or 600 feet back from the
river, and since the water has drained out, little trouble has been
experienced, for the land has settled firm and dry. At the south
slide, irrigation being maintained, there has been a continuous
movement ever since the railway was built across it. The central
portion has advanced toward the river about 800 feet since it
first fell. As the material is forced forward the river washes it
away, a new road-bed is built further back, and the line is moved
inland to it. This new bed gradually sinks and moves forward,
and the line must be continuously raised and moved in, to keep a
safe passage across the broken and moving ground. Such work is
in progress to a greater or less extent over the whole series of
some 5 miles or 6 miles of slide. The expense and difficulty are
so great that, to use the words of the general superintendent
of the road, it has become " almost impossible to work the
railway."
At Spence's Bridge, some miles down the river, a slide occurred
from the same cause on the west side of the river. It crossed^entirely
over the stream and deposited a number of acres of land on the
east side of a bench 25 feet above the river, between it and the
railway, covering an Indian burying-ground.
At one time the tract of land on which the town of Ashcroft
now stands began to move towards the river. The Dominion
Government bought out the farmers who were irrigating above
the town ; the irrigation ceasing, the movement soon stopped, and
the town has since remained in the position it then occupied.
Physical Geography.—The Province of British Columbia, extending from lat. 49° N. to lat. 60° N., forms the northern extension of
the Rocky Mountain region of North America.    The two principal Proceedings.]     STANTON  ON LAND-SLIDES IN BRITISH COLUMBIA.     9
mountain systems are the Rocky Mountains proper and the Coast
Ranges. These form the north-eastern and south-western boundaries
of an interior section of the province. | Between these two limiting systems are found, on the side next to the Rocky Mountains,
several less regular though often equally lofty mountain ranges,
which may be collectively referred to under the name of the Gold
Ranges, being west of the Selkirk Range, and between these and
the inner margin of the Coast Ranges lies the region called the
Interior Plateau." |
The Interior Plateau, across the southern part of British
Columbia, is nearly. 200 miles wide. It has an average width of
about 100 miles, with a length from south-east to north-west of
about 500 miles. The general level declines towards the north
and west, having an average elevation in its southern portions of
4,500 feet, sinking as stated to the north-west to a much lower
level, and having an average height above the sea of about 3,500
feet. Only in contrast to the lofty and rugged mountains which
border it, can this interior region be properly called a plateau.
" More carefully examined, it is found to consist, particularly in
its southern part, of numerous blocks of plateau-like contour,
separated by important depressions, and differing considerably
in their actual elevations. But there is every reason to believe
that, in the early Tertiary period, the area of the interior
plateau had become reduced by prolonged denudation to the condition of the nearly uniform plain. . . . Though never absolutely
flat, the surface of the country thus became an approximate
plain or what is called a ' peneplane.' It appears further that the
peneplane at this time formed has never since been entirely
obliterated, although it has passed through several stages of
elevation and depression, and has been subjected to more or
less deformation due to earth-movements. At certain periods it
has been an area of deposition of strata, and the theatre of great
volcanic eruptions. At times the natural forces of waste have
been engaged in reducing the superadded irregularities toward
the old plain ; but during the latter part of the Pliocene Tertiary
period, with the country standing at an elevation higher relatively
to the sea than at present, the greatest erosive changes, tending to.
the destruction of the ancient plateau, occurred. At this time
the great valleys by which the interior plateau is now conspicuously trenched—those of the Fraser  River, the Thompson
1 " Geological Survey of Canada," vol. vii. part B.    Annual Report, G. M.
Dawson, C.M.G., L.L.D., F.E.S., Director. 10     STANTON ON LAND-SLIDES IN BRITISH COLUMBIA.      [Minutes of
River and their main tributaries—were cut." 1 Thus, considering
its origin and former condition, the present appearance of this
interior plateau, when seen from a sufficient elevation, is that of a
number of high plateau areas, which run together everywhere in
the distance, shutting out from view the great lower valleys; and
while these areas differ considerably in elevation, they do not so
differ materially as between, adjacent parts. Even with the more
marked irregularities on its west or south-western side, this interior section may be considered in general as a plateau, and
particularly as a country peculiar to itself, not only as to its
origin and partial present form; but also with reference to its
general climatic characteristics, its vegetation, rainfall, &c.
Climatic Conditions.—Situated with its western border within
100 miles of the Pacific coast, where the rainfall at Vancouver
attains 62 inches, and at other points even exceeds 150 inches per
annum, the average rainfall of the larger part of the interior
plateau is only 6 inches to 12 inches per annum. The Coast
Ranges form an almost complete barrier between the Pacific and
the inner country, so that the moisture-bearing winds from the
ocean become desiccated before they reach the latter region. " A
study of these conditions will show how it happens that the
lower valleys and those nearest the Coast Ranges form the most
arid part of the district, while the humidity of many of the
higher parts of the plateau, particularly those situated to the
eastward, is at the same time still considerable, enabling them to
be the sources of perennial streams which may be employed in the
irrigation of the lower and drier tracts." The climate of many
parts of this section seems out of relation to the geographical
position of this far-north country. It is largely governed by the
close proximity of these mountain ranges. The difference of
temperature during the year, and even during the day, is very
great. In the valley of the Thompson River it ranges from 120°
in the summer to, at times, 50° below zero in winter.
Larger portions of the higher hills and valleys are covered
with dense forests, including among their trees the black pine
(Pinus Murrayana), white spruce (Picea Engelmanni), Douglas fir
(Pseudotsuga Douglasii), and balsam spruce (Abies subalpina). The
mountains range between 8,000 feet and 9,000 feet in height.
The timber limit is about 7,000 feet. Below this, for some
distance, the hills and valleys are but sparsely wooded on account
of the extreme depth  and long duration of the winter snows.
" Geological Survey of Canada."   Annual Reports, vol. vii. part B. Proceedings.]    STANTON  ON LAND-SLIDES  IN  BRITISH  COLUMBIA.    11
Below the denser forests, the valleys are but partially covered
with trees, for the opposite reason—the want of sufficient moisture;
while the lower and larger valleys, up to an elevation of about
3,000 feet, are almost entirely devoid of trees on account of the
extreme dryness of the climate. The lower valleys with their
slopes and terraces only produce naturally a few shrubs and
grasses, the most abundant being the bunch grass (Agropyrum
tenerum).
Geological Formation.—The geological formation of the section
within which the land-slides have occurred embraces the cretaceous
formation of the Ashcroft area, and the triassio formation lying
immediately south of that, with a small portion of the carboniferous
formation lying still further south. In considering the real nature
and causes of the slides, the solid geology does not have so important
a bearing on the matter as the present position and condition of
the superficial or drift deposits due to the glaciers, which now
partially cover, and at one time largely covered, the interior
country. In the case, however, of the two largest slides at the
Black Canon, which have already been described, the underlying
rocks (at that point cretaceous) are important in establishing a
correct estimate of these causes.
The Glacial Drift.—The most important and extensive deposits
of drift are the boulder clay and the white silts. The former,
which in many places resembles a partially cemented gravel,
consists of a paste of hard sandy clay silt, with a large amount
of sand or coarser silt; through it, in close proximity to one
another, gravel, stone and boulders are found, from the finest
pebbles up to boulders between 2 feet and 3 feet in diameter.
Some are glacial, marked and polished, but the larger part are
shaped by ordinary water action. The stones and boulders in
places have come from one locality, and in others they are of
varied origin. It is impossible to give the general thickness of
this deposit, though it is evident that in some low portions, and
in the valleys, it attains considerable thickness, and, on the other
hand, that the higher parts of the districts were never covered
with this deposit. The greater part of the boulder clay deposited
in the larger valleys was removed at a later period by denudation.
The white silts were deposited at a subsequent time. They
reached an extreme elevation of 2,500 feet above the present sea-
level, while the highest levels of separate deposits of these silts
are found at other distinct levels, in different localities, down
to 1,700 feet, showing possibly an uplifting of parts of the country
during  their depression.    The  extent of  this silt deposit was 12     STANTON ON LAND-SLIDES IN BRITISH COLUMBIA.      [Minutes of
at first very great, but much of it has been removed, as was the
case with the boulder clay, by denudation.
The western part of Canada was covered by the great Cordillean
glacier,1 which attained a maximum length of nearly 1,200 miles,
and over the higher tracts of the interior plateau of British
Columbia reached a general thickness of between 2,000 feet and
3,000 feet, while in the main river valleys (cut out during the
Pliocene Tertiary period) it must have been 6,000 feet in thickness.
Daring the maximum of this glacier, the whole region stood at a
much higher elevation than at present. Eventually there came
a subsidence of the mountain region, and a retreat of the Cordillean
glacier. During the retreat of the great ice-sheet lakes were
probably formed on its surface, which increased in size as the
subsidence progressed, and after their final drainage, and the
further retreat of the great glacier, the interior plateau was
covered by gradually deepening waters which were connected
with the ocean. It was during this retreat that the boulder clay
of this section is supposed to have been formed. The next great
change was a re-elevation of the whole territory and the forming
of the higher terraces of British Columbia. During this elevation
much, and in some parts all, of the boulder clay deposit was
washed out of the larger river valleys, such as those of the Thompson and Fraser, as the rivers for a second time cut a channel for
the waters draining from the lakes and glaciers, and pouring out
into the ocean now below their level. Later, in consequence of
this elevation, the country was again covered to a considerable
extent by glaciers, though these were local in character. Then
occurred a second subsidence, less than the first, but which lowered
the level of the region about 2,500 feet below the present one.
At this time, with glaciers of considerable size occupying the
mountain valleys, the land remained nearly stationary for a long
interval, and remarkable and important silt deposits, well bedded
and of considerable thickness, were tranquilly laid down in different
low tracts, scattered along the Cordillean region for a length of
some 1,200 miles. These have been termed the white silts. The
final retreat of the glaciers was not connected with subsidence, but
was either during, or soon followed by, a general elevation of the
region, and is supposed to have been chiefly due to a general
amelioration of the climate, the cause of which does not seem clear.
Following this elevation the present large rivers cut for'a third
time a channel for themselves, this time principally in the white
G. M. Dawson. Proceedings.]    STANTON  ON  LAND-SLIDES IN BRITISH   COLUMBIA.    13
silts, which had been deposited in their gorges and over the
remaining boulder clay. In some places the silt has been completely washed out of the original gorges, while in others it still
covers the whole floor of the valley. In most places it remains
only on the sides of the valleys, and stands in terraces along and
far above the river. These terraces of silt, next to the large
rivers, in places stand to a considerable height with only a thin
soil above them. Others farther back are covered by a heavy
deposit of re-arranged or secondary boulder clay, and again, still
farther up, are found large sections of silt over the original
deposit of boulder clay, these in turn being covered by the
secondary, re-arranged boulder clay.
