earth in its original position, it should be determined whether the
undisturbed portion of an embankment will support the weight when
unaided by counterforts with a foundation in solid ground at the toe of
the slope, trenches and drains upon the slopes, and perhaps a rough
stone bed below the seat of the slip acting as an open drain over part
or the whole of it. In any case provision must be made that there is no
localization of water between the original embankment and the filling or
the counterfort. In some soils, particularly those having seams of sand
or silt, the slipped earth frequently becomes displaced in layers, and
if allowed to remain, each bed will form a water seam upon which any
stratum can slide, and then the earth may not be at rest until the slope
is very flat. The removal of the whole mass is the cheapest remedy. The
surface left bare by the slipped earth should be trimmed, and all
fissures in it be filled so as to prevent any accumulation or lodgment
of water, but the slipped earth should only be excavated in short
lengths, as it may render support and keep part of the face covered, and
it should be remembered that although the upper portion of the fallen
earth may be the drier it may not be the most stable.
The system of removing the slipped earth, erecting rough rubble walls at
intervals of 20 to 30 feet projecting as far as the face of the original
slope, and then filling the intervening space with the material that has
slipped, when turned over and punned, has been successfully adopted. It
is advisable to cover with turf the replaced earth in the slope, unless
some other protection is supplied. When any signs of movement afterwards
take place a few additional counterforts, which, may also be made to act
as drains, will generally restore the equilibrium. This system relies
upon preventing movement in earth by separating the masses of the
slipped soil, and draining and supporting them in detail. The
foundations of the counterforts must be in the solid ground and not
merely below the seat of the slip. It may be impossible to drain the
site of a slip or the soil that has moved without dividing it into
portions, the chief object being to thoroughly drain the site and the
slipped earth, so that it is practically encircled with drains and any
water prevented from collecting in or upon it.
When the land is of little value and a cutting is in a mountain or
hill-side, it may be advisable to assist an extensive slip, provided it
happens before any public works are opened for traffic; and to remove
the earth by loosening it by the action of a stream of water until it
slides away, as draining or supporting it may be insufficient. Under
such circumstances it is the best course to adopt, especially if a
stream of water can be readily diverted to it as the unstable soil is
finally disengaged.
CHAPTER IV.
NOTES ON THE PERCOLATION OF WATER.—SYSTEMS OF DRAINAGE OF CUTTINGS AND
EMBANKMENTS IN DIFFERENT KINDS OF EARTH AND UNDER DIVERSE
CONDITIONS.—THE CONSTRUCTION OF CULVERTS, PIPE-DRAINS, TRENCHES,
DITCHES, AND CATCHWATER DRAINS.
With respect to the percolation and drainage of water in cuttings and
embankments, in cuttings the chief consideration is to gently extract
and conduct the water so as to avoid any accumulation or localised flow
beneath the original surface of the ground in order to prevent the
surface water eroding the slopes or collecting or forming a course,
saturating the ground outside them, and to ensure that the earth is not
more charged with water than when in its normal condition; for, as soon
as the state of absorption has reached that of dissolving or separating
the particles, however fine, aqueous action is likely to produce slips,
and a flow of water or vibration will supply the disturbing force
necessary to commence a movement. In embankments one of the chief
precautions is to obviate any flow of water upon or at a few feet
beneath the land upon which an embankment has to be deposited, as it
will disturb the feet of the slopes and the base, reduce the adhesion to
and the friction of the tipped earth upon the ground and form a sliding
surface.
An accumulation of water upon the formation must be prevented, and, as
in the majority of cases a railway or road is not level, any collection
of water at the commencement of an incline or at a change of gradient
should be provided against, and especially any localisation of flow down
the slopes from the formation; the main point being to keep a cutting or
embankment in a uniform state so that settlement is equable. By
carefully watching the effect of heavy rain upon the slopes and the
formation, the places where water amasses can be traced, and means used
to restore the surface to the same condition as the other portions of a
cutting or embankment. As water is the chief cause of slips, the
friction and cohesion of earth being impaired and, perhaps, destroyed by
it, it is obvious that at the time percolation, which varies greatly
with the seasons, is at its maximum, _i.e._ when the earth becomes in a
damp or wet state, slips are to be most expected, and particularly soon
after the commencement of the wet season. It is known that upon the
thawing of a heavy fall of snow, and of quickly succeeding and separate
falls of snow, percolation is great; also after heavy and continuous
rains, especially if the strata dip towards a river, and in the case of
springs whose yield depends more upon percolation than the amount of
rainfall, a wet winter will cause an increased flow some time after,
when the earth may become saturated.
If it could be determined at what depth in any earth in any state
percolation, evaporation, and meteorological influences would cease, and
also their effects, rules could be deduced for guidance in draining
cuttings and embankments. The manner of the execution of ordinary
cuttings and embankments is so dissimilar to that of filter-beds of
waterworks, that the experiments made for such purposes are only of
comparative value for the former works, for the condition as well as the
character of the soil affects its permeability by water. For instance,
in cuttings, with the exception of some surface disturbance during the
process of excavation, the removal of vegetation or a covering, and the
exposure of the ground to the atmosphere, &c., the normal state of the
earth is not much altered; but in embankments different soils may be
intermingled in a manner almost unknown in nature, the varieties of
mixtures of earth being most numerous, and they may be in every
condition of compactness, dryness, and dampness amounting almost to
saturation, and in any case, therefore, percolation is temporarily or
permanently increased consequent upon the earth having been disturbed
and loosened.
The general principles of the percolation of water are here only briefly
referred to, as they particularly concern slips in earthwork: but
obviously the quantity of percolated water greatly affects the stability
of a slope, for the surface water should not be guidelessly allowed to
soak into or be absorbed by the ground at the top, and so proceed
through and down the slopes, as then the pent-up water tends to press
out the face which may be temporarily sun-dried. As in excavating
cuttings the surface is bared and vegetation removed from the soil,
water has easier access, and unless the ground when excavated is
immediately covered as before, its normal state is not preserved. One
point of considerable importance is to ascertain whether in any cutting
or embankment percolation is uniform and regular; some infiltration will
necessarily take place, as water will gravitate from the top to the
bottom and will find the easiest course or line of least resistance,
which may not be at the lowest level.
As the amount of the annual rainfall varies greatly according to the
country, and, even in England, considerably in a small area, the earth
will be more affected in one place than another; for instance, 48 to 50
inches is approximately the annual average rainfall upon the extreme
S.S.W. coast of England, being greatest at the highest level nearest the
sea and to leeward; it diminishes gradually from W. to E. to from 26 to
24 inches, the minimum of about 20 inches being in Essex. The
differences of quantity must therefore be regarded; but such rainfall is
as nothing compared with that of tropical lands, for the fall often
continues many hours, and yet equals and perhaps exceeds an inch per
hour. The heaviest fall and its usual time of appearance should be
ascertained, as earthworks may have to be constructed in a peculiar
district where the rainfall may be more than double that of the average
wettest district, and it will often be much greater at the foot or the
top of neighbouring hills than on a flat coast. Local information from
reliable sources is the best guide when confirmed by general knowledge.
In the tropics 100 to 200 inches in depth of rain instead of 20 to 30
inches has to be treated, and frequently half the total annual rainfall
in England comes down in twenty-four hours. It is almost superfluous to
name that the protective works which would be amply sufficient in one
country may be useless in another, simply because of the variation in
the amount of the rainfall and the capacity of different earths to
resist or invite the percolation of water.
Obviously percolation will vary considerably with the seasons, and a
succession of wet periods or a continuous downpour will increase the
quantity of infiltrating water; but the effect of a fall may not be
experienced until some time after it occurs, as in districts or rivers
that are fed with water from the thawing of snow upon surrounding or
distant hills water reaches the lower tracts of country in hot and
sunny, and therefore generally dry weather, when evaporation is the
greatest, and not in winter. Again, there is generally very little
rainfall over flat deserts, but an excess upon mountain ranges which may
surround a desert, and especially in tropical countries experience has
proved that storms and rainfall are often local and extend over a small
area, one district being more liable to such a visitation than another;
they are also, as usual, of irregular duration and severity.
The position of a river may also affect the percolation of water more
upon one side of a valley than another, for a river seldom has its
course in the centre of a vale, but is generally nearest to the steeper
and higher side of a hill. The configuration of a country governs to a
great extent the flow and quantity of the rainfall that sinks into the
earth, as in a hilly country and in impervious soil the water is quickly
discharged into an adjacent river, taking the easiest course. In a flat
country and pervious soil rain percolates the earth and saturates the
ground, or reappears in springs.
As by drainage the retentive power of the soil is not allowed free
operation, water rapidly flows into the drains instead of being chiefly
held by the earth and watercourses, and channels are sometimes created;
and where the rainfall is heavy or occurs in a comparatively short time
floods may be caused, although the soil when drained, and therefore in a
drier state, absorbs more water than when undrained and in a damp
condition, water will pass through it quicker, and the discharge is thus
increased in volume and velocity.
Earth may be in a damp state, either from mere surface percolation and
accumulation of water, or from springs which may never cease to flow; on
the contrary, in rainless districts, from the almost perpetual daily
drying power of the sun, the earth is sometimes found to be firmest and
hardest upon the top, and for a few feet below it, than at greater
depths. Separate masses of vegetation usually indicate damp places in a
bare country.