Local Conditions.—The various actions during the glacial periods,
as related above, are almost entirely responsible for the condition
of that portion of the Thompson River valley in the neighbourhood
of the Black) Canon. The great gorge cut during the Pliocene
Tertiary was at first filled by the boulder clay. This is found at
the elevation of nearly 2,000 feet above the present river, and is
shown in cuts of the railway close to the water. When the river
cut its channel through the boulder clay, large masses of this
material were left on the east side of the river, both above and
below the Black Canon. The boulder clay here found is of two
kinds. The first deposit as first laid down, and the secondary or
re-arranged deposit as formed from the original by the action
of the river at its various heights, and also by the submersion and
re-elevation during the laying down of the white silts.
From a careful examination of this section it would seem that
possibly the ancient pliocene river ran, when at its lowest levels,
to the east of the present Black Canon, and east of the two
prominent points noted above, and that, when cutting its second
channel through the boulder clay, the current was thrown to
the west, cutting its way through the shattered shales west of
the two higher and hard sandstone points, leaving a great mass
of boulder clay to the east or back of them. This action of
the river would form, both north and south of the Black Canon,
great bays washed out from the boulder clay. These bays were
subsequently filled with silt, and the river, cutting its third
channel through the silt, followed in a more direct line to and
from the Black Canon. These are the conditions found at these
two points, great bays of white silt extending in the case of
the south slide, 1,600 feet back from the river into the original
mass of boulder clay, and covered by the loose gravel and secondary
or re-arranged boulder clay.    In the case of the north slide, this r
14     STANTON  ON LAND-SLIDES IN BRITISH  COLUMBIA.      [Minutes of
bay of silt extends back nearly f mile, while no silt, at low
elevations, is found at all across the whole section of the valley at
the point between them, that is, east of the Black Canon tunnel.
The silt deposit in these two bays accounts clearly for the peculiar
form and action of these two great slides, and for the immense
vertical drop in each case.
While the white silts were being deposited, there was much rearrangement of the boulder clay. At two points, one on the
south side of the north slide, and the other on an exposed section
of Nelson's Creek, are found deposits of silt on the original
boulder clay of the higher benches, and entirely separated from
the larger masses of silt below, and also covered to a considerable
depth with the re-arranged boulder clay during the latest
elevation of the territory.
Causes of the Slides.
I. Natural Precipitation upon the Lands in and around the Slides
themselves.—The natural precipitation upon these lands would not
exceed 6 inches per annum, and could have no effect upon the clay
or silt lying between 100 feet and 400 feet below the surface of
the higher benches. Even thousands of years of such precipitation could not reach the underlying stratum of silt. On a nearly
level plain, a few miles north-west of the city of Denver, in the
State of Colorado, where the surface drainage was not nearly so
perfect as on the benches of the Thompson River, and where the
precipitation is much greater than at that point, in sinking a shaft
through fine sandy soil for the purpose of opening up a coal mine,
it was found that for some 50 feet down to the rock the soil was
absolutely dry and flew into dust and floated in the air when
struck with a pick.
Natural Surface Drainage from the Watershed above the Land.—
Barns's Creek and Nelson's Creek, together with the depression
between them in which lie the natural lake and artificial
reservoir, designated in Fig. 4, Plate 1, Barnes Lake, cut the
tracts off completely from the mountain region to the east.
On the slopes above and immediately east of the north branch
of Nelson's Creek springs show upon the surface above a
ledge of bed rock running north and south near to the surface—
and at some points exposed—along the ridge next east of the
creek. No such springs or any evidence of drainage water show
to the west of this north branch of Nelson's Creek.    This north Proceedings.]    STANTON  ON LAND-SLIDES IN BRITISH  COLUMBIA.    15
branch, which is cut deep into the gravel and rock, and also a
smaller drainage system which abuts against and runs directly
north from the source of this same north branch of Nelson's
Creek, are, even in the driest season, as continual running
streams, in places on clean rock. These natural drainage systems
carry off all surface water from the mountain water-shed, and
also any that may flow through the soil from the springs above,
and being thus cut off it would in no way affect the lands lying
below and to the west of these drainage channels.
Natural Drainage by Subterranean Streams.—(Fig. 4, Plate 1.)
" This (Ashcroft) area is about 4 miles in average width, with a
length of about 11 miles, and for the greater portion of this length
the Thompson River follows its axial line. . . . No fossils have
been found in any part of this area, but its lithological identity
with the cretaceous formation of adjacent parts of the Fraser valley
is sufficient to fix its cretaceous age. The rocks consist of sandstones, conglomerates, and dark shales, the shales here apparently
predominating in the upper part of the series, and occupying in
the main the central part of the area, while the sandstones and
conglomerates are more abundant and characteristic in the lower
parts. . . . The sandstones are usually greenish or greenish grey
in colour, being largely composed of debris of the underlying
diabases and felspathic rocks, and seldom or never purely siliceous. . . . The shales are blackish, and their sombre outcrop
along the Thompson has given its name to the Black Canon. . . .
The rocks are nearly everywhere much disturbed and crushed,
and no satisfactory general section has been obtained of them.
Nearly all of the observed dips are to the westward, but as a rule
those on the eastern side of the middle line of the area are
comparatively low, ranging from 10° to 40°, while those on the
western side are often at angles of 60° and from that to vertical.
It is probable that the structure of the area as a whole is that of a
syncline, of which the western limb has been more or less completely overturned by pressure acting from the west." 1 One of
these minor folds is the black shale ridge through which the
Thompson River has cut its way and has formed the Black Canon.
This fold here lies on the eastern limb of the general syncline, its
trend being north and south, and having its anticlinal axis on the
east side of the river and just east of the railroad tunnel and
cutting.    This shale ridge is somewhat over  1  mile in length
1 " Geological Survey of Canada," Annual Report, part B, vol. vii. pp. 154
and 155. 16     STANTON  ON  LAND-SLIDES  IN  BRITISH   COLUMBIA.   .   [Minutes of
where the river is cut through it. Its abrupt termination is
shown at both ends on the west side of the canon where its wall
terminates abruptly in the boulder-clay deposit, and for that
distance and to the height of 100 feet and more is composed
entirely of black shale, dipping abruptly to the west, and at an
angle much greater than the general dip of the eastern limb of the
main syncline.
On the east side of the river, and forming the eastern side or
wall of this little canon are two lesser folds of this one " minor
fold." The anticlinal axis of one is at the point through which
the tunnel runs and the other at the centre of the open cutting
just north of the tunnel. The direction of these axes are nearly
east and west. The hard greenish sandstone rock under the
black shale is raised and exposed in the tunnel, and cut some
50 feet above the railway line, and in both cases it terminates
abruptly close to the eastern wall of the canon. On the top
of these points is a stratum, 20 feet to 30 feet thick, of the
black shale (on the wall of the canon the shale shows over
100 feet thick), dipping over the points very abruptly into
the river, and folding down north and south round these points,
with also some dips showing as inclined to the east away from the
river.
These facts and the additional evidence that immediately east
of the tunnel (Fig. 3, Plate 1) is the break of the south slide
and east of the open cutting is the break of the north slide, and
several hundred feet below the top level of these two elevated
points no rock appears, but simply a mass of boulder clay and
silt, led the Author to conclude that it is most likely that
the ancient pliocene river ran, when at its lowest levels, to the
east of the present Black Canon, and east of the two points
here described, and that when cutting its second channel through
the boulder clay, the current was thrown to the west, cutting its
way through the shattered shales, and leaving such "a mass of
boulder clay as is formed east of the canon. This ancient channel
was but a short distance east of the canon, for on this cross-section
east of the two slides, about 1 mile from the river, the rocks are
exposed in place and dipping again to the west.
The fact that the rocks are nearly everywhere much disturbed
and crushed ; the numerous indications of faulting; the volcanic
nature of many parts of the section, these volcanic rocks showing
both north and south of this point; the minor and lesser folds in
the immediate neighbourhood, with a possible older and lower
channel of the river, in which the great masses of silt and boulder 1
Proceedings.]   STANTON ON LAND-SLIDES IN BRITISH COLUMBIA.    17
clay which compose these two slides were deposited, and from
which channel there would be an outlet lower down into the
present river, would seem to entirely set aside any possibility
of underground streams upon or in the seams of the rock reaching
the silt and causing the slides in the manner in which they
occurred. Leaving out of consideration any possible older or
lower eastern channel, it cannot be ass'umed that at the two
particular places where the slides occur the underlying rock strata
are uniformly inclined and unbroken, and that over them or
through their seams the water reached the silts and caused the
slides; for it must be at once asked why this action did not occur
centuries ago.
Effect of Surface-Streams Sunning by the Land.—This effect is
intimately connected with the questions coming under the fifth
and last division of the subject, and can only be partially treated
as a separate   item.     Two streams — Nelson's Creek and   the
Thompson River—run by, and come in partial contact with, the
south slide.    In the case of the former, the present stream runs in
a deep gulch, partly (the north branch) in a trench cut down
through  the  boulder clay,  where  the   banks   stand  in  nearly
vertical walls, and further down the  stream it flows on clean
bedrock, and still lower over a bed formed on the debris of the
boulder clay and silt.      Even in the driest season this is  a
continual running stream fed by springs.    At its upper end on
the north branch, these springs come in from the east side, while
the west side is absolutely dry.    In its lower section they come
in from both sides, but more especially from the irrigated land,
which is much higher than the bed of the stream, as there is
a section of unbroken land lying between Nelson's Creek and the
slide itself, the only way this stream could affect, or could have
been the cause, or partial cause of the slide, would be by soaking
through this mass and into the underlying silt.    By two careful
measurements  of the  water flowing in   this  stream,  made in
November, 1896, one being at a little fall on bedrock above the
slide, and the other at the railway bridge, just before the creek
enters the rivers, it was shown that there was 18* 6 per cent, more
water running in the stream at the lower point of measurement
than at the upper, these points being about 1 mile apart;  and
further, that 90 per cent, of this increase came from the side next
to the slide and from under the present irrigated fields—a result
entirely opposite  from what would occur if this stream were
supplying the water   to flow into the slide and thus  keeping
up the present action.   It was also shown that at least 95 per cent.
[THE .INST. CE. VOL. CXXXII.] C r
18     STANTON ON  LAND-SLIDES IN BRITISH COLUMBIA.      [Minutes of
of the water running into Nelson's Creek during that dry season
was irrigation water from the upper fields, and that the surplus
water applied upon the fields and orchards to the east of the north
branch did not to any extent sweep down under the creek, but
came out into the stream and was thus carried to the river.
The Thompson River flows along and against the foot or toe of
all the slides, and in th£rr present condition, that of a broken and
moving mass, gradually pushing their way into the river, has
a more or less marked effect upon them. At the south slide, especially during high water, the action of the river is most marked.
The condition of the material, as described above, cut in every
direction by huge cracks and being forced into the stream in
detached and broken blocks of boulder clay and dry silt, invites
the water to enter the cracks and seams. Thus getting in behind
and around these blocks it readily melts the argillaceous silt, and
the rapid current washes the material away. The river, even
during high water, carries away only that portion that is delivered
to it in this crushed condition by the force of the continually
moving slide. It would be impossible, within any reasonable
limit, to place any protection from the river upon the toe of
this slide. The force from the millions of tons above is practically
irresistible.
That the river, running as it does at the foot of these slides, has
in no degree been the original cause of any one of them is clearly
shown by the fact that there are hundreds of places close by and
between the several slides, and at other points, where the banks of
the river are walls of boulder clay and even of the clean unprotected silt, which stand in almost, and in some cases in actual,
vertical cliffs between 10 feet and 100 feet in height, against which
the river has run for many centuries without any material effect,
and in no case causing slides where no irrigation has been done
above them (in places small cave-downs have occurred); while, on
the other hand, in every instance along the Thompson (and in
■other sections also) where irrigation has been practised above
such a formation of boulder clay and silt, after a few years of
application of the water more or less great land-slides have
occurred.
Artificial Irrigation of the Lands Upon and Above the Slides.—By
the testimony of Indians who had lived in the country all their
lives and that of some of the original white settlers, no slides had
ever occurred at any point along the Thompson River before the
white men began to irrigate the land, although one or two small
cave-downs existed before that time.   In every instance noted, these Proceedings.]   STANTON ON LAND-SLIDES IN BRITISH COLUMBIA.    19
slides occurred between 3 years and 6 years after irrigation began
at each point. In the case of the largest one, the great north
slide, the final catastrophe was hastened by the bursting of the
reservoir. A very large amount of water was necessary for raising
crops, on account of the sandy nature of the soil and the nature of
the subsoil. The topography of the several benches assisted
materially toward the final result. Each field being in the form
of a shallow basin, around which the irrigation ditches were built,
little of the surplus water was drained off, hence the greater part
of that not taken up by the plants or evaporation, ran towards the
centre of the field and soaked down in one channel.
As to the action of the water upon the peculiar masses of silt
which at present underlie the benches and terraces along the
Thompson River, a number of curious facts were noted in and
around the south slide, which at first seemed most difficult to
explain. The silt or imperfect clayr which lies in masses between
200 feet and 1,000 feet in thickness, is generally fine and of
uniform texture, and usually well bedded in horizontal layers of
from j- inch to 3 inches or 4 inches thick. In its natural state it
is hard and dry, like a soft sandstone, and, when held between
the fingers and struck with a light hammer, rings like stone. A
large piece of this- silt, however, placed in a basin of water dissolves after a few minutes and falls down, not in a lump as clay,
but it mingles with the water, forming a semi-fluid mass like
thick pea-soup. The same soft mixture was observed oozing
out at many points along the foot of the slide, forced out by
pressure from above; so the question arose, how it was that
this silt stood in vertical walls between 10 feet and 100 feet in
height along the Thompson River and Nelson Creek, with the
waters of these streams running along and against their base,
and at high water some distance up them, and yet they had stood
for ages, and were but little injured, except by slight atmospheric
disintegration ?
The silt is formed of three parts—silica in the form of coarse-
and fine sand, and alumina in two forms, first disintegrated felspar,
simply mechanically separated into grains, both coarse and fine also
in the form of sand, and constituting a large part of the mass, and
second, decomposed felspar or plastic clay. Under the action of
running water, the sand, both the silica and the disintegrated
felspar, is washed out, while the decomposed felspar is precipitated,
and forms a coating of true plastic clay upon the mass, which soon
becomes impervious to the water and is practically indestructible,
thus protecting the underlying silt from further action of the
C 2 r
20     STANTON ON LAND-SLIDES IN BRITISH COLUMBIA.     [Minutes of
water. The result of a chemical examination of the material will
be found in an Appendix. Mr. Warsap, the analyst, also suggests
the chemical action and assistance of the carbonic acid of the
atmosphere and ammonia, if present in the clay and the lime, in
forming this impervious coating. The mechanical action of separation and precipitation noted above accounts for all the peculiar
phenomena observed at every point in and around the slides.
The great quantity of irrigation water soaked downwards into
the mass of silt. It would absorb 53 per cent, of water without
changing its form, yet with only about 35 per cent, it would be
incapable of sustaining any great weight except in its confined
position. After the final breakdown, and its release into the
river with the continual application of water, and still being
under pressure, this semi-liquid silt, containing all its original
constituents, is forced out at the foot of the slide in great quantities. If a man steps on this ooze he is liable to sink into it,
while within only a few feet of such a spot lay, when examined
by the Author, a large block of the same silt which had fallen
over into the river in a dry state, and over which the last season's
high water had run ; it stood up 3 feet or 4 feet above the
level of the river beach, and the Author walked and jumped
upon it without making any impression. Breaking off a piece
of this block, it was found that, less than 1 inch under the
surface, the silt was in its original form, and easily dissolved in
water. In the river, under low water, were also observed great
masses of this silt which had fallen over into the river in blocks,
over the surface of which the river had run for years without
carrying them away. On the other hand, the backwater in an
eddy soaking through the cracks and getting behind and around
other blocks completely dissolved them, and the river carried
them away.
The bed of the lower section of Nelson's Creek, a rapid stream,
is formed of this precipitated plastic clay, whioh prevents any
water soaking out of it into the slide. But more noted than
these instances is the fact that, on the walls of the gorges, the
Thompson River and Nelson's Creek, whioh in many places are
composed of this same silt, is found the same coating of plastic
clay and lime—an imperfect cement—formed by the action of
the running water. Thus these walls have stood almost vertically
(in one place 100 feet high) for ages, withstanding almost entirely
the cutting effect of the highest freshets. It is true that these
banks of silt gradually disintegrate by atmospheric action upon
them, and in the dry season the scale of plastic clay becomes Proceedings.]   STANTON ON LAND-SLIDES IN BRITISH COLUMBIA.    21
baked and drops off; but as soon as the stream rises by its
assistance, Nature immediately erects a new bulwark against its
ravages.
There is no way of preventing the continual disastrous effects
of these land-slides upon the railway except by cutting off the
irrigation water above. This has been and can be made effectual.
The water so easily absorbed as easily drains out, and the mass
after a time again becomes firm and solid.
The Paper is accompanied by three tracings, from whioh
Plate 1 has been prepared.
I 22     STANTON   ON  LAND-SLIDES  TN  BRITISH  COLUMBIA.      [Minutes of
APPENDIX.
Vancouver, B.C., Nov. 14th, 1896.
Mr. Robert B. Stanton, M.A.
Dear Sir—The clay, or silt, you handed me for examination absorbs 53 ■ 5
per cent, (of its volume) of water without losing its form, and, after absorbing -
78 "5 per cent., it frets down and mingles with the water.
I have compared it with clay from Warnock, B.C., which takes 49 per cent,
and 70-5 per cent. When the clay, or silt, you handed me has absorbed 78 "5
per cent, of water, it seems to mingle with the water, whereas the Warnock
clay drops into a square heap and remains there. The following analyses of
this and other clays are given for comparison.
Yours truly,
H. J. Warsap,
Chemist.
Analyses of Silt and Clays.
Silt from
South Slide,
Mack Cation.
White China     Warnock,
Clay or
Kaolin.
B.C.,
Blue Clay.
Average
English
Blue Clay.
Silica (S202) ......
Alumina (AL203), and  Ironl
(Fe203) /
Total Sulphur (S03)     .    .    .
Lime(CaO)	
Water loss	
26-75
46-8
60-05
63 "35
56-60
37-7
31-08
18-50
6-8
..
1-02
4-32
3-24
..
6-25
6-60
6-61
15-5
1-60
7-23
[Discussion. 1
Proceedings.] DISCUSSION ON LAND-SLIDES IN BRITISH COLUMBIA.      23
Discussion.
Sir J. Wolfe Barry, K.C.B., President, was sure the members Sir J- Wolfe
would all regret that the Author was not present to further dr y'
elucidate his interesting description of the careful observations
which had led him to important deductions in connection with
the great landslides in British Columbia. The slides certainly
were on that immense American scale to which engineers were
not unaccustomed when hearing of what took place in that great
continent. The Paper was an interesting one to engineers, as
many of them might, on a small scale, have to deal with disasters
somewhat similar to those to which the Canadian Pacific Railway
had been exposed. No one could envy the engineer in charge of
that length of the railway; but it was very satisfactory to know
that by the careful observations of the Author, the cause of the
slides seemed to have been tracked by his scientific mode of
research. He was sure he was only speaking the sentiments of
all present in proposing a hearty vote of thanks to the Author
for his interesting contribution to the Proceedings.
Mr. G-. R. Jebb thought it would add to the interest and clearness Mr. Jebb.
of the Paper if cross-sections could be given, one, for example,
across the valley and across the north slide, showing the relative
levels of the river, the railway, and the country above; also
showing the thickness of the boulder-clay and' of the various
strata forming the sides of the valley.
Mr. Horace Bell remarked that the Paper was essentially Mr. Eell.
a geological one, and it indicated clearly the necessity of attention
to the geological features of any ground upon which works were
proposed to be carried out. The cause of these great slips and
the remedy for them appeared to be clear enough. The cause was
apparently constant irrigation, and the remedy was that of ceasing
it. It would no doubt have appeared to the Author that the
remedy would perhaps be wOrse than the disease, as it would end
in stopping the irrigation, and reducing the population and the
traffic of the Canadian Pacific Railway. The great slides that
had taken place on that railway were paralleled in some measure
by the enormous slides with which he had had to deal in the
Beluchistan frontier of British India. There they were due not
to irrigation but to peculiar geological conditions.     The line he 24:     DISCUSSION ON LAND-SLIDES IN BRITISH COLUMBIA.     [Minutes of
Mr. Bell, referred to connected the Indus Valley with the great military
station at Quetta on the Beluchistan frontier; and at an elevation
of 6,000 feet above the sea there was a valley which had rightly
been called " Mud Gorge," consisting almost entirely of disintegrated shales overlying the lower nummulitio limestone, as
shown in Fig. 5. The railway, which, perhaps, had been somewhat hastily located over the ground, had proved to be, both
at this point and at others, a frequent source of trouble and
anxiety to the Government of India. It was a connection between the Port of Kurrachee and the Afghan frontier, and any
defect in it could hardly be contemplated with satisfaction either
by the Government or by the public. The slides were mainly due
to the fact that the shales were permeated by beds of gypsum
and anhydrites, which, subject to subaqueous disintegration, forced
the whole body of the shales from the upper hills down to the
river, which drained  the valley, and the result had been that,
SOOO      FT     LEVEL
Scale 1 inch = li mile.
1. Nummulitic limestone; 2. Middle Eocene, black shale and bund; 3. Upper nummulitic
limestone; i. Pliocene sandstones.