As rain flows more quickly from non-absorbent soil, such as rock, and
slowly permeable earth, as the clays, than from porous soil, the surface
discharge is greater; and unless the water is guided, pools are likely
to be formed and weak places created, especially if the ground dips
towards a cutting. It is well to remember that a cutting being excavated
upon the side of a hill or upon table-land may change the direction of
the flow of the drainage waters, and an embankment may obstruct and
interfere with them, and should the strata incline towards the
excavation, it obviously favours a discharge of water into it.
If water permeates the soil or trickles down through fissures or veins,
it will continue to do so until an impervious layer is reached, when it
will be deflected and may become a current; therefore, whenever a
permeable stratum overlies an impermeable, and the impermeable earth
inclines, an increased flow may be expected, as also near the junction
of tributary waters with streams or rivers.
In a drained district, water will not usually be encountered in large
quantities until a depth is reached below the level of general drainage,
_i.e._, about the invert of the nearest drains or sewers, especially in
porous soils, but in the case of a pervious subsoil, such as sand or
gravel, or if the tides rise in an adjacent river or the sea to the
level of any foundations or above it, more water may percolate over the
site or drainage area than has time to flow away between tides, the
water will then rise, and systematic pumping becomes a necessity, unless
the volume of the ingressing waters can be sufficiently reduced by the
deposition of an impervious layer upon or in the river-bed or sea-shore,
or by sheet-piling or other means, or the flow confined, which may be a
risky operation in loose soil, depending upon its resistance to scour.
In clay soils, so far as slips are concerned, the action especially to
be feared is the trickling of water down fissures which may extend to
depths below the bottom in cuttings and create slimy surfaces,
disconnecting masses of earth, and finally offering a ready means of
movement, which swelling of the clay or vibration may complete. The
experiments of Mr. Baldwin Latham, M. Inst. C.E., on the absorption and
retention of water by clay soils, gave the following results:—The
stiffest clays retained the greatest quantity of water. Clay soils can
absorb and retain from 40 to 60 per cent. of water by weight. Marly
clays hold less water than the pure clays. In the case of loamy soils,
the percentage of water retained varied from 35 to 60 per cent. by
weight, the mixture of sand and clay, therefore, limited the amount of
water which it would naturally hold.
As in chalk soils fissures occur, the percolation of water and the
effects of the atmosphere through the pores causes movement, and even
crevices and breaks in rocks are not to be disregarded with impunity, as
they are channels of disintegration.
Most earths when dry attract water, but if they are regularly irrigated,
they require less moisture, depending upon the nature of the earth, and
slope and relative level of the land. All soils when broken, as in
embankments, absorb more water than when in an unbroken state, as in
cuttings; for instance, it has been found by experiment that clayey and
retentive earths will absorb about 7 per cent. more water, and light
porous soils about 6 per centum. Of course, the increment varies. Even
wet retentive soil, if handled, becomes considerably less impervious to
moisture.
Mr. Evans, F.R.S., has proved by experiment that percolation through
pure chalk is much greater than through ordinary top soil consisting of
gravel, loam, and mould, both being covered with turf, and that in
winter the average proportions of percolation are as about 1 for soil to
1·5 for chalk; in summer 1 for soil to 2·6 for chalk. The depth also to
which chalk will allow a passage of water is some 60 per cent. more than
ordinary top soil.
Rain will percolate through chalk or any open soil until it meets an
impervious stratum, or to that place which is in a state of saturation,
when the water must either flow away or the level of saturation must
gradually rise. This causes rivulets to burst out in places after heavy
rain, when the water has had time to percolate, and the rainfall has
exceeded the average, but such springs will cease when the local excess
has terminated. However, in cuttings it is the flow from fissures that
is to be feared, and their size may indicate the quantity of water that
may be expected to issue from them. It seems to be generally agreed that
the supply of water from chalk is derived from rain, which percolates
through innumerable fissures, and that in all rocks, whether limestone,
sandstone, granite, sand, or clay, it is by means of the fissures,
seams, and veins that the supply of water is obtained from rain, and
springs created.
With regard to the percolation of water through sand, it may always be
expected to be very considerable, and the soil may under certain
conditions become water-charged. Mr. Greaves, M. Inst. C.E., has shown
by experiments that the average percolation through ordinary top soil is
only about one-third of that of sand, but the evaporation from a surface
of ordinary soil was about four times more than from a similar surface
of sand, and also the amount of percolation in ordinary top earth was
small on the whole, and, perhaps, the percolation through ordinary
ground would be about 25 per cent. of the rainfall, but 80 per cent. in
average sand.