Cross-section through the "Mud Gobge.*'
for over 6 miles of line, there had been constant and gigantic
movements extending for a mile or more above the railway towards
the hills, which were about 10,000 feet above sea-level, thus
forcing the railway outwards towards the river. The treacherous
nature of such ground was well known to engineers, one of the
most prominent cases being that of the tunnel near Heilbronn in
Wurtemburg, which had given great trouble many years ago, and
resulted in a completely new line being made. The engineers
had tried their best to cope with the difficulty; they spent in
their zeal between £50,000 and £100,000 in trying to make the
railway stable, but they failed, as might have been expected. In
1893, however, the Government of India appointed a small
committee, of whioh he was a member, to determine whether those Proceedings.] DISCUSSION ON LAND-SLIDES IN BRITISH COLUMBIA.      25
operations should be continued, or whether each movement should Mr. Bell.
i be dealt with as it occurred. The committee carefully investigated
the whole matter on -the ground, and after considering the many
heroio remedies proposed, came to the conclusion, with which
probably the Institution would agree, that there was nothing to
be done; that the best course was to take each slip or movement
as it occurred, slewing over the line, and working with any
gradients, or curve up to 300 feet radius, and pushing the traffic
through as well as possible. In addition to this, the drainage
of the slopes, in order to remove the water from the grounds
quickly, was adopted, and since that time there had been comparatively little difficulty. The case, of course, totally differed
from that which the Author had described. The one case was
entirely a question of water, the other was a case of chemistry
and water; but the moral of both was that, before investing large
sums of money in public works, an endeavour should be made to
test, as far as possible, the geological and chemical conditions of
the ground to be dealt with.
Mr. W. R. Galbraith thought the Paper set forth the disease Mr. Galbraith.
and the remedy. The disease was a large amount of ground
moving probably on some stable bed inclined towards the river.
In regard to the northern and more serious slip, the Author had
said, that directly the irrigation—the supply of water poured
in—ceased, the slip was arrested; and he could not see why
the same remedy was not applied to the southern slip. It was
thought that the purchase of the property now being irrigated
would probably involve a large expenditure; but if the northern
slip was 155 acres, the land irrigated between the two slips,
the southern portion of that which seemed to be causing
mischief on the southern slip, could not be much more than
200 acres; and it therefore appeared that the Canadian Pacific
Company might long ago, instead of spending £100,000 in continual alterations of the line, have bought the irrigated land
and stopped the water. In the case of a railway slip in England
the first remedy would be to cut a drain round the back of the
slip to take the water off, and the second remedy would be to make
two or three drains so as to drain off the water below. The fact
that the mere cutting off of the water behind had been effectual
in curing the larger and more dangerous slip, seemed to indicate
that the same plan ought at once to have been adopted with regard
to the southern slip. He did not understand why that had not
been tried long ago. If this were the slip of a railway slope
a large bank of stone would be erected at the bottom of the dam, 26     DISCUSSION ON LAND-SLIDES LN BRITISH COLUMBIA.      [Minutes of
Mr. Galbraith. forming a heavy drystone wall to retain the slope.    In the present
case this was out of the question, and the only remedy was to
drain the slip and stop the water.    If that did not cure it he
should drive a tunnel through the slip at right angles to the river
and draw the water off.    He thought that would be much cheaper
and better than continually watching and moving the railway,
which must be dangerous.    A great amount of silt below became
impregnated with  water, and  the heavy mass resting upon  it
drove the silt into the river, forming a cavity, then the ground
settled and broke away into a great number of crevices, causing
the railway to be  exceedingly dangerous.    He thought   there
was a very simple remedy which might be tried at no great
expense—stopping the irrigation in the lower slip, as had been
done in the upper.    That was what the Paper really seemed to
imply; it was a very simple remedy, the amount of ground to be
dealt with not being great.   If that were effectual on the southern
slip the company would be saved considerable danger in working
the traffic, and a large amount of money in continual repairs and
alterations of the railway.
Mr. Hill.     Mr. G. H. Hill had had considerable experience in connection
with  works  constructed   for   waterworks   purposes   in   different
valleys where such slips had taken place.    In the case of the
Manchester Waterworks there had been, 45 years ago, a very large
slip, over which, in the Parliamentary plans, the water-courses
were intended to pass.   But when the ground came to be examined,
thoroughly,  there was some doubt whether   the  water-courses
could be carried across the slip (the area of which was double
that mentioned in the Paper), and  Mr. Bateman, the engineer,
advised the Corporation to call in the assistance of Mr. Robert
Stephenson  and Mr.  Brunei.     Those gentlemen  examined the
ground, spending the day on the spot, and they came to the conclusion that the proper way to deal with the slip, which measured
about J mile across, would be to drive a tunnel underneath it, and
then to drive headings in all directions where the water was to
be intercepted.    The tunnel had been driven, and a great number
of adits were put into the hillside from the tunnel; all the water
was extracted  from the slip, and it became  absolutely stable.
The water-course was then carried across the slip, conveying about
600 million gallons a day in flood times, and no trouble had since
been experienced.    It appeared from Plate 1 that, in making the
railway up the valley, the cutting away of the old land-slips might
have affected the balance, which never had been stable before the
railway was made,  and  that changing of the balance of the Proceedings.] DISCUSSION ON LAND-SLIDES IN BRITISH COLUMBIA.      27
material would probably start the slip again in motion. He had Mr. Hill.
himself seen such cases. A slip had occurred in the Ashton waterworks of 36 acres; the contractors, in making the embankment,
ran a wagon road at the foot of the hill for a distance of 200 yards
or 300 yards, and they started 36 acres of land in motion. In the
same way, in the making of the railway some of the old slips might
have been set in motion. The course he should have adopted
would be to begin near the river-level and drive a heading right
in, making adits in all directions and tapping the Water. If the
lower part was stable it would retain the upper part in its
position; but there could be no better way of creating a slip than
to bring water upon it for irrigation purposes. That would sink
through and, coming down to the rock or shale, a movement
would take place upon the surface which would be greased by the
admission of water. The only way to stop slips was to get the
water out; the mere fact of making drains round it would not, he
thought, be effectual.
Mr. H. Osburn considered that if cross-sections of the Thompson Mr. Osburn.
river valley had been given, as well as a description of the levels
of the land on each side of it, the cause of the slips could be better
judged. He had spent two or three days near Ashcroft, at the
back of the country where the slips seemed to have commenced.
He thought slips had constantly been in progress in that valley,
and he was surprised to find that the irrigation above had been
given as the cause. One could ride for the whole of a day in that
district and only pass one or two ranches, the greater part of the
country was entirely uninhabited. He had ridden on a mule up
the mountain side, and was able to form an opinion of the. country;
irrigation was only carried on in a very few places, and it seemed
to him very strange that the. slides should occur in those places.
There were natural streams and reservoirs, and no doubt they
sometimes broke bounds, and, rushing down, caused the slips. The
whole valley showed, however, that these slides were not unnatural;
they were going on constantly.
Mr. L. P. VErnon-Harcourt had had the opportunity in August Mr. Vernon-
and  September, 1897, of going to Vancouver by the Canadian -Harcourt.
Pacific Railway, and though, unfortunately, he did not see the
exact places described in the Paper, because the railway train
always passed them at night, going and returning, he had seen
places alongside the Eraser River, of which the Thompson River
was a tributary, where they had been putting the railway further .
back from the river.    The former course of the railway could still
be seen; and there had, no doubt, been slips to a certain extent in If
28 .   DISCUSSION ON LAND-SLIDES IN BRITISH COLUMBIA.     [Minutes of
Mr. Vernon- that part. If the slides described were really caused by irrigation,
Harcourt. j^ agree(j wjth Mr. Osburn in thinking that in a place like Canada,
where there were immense tracts of country perfectly unoccupied
in more favourably situated localities, such as the land offered for
sale in Manitoba at about Is. Qd. per acre, and far more fertile than
the places described in the Paper, it would be much cheaper to
buy out the people who hpd that small area of land and stop the
irrigation than to continue contending with the slides. There
were other places on the Canadian Pacific Railway where smaller
slips had occurred. He had noticed one especially in the Kicking-
horse River Valley, on the western slope of the Rockies, where the
sharpest curve upon the Canadian Pacific Railway had to be
introduced, where a tunnel had formerly been constructed for the
railway through a projecting spur, which, on account of the slides,
had to be given up. Apparently some trouble had occurred from
slides during its construction; and it was now abandoned, and a
sharp curve had been substituted for the railway, running round
the spur. That curve was only 262 feet radius. Mr. Cuningham,
in his account of that portion of the Canadian Pacific Railway,.
stated that the spur through which this tunnel had been formed
consisted of blue clay interspersed with layers of sand, with an
overlying mass of boujder drift.1 The action of water, he thought,
in that treacherous formation had caused the slip. They had,
accordingly, been obliged to put in round that spur the sharpest
curve on the line of 262 feet radius, the next sharpest being 573
feet radius, or what was known in America as a 10-degree curve.
Round that curve the railway went without an elevation of the
outer raiL because the curve being so sharp the projecting roofs of
the cars, if the rail were super-elevated, would touch one another.
A guard-rail had been put in, but it- was curious that on most of
the other sharp curves a guard-rail was not used. They had put
one in, in that case, he supposed, because they were not able to
elevate the outer rail. The train went round at a very slow pace,
and in that.way, with the help of the bogies on which the cars ran,
was kept on the line. There were also other places on the line
•besides the Eraser River where there had evidently been some
slips, for example, on the ascending slope of the Selkirks going
west, before getting to Rogers' Pass, where they were trying to
consolidate, some of the fallen portion of a large slip, in a steep
ravine above the railway, by directing a stream of water upon it,
as in some parts they had consolidated embankments upon the
1 Minutes of Proceedings Inst. CE., vol. Ixxxv. p. 108. Proceedings.] DISCUSSION ON LAND-SLIDES IN BRITISH COLUMBIA.      29
same principle. The great difficulty with regard to the Canadian Mr. Vernon-
Pacific Railway was that, on account of its not being able to follow ar
a regular definite river valley like that of the Columbia River, it
had to go through very narrow canons across the Selkirks and the
Gold Range, and therefore there was very little choice of route.
Along the Fraser River, moreover, the train ran for some distance
in a narrow gorge; and whilst fairly close to the river bank a high
cliff rose directly above the railway on the far side. All the way
down from the Selkirks, and most of the way from the Rockies,
there was very little possibility of changing much of the route of
the line. The Columbia River flowed in a very peculiar course,
so that although the railway followed along the wide Columbia
River Valley for a short distance on leaving the Rocky Mountains,
it had to leave the river again soon because it went so far north in
order to get round the Selkirks; and the railway crossed the
Selkirks by Rogers' Pass. The railway then descended again to
the Columbia River Valley where the river flowed south in the
opposite direction parallel to itself on the other side of the
Selkirks; and as the Columbia River continued its southerly
course down to the United States, the railway had to cross it
again at Revelstoke, and pass westwards across the Gold Range
through the Eagle Pass. There again, though there was but little
elevation, the pass was narrow; whilst down the western slope of
the Selkirks there was the contracted Albert Canon, so that there
was very little opportunity of modifying the route of the line.