Experiments have also shown that the absorbent capacity of sand
decreases regularly according to the fineness of the grain, and that
“some sandy soils will not absorb more than 20 per cent., but sandy soil
containing peat, as moorland, as much as 80 per cent., both computed by
weight.”
The quantity of water absorbed by loamy soil will vary considerably
according as clay or sand preponderates in the mass. Earth may become so
mixed with coarse or fine sand that, when saturated, it approaches the
condition of a quicksand.
The general effect of percolation has been briefly described as follows.
Upon water entering the pores of an earth it displaces the air or liquid
previously present, forcing the former upwards into the atmosphere, and
the latter downwards.
Having briefly referred to the percolation of water in cuttings and
embankments, the drainage is now considered. It must be either
precautionary, _i.e._, to prevent a slip, or remedial, _i.e._, to drain
a slip.
The aim of any draining operations to prevent slips in earthwork is to
search for the source of water discharge, to tap and gently conduct it
away and prevent it reaching, accumulating, percolating, or being
confined within the slope of a cutting, which it may then reduce to a
pulpy condition; its free effluxion being most important, as also the
lessening of the percolation of rain and surface-waters. The drainage of
cuttings or embankments may consist of wells, culverts, closed or open
channels, pipes, and tile drains of every reasonable and economical
form, and may be placed in various positions. To describe them and the
different systems of draining is to open up a subject requiring several
volumes; here the endeavour is made to indicate whether elaborate or
ordinary drainage is required, or mere water-tables and surface drains,
and care in the process of excavation and deposition, protection of the
slopes, and in giving them sufficient inclination to prevent movement.
If possible and time allows, the drainage in treacherous soils should
always be commenced before the main excavation, and, in any case,
simultaneously.
Rock and solid impermeable earth may merely require to be surface
drained, but all treacherous and porous soil, deep draining; and
granular soils, which usually exude water from the whole mass, demand
different treatment to those earths which discharge water at particular
places; but it may be most difficult to drain a mixed soil, such as
sandy loam and silt. With the exception of a counterfort and drain at
the foot of a slope, and an impermeable catchwater drain upon the slopes
and top drains, to prevent and lessen surface percolation, the best
method to adopt in earth of this description may be to sink wells at
intervals to intercept the flow or percolation of any ground waters; to
attempt to drain or draw out the water in the soil will end in
comparative failure. To reduce the volume of the percolating waters is
the object to be attained, and then evaporation, vibration, which tends
to shake down water, and time may gradually convert the earth to the
desired drier condition. The wells can be filled with broken stone or
coarse gravel to support them, and prevent their closing.
It is an advantage to prevent the percolation of water into soil that
will not readily part with it, such as the clay earths, as it may be
economically impossible to drain or restore the earth to its normal
condition, and should the strata be upheaved, intermixed, and of a
permeable and impermeable character, a scientific application of
drainage can alone succeed. When the source of the water is ascertained,
it can be seen whether a complete system of drainage is necessary over
the whole of a cutting or only a portion of the slopes. Dampness and the
egression of water may be merely local; if so, by boring a hole and
inserting a drain into a slope to tap the flow it may be cured, the
surface being made dry by a layer of ashes or other absorbent material.
Pipes or tile drains may be sufficient when springs exist, or the flow
of water is local, and in loose soils it is especially advisable to
provide openings for cleaning any covered drains. Brickwork, masonry,
concrete, pipes, or other rigid drains, may not be suitable for ground
likely to unequally subside, as they will probably crack or leak, and
loose-jointed pipes or over-lapping tile drains may be required. In
treacherous clay soils surface longitudinal and transverse drains will
most probably be insufficient, and deep draining of the mass be
necessary, also the ground upon which earth is deposited will require to
be drained and a layer of rubble stone placed upon it, a cutting or
embankment being divided into small drainage areas by deep open dry
stone trenches.
Water may not sufficiently percolate into a hill, either because of its
surface being covered with vegetation, or the soil being of an
impervious nature. It may then flow down the slope of a cutting which
will probably be bare and unprotected. To prevent slips the discharge
must be carefully controlled and led away, particularly when the
formation is drift soil upon rock, or the earth will be liable to
saturation and degradation; also to prevent a flow of water under its
seat, and upon the natural ground, should an embankment be deposited
upon it.
When a slope upon the hill-side of a cutting is of considerable length
and steepness, it is advisable to bench it and divide it into a series
of terraces and short slopes, and to provide catchwater drains, with
impermeable surfaces so as to prevent any surface water attaining a high
velocity, and scouring power. All surface water upon the side of a hill
should be controlled, and catchwater drains may suffice to do this.
Should an impervious stratum be upheaved so as to make a reservoir wall
for water under the seat of an embankment, it is useless to
surface-drain the valley side of it in sidelong ground, as it will not
affect the waters that trickle down the hill, which will be dammed up to
the top level of the impervious stratum, and may saturate the seat of an
embankment and cause a slip. In such a case through drainage must be
created from the hill to the valley, and the impervious upheaved cap
must be pierced so as not to interfere with the passage of water.
In countries where there is a certain dry and rainy season, the
necessary provision required for drainage must not be computed from the
visible effects of the rainfall soon after the wet season has commenced,
but the maximum flood may be discerned when the earth has absorbed or
retains the moisture evaporated during the dry season, and becomes
water-charged. In the tropics or exceptionally wet districts the only
effectual method of draining may be to divide the area into small
portions, as the rainfall may be so great, sudden, and continuous that
unless it naturally flows into a channel, which should not be diverted
or its course be materially altered, it will be impossible to control
the waters. In an exceptional situation where a railway must be located
in a ravine and close to a river, and the material of which an
embankment is made is compact earth or the ground firm, it may be
advisable to allow the overflow waters of a river, or the flow of
surface water towards it, when the extreme flood level is known, to
gently pass over a line of railway, it being kept at such a level as not
to impede free working, and to ensure that any back water is not
prevented from escaping. If not, an embankment may slip and require
extensive and frequent bridges, culverts, or drainage channels in order
to provide sufficient waterway, and to attain permanent stability of the
embankment.
In all cases it is necessary to determine the depth to which the drains
must be placed to be effectual, and their position, extent and number,
and it should be remembered that the fewer there are the greater will be
the velocity and discharge; also if many small drains are inserted the
soil may stand, but if only one or two are made the surface may succumb
to the erosive action of the flowing waters. The provision of a
water-table or the mere surface drainage of a cutting, or the seat of an
embankment may be of little use, as the superimposed soil may slide upon
a stratum, and unless this bed is thoroughly drained rain may quickly
destroy the equilibrium. Of course, in the case of a slip, the drains to
be effectual must be placed below its level and down to the layer upon
which movement has occurred, especially in clay and retentive and
impervious earths, for instance, surface drainage in yellow clay is
almost useless.
Should the source be known from which the water issues the drainage may
be local, and if a spring be unsealed it may be necessary to insert
pipes in the slope, for until the spring is tapped and guided no system
of drains may be effectual.
By inserting a stand-pipe over a spring, the height to which the water
will rise will approximately show the head-level of the supply. If
possible, this should be ascertained, as it may happen that the water
can be drained by gravitation in pipes without much excavation being
required in the slopes or formation; but care should be taken that no
water is allowed to settle or accumulate for the purpose of its being
conducted away unless upon a protected surface.
With regard to catchwater drains upon the cess at the top of the slopes,
they should be cut before the excavation is commenced; and it is
important to remember that instead of their affording protection by
guiding the surface waters, which would otherwise proceed towards the
slopes, they may increase the percolation by localizing the water and
allowing it to accumulate and find its way to the slope; and in sidelong
ground it may, therefore, be necessary to protect their valley more than
their uphill side, as the surface water will impinge against it; but
when they are practically impervious and gently direct the surface water
they are advantageous, and in permeable soil, unless they are so
constructed as to be impervious, perhaps it is better to have none. They
should be as reasonably distant from the top of the slope as is
convenient without weakening the foundations of the posts supporting any
fencing, and in order to quickly discharge the water and lessen the
chance of their becoming choked by detritus or ice, they should have
considerable inclination. As the adoption of even a moderate fall in the
drains may erode the side ditches and cause water to percolate to the
slopes and make a water-seam, it may be necessary to protect the bottom,
and as the depth of the side drains in order to be effectual may be
considerable, according to the character of the soil, the sides may also
require to be covered and supported. In cuttings in many soils
sufficient stones can be picked out to cover the surfaces of the side
drains, and they can be roughly packed, the smaller stones being rammed
into the interstices between the larger, which will gradually become
filled; also a covering of brushwood, rammed earth, or puddled clay can
be used, or other expedient which occasion may suggest. The inclination
must not be steeper than the natural or protected bed can bear without
the water scouring it, and yet should be sufficient to prevent any
deposit or choking, and the drains should be cleared regularly, and
especially in the autumn in England or before the wet season commences,
and all depressions in which water can accumulate should be levelled in
order to assist easy discharge. Small open drains become choked somewhat
easily, and it is therefore advisable to make them according to the
nature of the soil, situation, and requirements not less than 1 foot 6
inches to 3 feet in width at the top and 1 foot at bottom; they may be
from 1 foot to 3 feet in depth. Care should be taken that the bends are
not too abrupt or the water may make its own course. A gentle curve
considerably increases the flow. The angle of a bend should be as easy
as possible, and not exceed 26° or 2 to 1.