How far it would have been possible to have avoided the slides, if
the geology of the district had been thoroughly known, by carrying
the railway on the other side of the Thompson River he could
not say; but it was quite possible that it might have been done,
because a little below the confluence of the Thompson River and
the Fraser River was crossed by the railway, and the line kept
along the western and the northern side of the Fraser River, from
thence to Vancouver, and he thought it just possible that though
they could not materially change the route of the line, they might
have gone on the opposite side of the Thompson River at the site
of the slides. The best course now appeared to be to stop the
influx of water by buying out the proprietors of the ranches above,
and arresting the irrigation where injurious.
Mr. E. Benedict found it difficult to locate the same place on Mr. Benedict,
the different maps given in Plate 1.    If the degrees of latitude
and longitude were given this would be facilitated.    Most of the
speakers appeared to have overlooked the last paragraph in the
Paper; the Author did not mean to stop all the irrigation, but 30     DISCUSSION ON LAND-SLIDES IN BRITISH COLUMBIA.    [Minutes of
Mr. Benedict, to intercept the water just before it reached the railway, thereby
making the toe of the slip solid, so that the top would not be
inclined to move. It appeared to be a very exceptional place,
such as was not likely to be found anywhere else, and the causes
of the movement were clearly set forth by the Author. The water
in the case described was carried on to the land by small rudely-
constructed ditches built almost entirely by the farmers, so that
they were not watertight.. That was the first' step towards the
slip. It was seen at Ashcroft that by stopping the irrigation the
movement was stopped. It was also said that nearly all the dips
were to the westward, and this also tended to start the slips. He
thought it was of very little use to discuss the slips that were so
exceptional and unlikely to occur elsewhere; besides, the Author
himself had pointed out the remedy, and he thought there was
no more to be said.
Colonel Colonel Pennycuick, R.E., was, like other speakers, in some
Pennycuick. difficulty on account of the absence of cross-sections. He imagined
there was a deep bluff immediately above the river, with a tableland on the top. In that way the water was distributed over the
surface, trying to get down to the river just under the boulder
clay, and got to the sand below. In irrigation works in Southern
' India much the same state of things was often experienced, though
on a much smaller scale, and the remedy in every case had been
simply drainage. In other words, the water had to be taken out
as soon as it got in. In many cases of violent slipping of the
rear slopes of embankments, the remedy had been to drive drains
into the bank as far as it was safe to go, and to let the water run
out freely without passing through the earth. He could not
help thinking that something of the same sort was possible in the
present case, but without sections it was impossible to say what
the proper remedy should be. In the concluding paragraph the
Author had stated exactly the remedy he proposed—not to stop
irrigation, but to cut off the water, to take it by properly-constructed watertight drains down into the river instead of allowing-
it to get under the surface of the soil. He knew of one instance,
in one of the large tanks of the Madras Presidency, which had an
embankment about 40 feet high, where the rear slope slipped so
badly that it forced a road which ran along the foot of it 30 feet
out of its proper direction, pushing the whole road into the
rice-field below. That had been cured by the simple process he
described. They cut away the rear slope as far as they dared and
drove drains in, running transversely to the bank, and connected
by longitudinal drains, which collected all the water that leaked in 1
Proceedings.] DISCUSSION ON LAND-SLIDES IN BRITISH COLUMBIA.      31
and carried it out harmlessly.    That was twenty-five years ago, Colonel
and he believed the bank was now as safe as could be desired. Pennycuick.
Sir John Wolfe Barry, President, thought some idea of the Sir John W< lie
<jross-section could be formed from a passage of the letter-press in Barr7«
the Paper, which stated that the bluff, where the break-off took
place, farthest from the railway appeared to have been about 400
feet in depth, so that it might be imagined a considerable cliff had
there been formed to begin with.
Correspondence.
Mr. James R. Bell had had occasion to study the problem dealt Mr. Bell,
with by the Author on a no less gigantic scale at the Mud-Gorge slips
in Beluchistan, where in earlygeologic time's a dam of solid limestone
rock some 2,000 feet high must have upheld a lake of mud some
10 miles long and 3 miles or 4 miles wide. How far the sides of
that lake squeezed together and pressed up the centre of the clay
lake hundreds of feet above the dam, or in what other way the
clays were upheaved above their original level, their crests on
either side of the valley were now far higher than that of the
Chappar Mountain which formed the dam. The dam had not
failed as a whole, but it had seemingly had a tunnel bored through
it by (probably thermal) springs, and the roof of the tunnel falling
in while the bed scoured deeper and deeper, the mountain was
now cleft in twain by the famous Chappar rift, one of the most
remarkable canons yet encountered by any railway. The present
Mud-Gorge Valley had been formed in the clay by the action
of a small stream, which had carried away the mud from its bed
through the rift. The bed of this stream was alternately raised
locally when squeezed up by slips and lowered by cataract-like
retrogression of the river-levels. Here and there narrow strips of
level" bottom " occurred beside the river, but for the most part its
banks conjugated every tense and mood of the verb " to slip " except
the past pluperfect. The valley was now about 3,000 feet deep
and about 3 miles wide; and beyond the fact that in a recent
sequence of droughty years there had been less trouble than in
average rainfall, there was no reliable indication of amelioration.
The case might admit of remedies, but such skill as the Government
of India had been able to bring to bear on this crux had not yet
offered any more promising remedy than to watch the place, and
put in deviations of the line as stoppages threatened. It turned
out (now that the country was getting settled beyond-the valleys 32 CORRESPONDENCE ON [Minutes of
Mr. Bell, occupied by the railway) that this part of the Suleiman plateaux
teemed with rifts even grander than the Chappar, and doubtless
with mud-gorges of corresponding mobility. It was especially
noteworthy that both in the Mud-Gorge Valley and in that under
consideration the clays contain a considerable proportion of
gypsum in crystals, liable to expand when slaked by water, and
thus to create "slip-planes" analogous to the greased launching-
ways used by ship-builders. Although the railway through the
Chappar rift offered grades of 1 in 40, the Government of India,
in view of the vast military and political importance of a railway
to Quetta, had recently determined to build at great cost an alternative line with ruling gradients of 1 ki 25 in another valley,
whose paramount advantage consists in its having well-nigh
completed its processes of denudation in their more active phases.
He alluded to the Mushkaf-Bolan Railway, recently described by
Mr. James Ramsay.1 Compared with Indian experience in a hostile and newly-acquired territory, Mr. Bell was very favourably
impressed by the thoroughness of the geological diagnosis, whioh
the Author quoted from Professor G. M. Dawson. He saw little
need for the hypothesis that the ancient pliocene river scoured out
the beds of the lakes of white silt that now underlay the boulder
clay, but that was a mere detail. The Author appeared to consider that it was necessary for water to entirely saturate vast masses
of such silts before any motion would occur. On the strength
of Mud-Gorge experience, any such extent of hydration as that
indicated on p. 6, where the catastrophe was finally attributed to a
large body of the silt being saturated till it could not support itself,
far less the superposed clay which spanned it like a bridge, seemed
quite unnecessary. It was quite clear that at Mud-Gorge the
slip did not occur because more or less saturated and semi-liquid
silt had found an outlet, but, on the contrary, slurry only found an
outlet in consequence of a slip being established. He was far
from contesting, in offering this experience, the practical view
that water was the proximate cause of all slips, and that buying
out the irrigation might effect a material amelioration of the Canadian Pacifio Company's difficulties. Indeed, he was only at a loss to
conceive the arguments that had seemingly prevailed for years
against this treatment and until £100,000 had been spent in
remedies of an imperfectly successful class in the teeth of a neighbouring example. Stopping the irrigation would certainly check the
slipping of the valley flank, whether it checked those slips that
1 Minutes of Proceedings Inst. C.E., vol. cxxriii. p. 282. 1
Proceedings.] LAND-SLIDES IN  BRITISH COLUMBIA. 33
immediately affected the line or others more remote in the first Mr. BelL
instance.
l Mr. H. J. Gambie1 observed that on p. 7 the Author stated that" at Mr. Cambie.
times the road-bed has sunk 4 feet, and has moved out twice that
distance in a night." His experience as engineer of the western
division of the Canadian Pacific Railway led him to the conclusion that this statement would convey an erroneous impression of
the safety of the railway. A movement had occurred in 1894, but
it was not nearly so rapid as that described, and no such motion
had occurred before or since that time. It was also stated by the
Author that " this section of 5 miles or 6 miles . . . has cost the
Canadian Pacific Railway £100,000." Such an assertion was not
warranted by the evidence, and it was doubtful in his mind if the
cost had reached one-fifth of that sum. Again, it was stated that
"at one point a train-load of tea was . . . completely lost, by
... an extra amount of water being put on an already saturated
field." No tea-train had been wrecked within 100 miles of the
slides referred to, or anywhere on account of irrigation. On the
same page the Author stated that " after the watchmen had passed
over the line ... a west-bound train came suddenly upon a
section of the line sunken out of sight. The train fell into the
river and the engine-driver was killed." In 1886 a slide occurred
which disconnected the line, but no watchman was then kept on
that part of the railway. The engine ran a short distance down
the bank, and the driver was scalded, but he was still at work on
the line. Of the rest of the train, only the front end of one
carriage left the metals. He also considered that the Author's
statement that " at one time the tract of land on whioh the town
of Ashcroft now stands began to move towards the river " (p. 8)
was misleading; it had not, so far as he was aware, shown signs
of moving.
Prof. Boyd Dawkins, F.R.S., agreed entirely with the general Professor
conclusions of the Author as to the origin of the landslips and aw ms"
the best means to be adopted for preventing their recurrence in
the future. It was perfectly clear they had been caused by the
access of water to the argillaceous silt underneath the glacial
strata which formed the surface. It was equally clear from their
having followed the establishment of irrigation works, that they
were due to the introduction of water in sufficient quantities to
1 This communication was received subsequently to the remainder of the
Correspondence, and too late to be placed before the Author for comment or
reply.—Sbo. Inst. CE.
[THE INST. CE. VOL. CXXXH.] D
J 34 CORRESPONDENCE ON [Minutes o
Professor soak down into the silt, whioh naturally was dry and hard, and
Dawkms. ultimately to carry it away in the direction of least resistance into
the nearest river. Had the natural regime not been disturbed
there was no reason to suppose they would have occurred, as
there were no traces of them in the strata -which had not been
artificially dealt with. The best way, and in these strata the
only way, to stop them'was to restore the natural conditions by
putting a stop to irrigation in those portions of these strata which
.commanded the railway. Similar slips, it might be remarked,
had from time to time occurred in Great Britain from the access
of water to sandy beds resting upon clay, and had been remedied
by cutting off the surface water from the beds in question. This
was done many years ago in Bath, under the direction of William
Smith, father of British geology. The solid geology of the valley
of the Thompson River, carboniferous, cretaceous, triassic and
other, had nothing to do with the cause of the slips.