In sandy and loose soils, if unprotected, open drains may be difficult
to maintain even when filled with broken stone, and covered or pipe
drains be necessary, and those loosely filled with stones or faggots may
not succeed; in any case no run of the sand must be allowed, and it is
advisable to rapidly construct them. In peaty soils, from subsidence of
the ground consequent upon draining, the drains often become choked. The
depth of a drain in the formation may require to be deeper than
elsewhere, as at the toe of the slope, the weakest part, the water will
generally be most abundant.
Upon railways, the advantage of thorough drainage of the formation as
regards the stability of the permanent way and reduction of the cost of
repairs is proverbial, but here is only named in its relation to the
prevention and the reparation of slips.
When field drains are intercepted in the slopes, drain pipes or timber
ducts should be joined to them and be connected with the general
drainage of a cutting. Draining the slopes, providing outlets for the
water, and also support to the earth can be effected by means of a
counterfort of permeable material at the toe of the slope, with its
foundation a few feet below formation level, with open drains at right
angles or obliquely to it extending from the foe to the top of the
slope.[1] These open drains and trenches can be filled with stones,
gravel, hard chalk, ashes, brushwood and gravel, broken bricks, burnt
clay or other suitable material; and a simple covering of picked ashes,
&c., over a moist place in a clay cutting of little depth may be
sufficient. The distance apart of such trenches in clay cuttings
generally ranges between 10 and 33 feet; their location must be governed
by the consideration that they exist in order to prevent a localization
of water in the slopes: their width is usually from 2 feet to 8 feet,
and most frequently 4 to 6 feet, and the depth from 1 foot 6 inches to 3
feet and upwards, below the surface of the slope, but in very wet
embankments a width from 6 to 10 feet and of such a depth as the soil
will allow without extensive support. If the earth has not slipped, and
it should be found that the ground is wet for from 6 to 8 feet or so
beneath the surface, and the depth of the trench is made about half that
of the wetted soil, it is sufficient to collect the water, but if it has
slipped most probably such drains will not be effectual until carried
down below the seat of movement. At the foot of a slope these drains
should be connected with longitudinal channels parallel to the formation
to gently convey the water to the nearest outlet. The location, distance
apart, depth, width, and direction of the trenches must be governed by
the nature of the soil, the depth of a cutting or height of an
embankment, the area of the surface to be drained, the quantity of water
in it, the presence of water-seams and weak places, and by other minor
considerations.
Footnote 1:
_Vide_ Chap. VI., pp. 111, 112.
With respect to the material with which the trenches should be filled, a
uniform substance having considerable power of absorption, and but few
particles between the interstices, is to be preferred, as the trickling
of water and vibration causes the smaller material to fall towards the
bottom of the trench, which, therefore, may become partly choked, and
free drainage be interrupted at the toe of the slope, the most
vulnerable place. As sand is always present in unwashed gravel, it will
gradually flow or fall to the base of the trench and will prevent equal
drainage, but properly burnt clay, being a more uniform and fragmentary
substance, is better than sandy gravel, as it is dry and porous; but
much depends upon it being well burnt or it may weather. Ashes and chalk
are excellent collectors of water and are usually of an even character,
but ashes are better than chalk, as the latter material, unless very
hard, is liable to become disintegrated by the action of the atmosphere,
rain, and frost. The difficulty with respect to ashes is to obtain them
in sufficient quantity, free from dust, and of the requisite size. When
weight is required as well as drainage, burnt clay, gravel, or chalk is
to be preferred to ashes. Burnt clay, although burned upon the site, is
generally a more expensive material than gravel, but the amount of
moisture in the clay will principally determine the quantity of fuel
necessary to burn it, and therefore the cost. If shale is present, by
thoroughly igniting it the expense of burning may be nearly reduced to
that of lighting and turning over, as it will usually burn unaided. All
trenches should drain into pipes placed below the formation or into open
drains at a sufficient distance from the toe of a slope as not to
deleteriously affect it, in order that the water may be controlled and
gently conveyed to an outlet.
The chief objection to open drains is that all excavated trenches or
inserted drains in the slopes destroy the cohesion of the earth and aid
in detaching portions of the surface. If the cohesion and adhesion of
the soil were the same under every condition, this would be a cogent
reason against the system, but, as in earthwork, every degree of
moisture from dampness to saturation may be attained, the cohesive power
is a very variable quantity, apart from the effects of vibration; and
also open drains undoubtedly do cause a slope to be drier, and moderate
local humidity, and therefore increase the cohesion and general
stability of the part drained; the only fear being that from inattention
they may become choked; then they are dangerous, as they will
permanently collect and retain water instead of temporarily retaining
and gently guiding it. A careful consideration of the circumstances may
much reduce this objection by indicating whether it is advisable to have
only a few deep, or several small surface drains. Provided proper
precautions are taken, experience indicates that filled-in trenches in
the slopes are generally successful, and certainly are simpler and
cheaper than a retaining wall at the toe of a slope. In pervious soil it
may be economically impossible to drain the slopes unless they are
divided into sections, and should the material in the trenches be
well-packed and pressed down, it may even increase the friction between
the separated portions.
In connection with the drainage of cuttings may be named that excavation
should not be allowed to be cast upon the cess unless some distance from
the edge of a slope, and only temporarily for purposes of ballasting and
metalling; as such spoil-banks increase the load and localize the water
to be drained.
A more complete system of drainage may be necessary than those
previously named, consisting of a combination of wells, open, or holding
filtering material; pipe-drains or filled-in trenches, wells with a pipe
leading to a catchwater drain, &c., or other usual methods of land
drainage. For instance, it may be discovered that water issues from a
spring outside the fence or cutting; if so, in order to drain the slopes
it may be necessary to sink a well to a stratum below the formation
level so as to tap the spring, thereby preventing an exudation of water
upon the slope; this is a better plan than drawing the water into the
slope and then draining it.
When pervious earth overlies an impervious stratum, _i.e._, gravel or
sand upon clay, rough-filled wells at intervals inside the fence
extending 3 or 4 feet into the clay, with an outlet drain, may be
required to prevent a flow of water upon the clay and a wetted surface
upon which the gravel can slide; and it may be necessary to have a cess
on the slope between the top of the clay and the bottom of the gravel or
sand, with a catchwater drain upon it, particularly in a cutting in
sidelong ground.
Should the soil be silty sand, or be charged with water, consequent upon
the formation of the country, it may be impossible to drain a cutting
without a complete system of wells, catchwater drains and pipes, and
even then it may be difficult to separate the water from the earth. In
building a drain-shaft it should be remembered that it may not only be
subject to a compressive strain, but also to transverse strain and
flexure from different pressures of the earth at various depths,
especially when the soil is not the same throughout, and unequally damp.
When “boils” occur in sand-cuttings, perhaps the cheapest expedient is
to place a shaft over the boil, weight the bottom sufficiently to
prevent a movement of the sand, but to allow the water to escape, and
make a discharge outlet after having ascertained its head level: _vide_
Chapter XII. On no account should a spring be stopped, as such action
will result in its diversion to some other place; but the water flowing
from it should be guided and discharged. Weighting may arrest a slip in
any sandy soil, also clay or any impervious material placed upon the
sand, or sinking a well outside a cutting to a depth of some 5 feet
below the bottom may effect a remedy by abstracting the surplus water,
but care must be taken not to disturb the sand.
Slips in embankments frequently occur from the percolation of water
through the formation to the slopes, and so to the toe, the lower
portions become disintegrated by moisture and the effects of weather,
and cause the upper parts to slide or move. To lessen percolation and to
prevent an accumulation of water upon the formation, it is usual for its
centre to be raised a few inches above the level of the top of the
slope. This is undoubtedly a good practice, as it also tends to drain
the ballast, but it may be nullified in time if the entire width of the
formation be not covered with an impermeable layer or with ballast; for
when an unprotected space remains between the toe of the slope of the
ballast and the top of the slope of an embankment, water is liable to
percolate through the cess and cause a slope to become wet and unstable;
particularly so if the ballast is broken rock and has side walls instead
of slopes, as then a depression will probably be made by the platelayers
or signalmen walking upon the cess. In all cases where the material is
treacherous or likely to slip, it is advisable to cover the top of
embankments of considerable height throughout their width with ballast
or some impervious soil, provided the permanent way is also properly
drained. This is the simplest precaution to take respecting the
preservation of the formation level or summit of an embankment. All
grass, dirt, and refuse should be regularly removed from it and anything
that obstructs free drainage. The nature of the ballast also affects the
evenness of the surface of the formation, as if it consists of broken
rock, the equal and regular packing of the sleepers is not so easily
effected as with gravel ballast; the sleepers are frequently not
uniformly supported throughout their length, the pressure upon the
formation is localized, depressions are formed and water collected, and
slips and subsidences in soils of a treacherous nature may be induced
from this cause, as the equilibrium is soon disturbed. The formation
should be so drained and constructed that water cannot percolate to or
cause the surface to become soft and work up into or through the
ballast, or a state of unsettlement will be produced by water soaking
through the ballast to an embankment, and so saturating part of it and
forcing out the upper portion of a slope. In all close granular ballast
cross channels should be made to lead away the surface water. Transverse
open tile-drains may be required leading to an impervious channel. Water
has been known to percolate through a considerable depth of ballast when
added to restore a sunken embankment, even through as much as 7 to 10
feet when two falling gradients induced a flow of the surface waters to
one place. In certain situations it may be necessary should an
embankment be of clay or treacherous soil when wet, to cover the
formation with an impervious stratum to prevent percolation to the
embankment, and to thoroughly and separately drain the ballast placed
upon it.