Mr. Crowell. Mr- Foster Crowell remarked that artificial disturbances of
soil formation were rarely produced upon such a vast scale as in
the cases so well described in the Paper; and the occasions were
still rarer in which irrigation works might bring about destruction
of such magnitude as occurred on the Thompson River. Nevertheless these land-slides formed striking and valuable object
lessons to every engineer who had to deal with earthwork, and
especially with the location and construction of railroads in new
countries. In these cases the ultimate controlling cause was the
admixture of water with a formation that was stable only when
dry, thereby disturbing the conditions of equilibrium which had
been reached in the processes of nature. Other causes might
produce similar results in any given mass of soil that was in
a state bordering on motion; among such causes were such
obvious ones as the excavation of natural support and the
imposing of the weight of embankments. He had seen a
number of cases wherein the last-mentioned cause had been
productive of great and long-continued damage to surrounding
interests, which could have been avoided by due attention to the
Roman dictum " quieta non movere " on the part of the engineer,
either by adopting a change of location or by due precautions in
distributing the additional weights. He would quote three instances ; one occurred at Point of Rocks, near Brintou, on the
Pennsylvania Railroad at the time of the rebuilding under his
direction of the line to accommodate four pairs of rails, where
formerly there had been but two; the original location had been
balanced so that one line was established upon an excavated ledge
and the outer one upon embankment, the formation being argil- Proceedings.] LAND-SLIDES IN BRITISH COLUMBIA. 35
laceous rock with a very steep escarpment; the permanent way Mr. Crowell.
was elevated about 70 feet above the foot of the cliff, extending
from which were clay bottom lands of Turtle Creek—a sluggish
stream which at that point was about 150 feet in width, and was
spanned by an iron highway bridge resting upon substantial stone
abutments; between the cliff and the stream was a village built
along both sides of a single street extending parallel to the direction of the railroad; in building the two additional lines it was
found desirable to place them symmetrically with reference to the
old centre line and that brought the toe of the new slope, composed of rock from the cutting, close to the backs of the houses
which were protected from it by a properly-designed retaining
wall of masonry; the only feasible alternative would have been an
undesirable tunnel or gallery through the Point of Rocks; soon,
but not immediately, after the new embankment and retaining
wall were completed complaints were received that houses in the
village were being displaced because of them, but check measurements from the centre line proved that no movement had taken
place in either the wall or the embankment, and the complaint was
for the time dismissed; but there actually was a movement of the
houses, on both sides of the street, some of which being of brick
were ruined; those of wood were eventually moved back to their
former positions; the entire material of the street and of the
highway leading at right angles to the bridge, the near abutment
of the bridge and the superstructure were all moved laterally
several feet; the back wall of the far abutment was broken off
above the bridge-seat by the movement of the superstructure, but
beyond that no damage was apparent across the stream. Further
investigation showed that the additional weight of embankment,
acting vertically within its limits, had disturbed the equilibrium
of the clay formation, on which a portion of it rested, and the
pressure had expended itself in the line of least resistance laterally
at a lower elevation than that of the toe of the embankment; the
remedy, if it could have been applied in time, would have been
to either avoid the overloading or carry the support down to the
bottom of the clay. The second instance was that of a high embankment, also on the line of the Pennsylvania Railroad, across a
level valley where a lateral movement, similar in its manifestations
to that just described, was still active very recently, and had not
ceased since the road was constructed, nearly fifty years ago. The
third had come under his observation during an examination of
the works of the Panama Canal, in 1889, several years after the
abandonment of the excavation of the primary Culebra cutting.   In
d 2 36 CORRESPONDENCE ON [Minutes of
Mr. Crowell. preparation for the disposal of the spoil a series of parallel railways,
built on successive benches at the mouth of the cut between
Culebra, and Paraiso, had been provided; some distance farther on,
the line of the Panama Railway crossed the valley at a much
lower elevation upon an embankment; comparatively little of the
material had been taken out and deposited when the work terminated, but enough had been done to start a gigantic lateral
movement in the entire width of the valley that not only wrecked
all the extensive system of spoil railways, but displaced and
destroyed the Panama Railway itself, so that for years thereafter it was necessary to maintain constantly a large force
of section men to restore the line after the passage of every
train, as was done at the time of his visit, and under his observation, twice in one day at an interval of rather less than eight
hours. He had frequently seen cases of slips of serious nature
resulting from unnatural saturation of the soil, though nothing
approaching in magnitude those which the Author gave, and he
had been the instrument of contributing to the cause of some of
them by failure to properly intercept and lead off drainage water,
or to take steps to prevent it from concentration in injurious
quantities. These were often regarded as needless refinements of
construction, except in cases where outside interests were to be
considered and protected, but experience showed that no part of
the cost of any earthwork was better expended than that judiciously
applied to the matter of drainage. The Paper showed forcibly
how widespread might be the results of a seemingly trivial
agency, and how important it was not to lose sight of the
destructive elements in the forces of nature.
Professor Professor Jules Gaudard, of Lausanne, thought the Paper shed
Gaudard. new light on the causes of instability of ground subject to the
percolation of water. The slides on the banks of the River
Thompson were remarkable for their vast extent, for the fact of
their being a result of artificial irrigation, for the length of time
which had elapsed between the originating cause and the ultimate
effect, and for the continuance of the movement as long as irrigation
was carried on, so that the only remedy appeared to be to stop
this irrigation, the extent of the slides being too great to be dealt
with by drainage. Irrigation, in addition to its direct value for agricultural purposes, frequently formed a safeguard against disastrous
effects of flood, as it permitted the retention and storage of a.t least
a portion of the superabundant water, which, if allowed to flow away
direct, would become destructive, instead of being gradually dissipated in agriculture and by evaporation. But it might be seen here
that, in combating a danger in one direction, the effect might be to Proceedings.] LAND-SLIDES IN BRITISH COLUMBIA. 37
bring another into existence, although the original intention was Professor
simply to fertilize a district otherwise arid. This danger did not Gaudard-
arise from that portion of the water which was actually utilized
for agricultural purposes, viz., for the fertilization of the layer of
vegetable soil, as the slopes of this layer were not so steep as to
be capable of sliding, but was due to the remainder of the volume
filtrating into the ground, and, having penetrated the permeable
sub-soil, reached a stratum, the character of which was altogether changed by the fluid. It might, however, be easily
imagined that it would be difficult to deal effectually with this
troublesome and disaster-working waste, for the ideal arrangement for a network of irrigation-channels was that the latter
should gradually diminish the supply from its point of origin to
the limit of the area affected. Although the extreme ramifications
were laid out, as they should be, strictly in proportion to the
demand upon them for irrigating the surface-layer of vegetable
soil and no more, this did not apply to the channels nearer the
point of origin of supply, which served as main ducts, or partially
so. The beds of these ducts might, however, be lined with
puddled clay. The delayed production of movement of land-slides
had been exemplified in other instances. On the railway from
Brunoy to Bois-le-Roi, in France, movements had taken place more
than 4 years after the excavation of the cutting which had caused
them. This might be explained by the extreme slowness of the
process of percolation into a porous substratum (at a considerable
depth) and without outlet; there it was subject to various influences,
such as capillary attraction, &c, besides the action of gravity.
This latter was far from being aided by the high pressures exerted
by hydrostatic columns in deep fissures in the ground, for, as the '
liquid reaching the bottom of such a fissure penetrated further, it
must open out a space to occupy; it could not do this by heaving
up the ground, as- its weight was less than that of the latter. The
only way for it to penetrate a dry soil was by expelling the air in
bubbles; but, in addition to the fact that the ascent of these
bubbles became the more difficult in proportion to the distance
they had to traverse, which was constantly increasing with the
advance of percolation, the pressure itself diminished their volume
and, consequently, their ascensional power of escaping through
the liquid. Thus the pressure of a column of water, which acted
so powerfully in detaching solid masses, imperfectly connected,
seemed to have but little effect in accelerating infiltration in the
depths of the earth; it was evident that it was not until more or
less relative displacement of the component parts of the mass, producing disintegration, &c, took place, that the water could pene- r
38 CORRESPONDENCE  ON [Minutes of
Professor trate further and the process of infiltration became more and more
Gaudard. glanced. Qn the other hand, although the flow was so imperceptibly small, it was also unceasing; and the small supply
necessary kept it constantly in progress, and the water being
unchanged, and practically immovable in the subterranean interstices, it had the effect of reducing the silt to slurry without
removing any of its constituent elements. A soil with an inclined
surface could only keep its form by the power of cohesion; all
moisture, although it might be only partial, penetrating deeply
into it had an inevitable tendency to reduce it to the level. Let
it be supposed that from a ditch A, Fig. 6, there descended, through
a permeable soil, filtration following the line A B, which at B
reached a deposit of silt; there it spread out in all directions
over the surface C D, and liquefied the silt to some depth, formed
a semi-fluid layer or pocket OBDE, and consequently produced a
change in the general conditions of equilibrium. On account of its
incompressibility, a confined liquid was able to support a uniform
load, whatever might be the pressure, but
Fig. 6. . as it had lost its cohesion and frictional
resistance, it had become incapable of sustaining an unequal  load on the various
points of a level surface, since it now followed the laws of hydrostatic equilibrium.
Although, in the solid state, the layer C D
supported  at its  extremities the varying
heights of D D' and C C of the permeable ground, it could no
longer do so when it had become liquid; it then tended to become
changed in form and to move, so as to lower the general centre
of gravity of the mass.    The superincumbent mass changed its
shape; it sank at D', it rose at C; the fluid layer ran towards
the lower end, it rushed from D, where it was compressed; it
flowed to C, where it expanded and set up fresh percolations and
opened up a passage for them.1    This natural process went on and
on; the subterranean liquid layer, or pocket, continued to increase
in magnitude and to advance nearer and nearer to the valley, the
movements at the surface of the ground continuing at the same
time without cessation, and  if in place of a  regular slope the
natural surface found a series of steps, the movement, instead of
1 The existence in certain places, notably in some districts of Algeria, of
veritable subterranean lakes, appeared to be established. It seemed certain
a priori that the surface of the ground above these sheets of water ought to be
flat or nearly so; every elevation would not fail to sink in expelling the water
situated below its heavy mass. Proceedings.] LAND-SLIDES IN BRITISH COLUMBIA. 39
being continuous, would progress in a jerky or intermittent Professor
manner, a character which might be equally caused by the hetero- Gaudard-
geneity of the ground. As the water carried with it the silt
which it had already dissolved, it was, in fact, a transport of solid
matter, which was carried on under ground and which tended to
reduce to the horizontal the visible surface. For this to raise itself
at C, above the original extremity of the saturated layer, it
was necessary, however, that it should be sufficiently supported at
the lower end. Should it happen that the point of upheaval, as it
was displaced, approached the cutting of a road or railway or the
ravine of a river, affording no support on the lower side, then the
result would be a sudden sliding of the whole mass which had
been rent vertically by the subsidence at the upper side; the
mass would be more or less overturned and broken up in every
direction by deformations and undermined at its base by the
formation of a slippery plane surface, lubricated throughout by
the slurry. In these sudden and extensive subsidences, which
were presaged by comparatively small movements, the settlement
was general over the whole surface;
occasionally, however, it was noticed ig' ''
that the subsidence was unequal in j§|||      -»<*»»«.»»»»
extent, and, in the case of small land- //   J^r
slides, a portion, originally horizontal        •-—~/    ^^
A B, Fig. 7, would, after the slide has   osCstTL^r
occurred, assume the inclination A' B'
sloping downwards towards the hill at the back; the face slope might
also bulge outwards near the foot, the acquired momentum also
permitting of its partial ascent of the counter scarp. In the case
of the Canadian Pacific Railway, the origin of the land-slide being
on the uphill side of it, it was altogether out of the question to
attempt a remedy by carrying out simple works on the lower or
valley side. Suppose, for example, an immense retaining-wall
were constructed in the slope of the cutting, or in that of the
river, it could only retard, to a very slight extent, the general
subsidence, and as the surface of the ground would still possess a
tendency to become horizontal; by travelling downwards, the
upheaval of the lower ground would ultimately extend to the
neighbourhood of the wall, and even if the latter were not bodily
carried away, it would not prevent the disintegrated ground from
being squeezed over its summit and obstructing the cutting or
river. In order to grapple with the evil at its source and stop the
slides, it would be necessary to drive a heading or culvert at B
{Fig. 6) parallel to the irrigation duct A, with suitable gradients, 40 CORRESPONDENCE ON [Minutes of
Rrofesscr so as to intercept the percolation and allow it free outlet, the
Gaudaru. water would then be carried away without doing any damage.
This subterranean aqueduct could prevent the water coming into
contact with the silt by a lining of cement, but even if the water
flowed over the silt, unprotected, it might be fairly assumed that,
forming a rapid current, its effect would be to cause no more
erosion than the River Thompson below did upon its banks, and
that a protective covering of pasty clay would be constituted. Vet
the custom was to put a concave bed of cement concrete, as more
secure. Unfortunately, it was easily explainable why, in the presence of such extensive land-slides as those at the Black Canon,
recourse to treatment by drainage had been avoided, although it
had so frequently done good service upon railway works elsewhere.
In presence of the absolute necessity of keeping the permanent way
of the Canadian Pacific in running order, it would appear as if
for the time being at least, the cutting-off the supply of water was
contemplated, as soon as it was feasible to do so, although it
would entail a serious loss of money to sacrifice in this way the
benefit of an established system of irrigation. It must also be
remarked that drainage -culverts, especially in those portions of
the system where their direction was perpendicular to that of the
general movement of the ground, run a great risk of being
disturbed and broken, in which event they would be rendered
worse than useless. Where there was a probability of this occurring
it-was advisable to construct the culvert in such a manner as to
be accessible throughout its length, so that it could be kept in
repair and free from obstruction.
It was upon railway-construction works that considerable
' success had been attained in dealing with the slips which so
frequently occurred in the slopes, but the circumstances in these
cases were different from those of the valley of the Thompson,
and, in some respects, opposite in character. It was not, as in the
latter case, the artificial introduction of water into an area
unsuitable for carrying it off which constituted the disturbing
element and the direct cause of the land-slides (the valley and
ravine of the river having existed before), but, on the contrary, in
the case of railway construction, it was the subterranean water
which was originally present, a state of things having been
evolved under which the earth remained stable; further, the
collecting of these waters, due to natural topographical circumstances, could not generally be checked; the disturbing element
was the excavation of a cutting, which had upset the equilibrium
of the ground, and it was for this reason that works of consolidation, Proceedings.] LAND-SLIDES IN BRITISH COLUMBIA. 41
confined to the immediate vicinity of the part interfered with, Professor
could, at a moderate expense, prove efficient; for the disasters to au ar '
be feared took place by commencing at the critical point and
extending farther and farther; a primary partial slip was
succeeded by a second of greater extent* and so on, so that by
stopping the development of the first, the occurrence of the others
was prevented, and thus it was only in a small area that provision
had to be made for the free passage of percolating water, which
would soften beds of silt, &c. The system of buttresses or
transverse walls of dry stone, built at right angles to the slope
of the cutting or the bulging hill and having a culvert at their
base, was recognised at the present time as one of the best, since
it supplied a method of drainage which dried and consolidated
the surrounding soil, and, at the same time, a force of frictional
resistance capable of withstanding the pressure which tended to
push the slope into the cutting. With this object these walls,
which intersect the unstable mass at regular intervals, should be
carried down and founded upon a solid bed, so that they would not
yield to the tendency of the adjacent mass to slide. This being
so, each distinct portion of earth slipping could not slide without
rubbing laterally against the faces of the two buttresses which
enclosed it. This friction would afford sufficient resistance under
two conditions, viz., (1) That the mass of earth be not converted
into slurry, but preserve a certain amount of cohesion ; (2) While
being sufficiently cohesive, the mass of earth must yet exert
against the buttress a lateral thrust great enough to produce the
necessary friction. With regard to (1), it was necessary that the
buttresses should be carried down sufficiently deep into the side
of the hill, and be sufficiently near one another to perform their
function of draining the space between them. Where necessary
they might be connected together by a culvert or longitudinal
waterway (i.e., parallel to the railway) laid out with a falling
gradient towards each buttress. However, where these drains ran
the risk of occasionally becoming choked or broken, it might be
preferable to do away with them, at the same time bringing the
transverse walls closer together; they must, however, always be
founded on the solid ground and made as accessible as possible. If
the slope, although not sliding, exhibited in wet seasons a certain
viscosity, the surface between the two walls which contained and
retained them might swell and become convex. To counteract this
deformation, a layer of stone pitching, either covering the whole
slope or in the form of a pointed arch on plan, might be employed.
)f massive stone restricted the tendency to swell out. 42
CORRESPONDENCE ON
[Minutes of
Professor It was a good plan to concentrate the weight by thickening the
an ar . pjtghjng a^ the centre, so that the greatest pressure was at the point
of maximum upward thrust.    Condition (2) was affected by the
well-known uncertainty and difficulty of estimating the thrust of
earth.    Now instead of giving to these walls a constant thickness
throughout, as was the
^98- & usual custom, it would
seem   better  to  widen
them out, that was to
gradually increase their
thickness from theupper
to the lower part of the
slope, so that the mass
of earth (enclosed) between two consecutive
buttresses would assume
a trapezoidal  form in
plan.     It would   then
form   a wedge,  which
could not descend without   becoming   broken
up.   For similar reasons
it would be advisable
to give a batter to the
upright   faces   of   the
walls, as the tendency
of the earth is to descend, and  this batter
aids in sustaining it.    Fig». 8 showed the arrangements described.
The pitching is shown as only covering part of the slope.
fessor    Professor Edward Hull thought the Paper supplied an excellent
instance of the essential interdependence of geological conditions
and  engineering art.    It  was remarkable that  in   this far-off
region of Canada the same persistent enemy to engineering work
was present as had to be faced in the British Isles; he referred to
the boulder clay of thS Glacial epoch.    Engineers who had deep
railway cuttings to construct or maintain in this deposit in Great
Britain knew what a treacherous material it was to deal with;
and  the same appeared to be the case in North-West Canada.
When first cut into it was often so solid and tough as to be able to
stand like a wall; but a few months' exposure to rain and frost
initiated a process of sliding which was interminable; and so it
appeared along the line of the Canadian Pacific Railway.    This
SECTION
I
1
P
Hull Proceedings.] LAND-SLIDES IN BRITISH COLUMBIA. 43
arose from the heterogeneous composition and absence of regular Professor
stratification in the boulder clay due to its glacial origin. He u '
was not certain that the means described by the Author were the
best for getting over the difficulty. The expense of the process
of keeping the line clear over the " north and south slides " was
great and continuous, while sudden slips and subsidences, like the
case described in the Paper, would probably recur frequently. If
possible, the best solution of the difficulty might be to avoid the
enemy by tunnelling in the Cretaceous strata—from some point
north of Nelson Creek to the eastern bend north of the Black
Canon. The distance would only be 1J mile, and the work might
be spread over several years without interfering with the present
traffic. The account which the Author gives of the physical
changes which had passed over this part of the Continent were of
great interest, showing uprising and depression of the land, the
filling to some extent, and re-excavation, of river valleys, the
changes of climate, and the advances and retreats of the glacier
• ice in the Pleistocene epoch. These, changes had to some extent
their counterparts in Eastern America and Western Europe. From
recent investigations 1 by Professors J. W. Spencer, A. Agassiz,
Warren Upham, and other American geologists, it had been
shown that the eastern borders of the American Continent had
undergone great changes of level after the close of the Pliocene
period, consisting of uprises of several thousands of feet, and
subsequent depression, as shown by submarine soundings. The
Paper showed that representative changes (not necessarily absolutely contemporaneous) had taken place on the north-western
sea-board, and these went far to account for the occurrence of the
glacial conditions of the Pleistocene period themselves.
Mr. Malcolm Paterson remarked that the porous gravel deposits Mr. Paterson.
which formed the bulk of the material of these great land-slides
seemed to be those known to English geologists as " Kaims," or
" Eskers," in which it was supposed that the substance of moraines,
or boulder clay—the "till"—was blended with marine gravels.
Glaciers had descended to the sea-level and launched themselves
and their burden of debris into tidal waters, by whose action such
debris had become so washed and scoured that the angular boulders
had become rounded into shingle, and the " till" had been largely
changed into a perfectly free and porous gravel, with pockets of
argillaceous sand and thrown into mounds and terraces. He had
recently driven a short tunnel in an " Esker," parallel with and
1 Spencer, " Reconstruction of the Anlillean Continent," Geol. Mag. 1897;
Upham, " Cause of the Glacial Period," Trans. Viet Inst., 1897. 44 CORRESPONDENCE ON [Minutes of
Mr. Paterson. 12 yards or so from the River Aire at Shipley; in the heading of
which the river-water freely entered and as freely flowed out, as
the river-floods rose and sank. The movement of so vast a mass
of porous material as that described in the Black Canon upon its
inclined bed of silt was a natural result, such silt being converted
by saturation under enormous pressure into a kind of soft soap,
upon which the " slide " would be aided by the hardness of the
material below, it. With so slight a rainfall, the natural stability
of the gravel and its bed of silt, as explained by the Author, was
clear, as also its failure under the artificial deluge caused by
irrigation over a surface so porous.
Mr. Reade. Mr. T. Mellard Reade thought the land-slides described in the
Paper had occurred under very unusual conditions. The silt
which appeared to be the cause of the trouble, whether as boulder
clay forming the matrix of the boulders or as a secondary deposit
without boulders, was described by the Author as being, under
ordinary conditions, perfectly dry except near the surface. In
every deposit in Great Britain, boulder clays, silts, sand, gravel, or
even rock, held a large percentage of water. It was well known
to those who had examined clays, and mechanically analysed
them, that a moist lump of clay, if thrown into water, would melt
very slowly, whereas, if thoroughly dried, and then placed in
water, it would disintegrate and fall to pieces immediately. The
resistance of moist clay to disintegration was shown by the clay
boulders, which were often formed by the rolling of fragments of
clay on the seashore. The abnormally dry silt or boulder clay
composing the terraces of the Thompson River, when subjected to
artificial irrigation, evidently broke up in a similar manner to
dried lumps of British boulder clay when immersed in water.
That this was the true explanation of the great slides was shown
by the fact, mentioned by the Author, that when the clay or silt
naturally received a thin coating of moist clay it would withstand
the action of water in a remarkable manner.
Mr. Robson. Mr. John J. Robson had practised in British Columbia during
1888-89 and had a fair knowledge of the south part of the province. The cause of the slips was, indeed, no ordinary one, and
apparently only by careful observation and experiment in the
laboratory could the peculiar action of this silt when saturated
with water, and likewise when exposed to the elements, have been
discovered. Apart from the dangerous nature of driving tunnels
so as to drain the slip in loose moving earth, it would be necessary
that such tunnels should be low enough to drain the bed of the
slip in order to be effectual; but as the bed of the slip was at or
about low-water level of the river it appeared impracticable, and Proceedings.] LAND-SLIDES IN BRITISH COLUMBIA. 45
even if it were possible another danger would arise when the Mr. Robson^
spring freshets occurred; the river, then rising as much as 70 feet
or 80 feet would again surcharge the bed of the slip with water
by means of the drainage tunnels, and the last state of the case
would be worse than the first.    In the final paragraph the Author
pointed out the only remedy, viz., to stop the irrigation, and in
this he strongly agreed.    This he interpreted as an absolute
stoppage of the  irrigation   ranches,   and   not  only cutting off
the water.    The latter course was impossible, in consequence of
the open and porous nature of the top soil, underlaid with sand
and gravel.     On such land the water soaked in and found its
nearest way by underground channels into the back and bed of
the slip; and there was also the leakage from  the irrigation
ditches, which were very rudely constructed by the farmers, and
were seldom water-tight, so that the only remedy was to entirely
stop the irrigation.    In the Paper it was stated that the two
largest slides ooourred before the construction of the railway, and
he would have thought that when it was found to be imperative
to carry the railway across these slides, and the cause being
known, that the railway company would have at once bought out
the ranchers instead of continuing such costly maintenance works,
for in those early days of British Columbia all the ranchers of the
district could have been bought out for one-half the money which
had already been expended.    It naturally arose whether these
land-slides could have been avoided in locating the railway, and
in this case he did not see that it could have been so arranged
without carrying it at a higher level by some 300 feet or 400 feet
than at present, which, apart from the inconvenience of such a
route, would have been a serious obstacle to the general downgrade from Donald to Vancouver (a fall of 2,500 feet in 458 miles).
In other cases, however, the railway had been carried over old
land-slips which might have been avoided; he particularly referred
to one near Port Harvey, on the Fraser River, some 50 miles east
of Vancouver, which he had examined; and, although no further
subsidence might have occurred, yet he considered that engineers
were not justified in running such unnecessary risks in order to
save -^ mile of cuttings.    The  Pacific section of the  Canadian
Pacific Railway was intensely interesting to the engineer.    The
route was beset with natural obstacles and difficulties, and some
idea  of the  nature of the work might be imagined from the
fact that between Donald and Vancouver there were over 1,900
bridges, some of which were over 200 feet high and others over
2 miles  long.    Indeed, no  engineer would have selected such
a route, as there was no doubt that a much easier one existed 46 CORRESPONDENCE ON [Minutes of
Mr. Robson. farther north; but political motives had influenced the selection.
The Author touched upon the topography and climate of the
country; respecting the former it was a veritable sea of mountains,
whilst the interior plateau was only a comparative term, indicating
a stretch of high hills and undulating country lying between the
more rugged and lofty mountain ranges; it was, indeed, the
roughest and most rugged country in the world. When, in
addition to the foregoing conditions, the dense forests on the
Pacific coast were added, with the trees between 200 feet and
300 feet high, and fallen timber (the accumulation of centuries)
so thick in places it was possible to go for miles without touching
the ground, the work of the engineer was difficult and arduous in
the extreme. The climate was, however, so delicious that such
drawbacks as the above were forgotten; it might well be likened to
an improved Devonshire (on the coast), the greatest heat registered
being about 90° F., whilst in the winter it seldom fell so low as
20° F. The climate was, as thel Author stated, rather less
genial in the interior, but taken as a whole he had never heard
of nor experienced a better.
The Author. The Author, replying in writing to the Discussion and
Correspondence, thought the last paragraph of the Paper had been
misunderstood. It had been written with the experiences of the
Ashcroft, and with the Great Northern slide fully in mind, as
well as the nature of the material to be dealt with in the slide,
and hence by " cutting off the irrigation water above " was meant
the absolute stopping of all irrigation in the neighbourhood of
the slide, for the reason that the nature of the material, as
described in the Paper, was such that no other means suggested
would be effectual. It was not necessary to add to the description
of the nature of the material composing the slides, or the peculiar
action of the water upon it. The ease was most clearly and forcibly
expressed by Prof. Jules Gaudard. He felt gratified at the kind
and complimentary discussion of his Paper; but he could not
agree with some of the opinions expressed as based upon the facts
he had stated. He had had much experience in dealing with
ordinary land-slides while in charge of the construction of a
portion of the Cincinnati Southern Railway through the coal
measures of the Cumberland Mountains in the States of Kentucky
and Tennessee, during the years 1874-1880, and he had made a
somewhat careful study of the great mud tunnel upon that road.
These slides, in most instances, had been caused, during the
building of the road, by cutting through the lower edges of the
strata, when the heavy rains of the winter wetted the layers of
blue clay, thus lubricating the inclined plane on which the super- Proceedings.] LAND-SLIDES IN BRITISH COLUMBIA. 47
incumbent mass was resting, when in some instances hundreds The Author.
of acres slid down the mountain side into the valleys below. In
other cases solid masonry tee-abutments, built upon clean and
solid bed rock, were carried off their foundations and moved
bodily down the hill by the force of a slide behind them. These
cases were similar to those described by Mr. Foster Crowell, and
to these in general were applied the usual remedies, such as drain
ditches above, tunnels, drifts and drains below, with very satisfactory results. None of these remedies, however, would be of
service if applied to the great slides under consideration in
British Columbia, on account of the nature, condition and position
of the material composing them. The cutting of a drain ditch
around the back of the slip to take the water off would be more
than useless: (1) The water causing the damage was that applied
upon the cultivated fields (p. 7), and after being once applied
could not be collected into a drain, for it rapidly percolated downwards into the boulder clay and silt underlying the top soil (p. 8).
(2) If the water was collected into an ordinary ditch it would
work more damage than good in such clay and silt. This method,
a ditch filled with water running around the back of the mass,
was that used in England, and especially on the great clay banks
of the Isle of Wight, for the purpose of bringing down great
masses of clay for commercial purposes. If, as had been suggested,,
this ditch, and also the small canals leading to the farms, were
lined with puddle, cement, or asphalt, and the water kept in them,
no irrigation could be accomplished, for, as soon as the water
should be taken from the ditch and spread upon the fields, the
damage would be done. Hence this remedy, if carried back far
enough to be effectual, meant simply cutting off the diseased
tail of the dog close up behind his ears—which was the true and
only real remedy. Careful study of the nature of the. material,
as explained in the Paper, and so forcibly expressed by Prof.
Jules Gaudard, would show that no system of tunnel drains, drifts
or surface drains through or at the lower side of the slide would
be of value in such a moving mass of silt, there being no " stable
bed inclined towards the river " on which it slipped; but " the
mass would be more or less overturned and broken up in
every direction by deformations and undermined at its base by the
formation of a slippery plane surface lubricated throughout by
the slurry." If such drains and tunnels were partially successful
for a time, they would not be permanent, for in such material no
system of tunnel drains would carry off all the water unless built
so close together as to form one complete cellar under the whole
mass, which would be out of the question.    Hence a portion of 48 CORRESPONDENCE ON LAND-SLIDES. [Minutes of
The Author, the water would sink lower—even below the level of low-water
in the river. After a time a new " subterranean liquid layer or
pocket" would be formed lower down, and the whole operation
would be repeated. As stated in the Paper (p. 13) and remarked
by Prof. Boyd Dawkins, " the solid geology of the valley ....
had nothing to do with the cause of the slips." But the position
and the condition of the solid geology at this time underlying
the great mass of silt became a very important matter of study
in determining the proper methods to be applied for curing the
evil as it now exists. Therefore he could not agree that the
position and action of the ancient Pliocene river was unimportant.
It was only by the study of every known detail, both ancient and
modern, in such a problem as that under consideration, that a true
knowledge of the facts could be arrived at, or the proper estimate
of the proper remedies formed. It was from the study of this
ancient Pliocene river bed that had led him to conclude that no
tunnels or drains from below would be effectual, for on examining
other details it seemed that the mass of silt under the slides
extended below the present bed of the Thompson River, and was
being forced up into the river at its edge. The neglect by the
Dominion Government of the mere detail that above this one old
' south slide a 35-acre field was being constantly irrigated—a field
at that time worth perhaps £35—had cost the railway company
some £10,000. It was not his purpose to criticise the Dominion
Government, which first built the railway, or the Canadian
Railway Company, which had since worked it; but, in justice to
■ the engineers who first built and had since had charge of the
railway, it must be said that they at first and continually
recommended the purchase and wiping out of the farms entirely.
Whether this want of action had been due to false economy,
neglect of detail, or political necessities, was unknown. The
possibility of locating the railway on the opposite side of the
Thompson was out of the question. The nature of the ground
was the same, and, since being irrigated, large slides had
occurred also on that side. As far as the old slides were concerned, the location was correct; but the land should have been
bought at first, and the farms, as such, wiped out of existence.
The absence of sufficiently accurate borings precluded his illustrating his reply by cross sections through the slides; he did not,
however, consider they would add to the information he had given
in the Paper in elucidating the subject. He could only repeat
his conclusion that there was no way of preventing the disastrous
effects of these land-slides upon the railway except by stopping
entirely all irrigation of any lands whatsoever above them.  /7y/   /-.
OBSERVED
□   □   n   □   ra
R . B    STANTON.
Minutes of Proceedings of The Institution of Civil Engineers. Vol: CXXHI. Session 1891-38 Part H.  Bi 

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