At the base of an embankment a ditch should be cut upon the higher side,
or both sides, as near as convenient to the fence. When in addition to
the interception of any surface waters by an embankment the ground is
very retentive of moisture, it may be necessary to drain the seat; with
this object trenches can be excavated at intervals at right angles or
obliquely to the centre line of the embankment, and be filled with some
hard filtering substance, such as stone or gravel, so as to effect and
control the discharge of the waters. Should this be too expensive a
method to adopt, and always provided the surface waters are prevented
from flowing or trickling into the base of an embankment, the ground
might be excavated so as to equally incline downwards towards the
centre, the level at that point being 1 foot to 2 feet below the toe of
the slope on each side, according to the width of the base of the
embankment; a small trench being cut in the centre and filled with
stones, and covered at the top with brushwood or hurdles or other
provision necessary to ensure it being permanently an effectual water
channel, with occasional or other drains to lead the water to the
nearest culvert or side ditch. When it is found that water passes over
the surface of firm soil upon which an embankment is deposited, the
water must be intercepted and led away; and should an embankment be of
retentive earth, in order to tap the water that has flowed and
percolated into it, and to restore the earth to its normal condition and
prevent slips and subsidences, it may be necessary to sink shafts to a
depth of a few feet below the seat of an embankment until the mass is
drained.
With regard to culverts, a settlement or slip of an embankment over a
culvert may unequally strain, fracture, and displace portions of it, and
therefore interrupt the flow of the drainage waters, which may then
reach the seat of an embankment and cause it to be in a dangerous
condition. They are usually necessarily placed at the deepest point of
an embankment, and consequently the most difficult to make repairs. In
such a situation they should be built sufficiently large to allow of the
easy passage of a man, in order that due inspection may be made, and be
constructed of materials of a durable character. Should a naturally firm
bank exist on one or both sides of a stream, it should be stripped of
all plant growth and decaying matter, and be preserved in order to form
a natural wall to relieve a culvert from side pressure, but firm or hard
material must be inserted between the back of the wall and the face of
the stream bank so as to support the wall against the pressure it
receives from the arch. This leads to a consideration of the best form
of culvert. In clay soils, and those which exert pressure from
expansion, especially if the culvert is surrounded by clay earth, the
circular is generally considered to be the best form, and this has been
proved to be so in tunnels in similar soil, with splayed wing-walls to
assist and guide the flow and help to keep a clear entrance. In granular
soils, such as dry sand or gravel, the earth acts differently and more
in accordance with the angle of repose theory of pressure, therefore the
strain upon the arch would be the greatest, and its thrust must be
counterbalanced at the sides, the strain upon the invert being probably
very little and due to the tendency to an overturning movement of either
of the straight walls. However, in culverts as in tunnels, it is
impossible to say the exact amount or direction of the strain, although
it may be approximately computed.
Should it be necessary to erect a culvert upon soft ground, as much of
it as practicable should be excavated and a concrete foundation be
placed thereon; it is also advisable to allow an extra length to that
required by the calculated slope.
A culvert should have an invert unless upon a hard rock bed, and care
should be taken that there shall be no leakage at the springing of the
inverted arch or under it, or beneath the sidewalls at the level of the
surface of a flat stone drain or below it.
The banks of a stream or watercourse should be inspected occasionally,
especially on the up-stream side of a culvert, in order to note whether
they are stable or crumbling away, as then the course of the stream may
be widened or diverted, and so erode the toe of an embankment and cause
a slip of earth. Splayed wing-walls are a protection against such a
danger besides aiding the flow of water through them, and they also
lessen the chance of a damming back of the water and undermining, as
they increase the discharge; for instance, a splay of 53° has been found
to increase the flow about 25 per centum. The surface of the material of
which a culvert is constructed should be as smooth as practicable, so as
to reduce the friction of water flowing past it, which, in the case of
unplaned timber, cast and wrought ironwork, ashlar masonry, brickwork,
and concrete, is about the same, but is some 30 per cent. more when the
surface consists of rubble masonry.
When an embankment crosses a narrow valley, in which no watercourse
exists, instead of a culvert, a bed of loose stones has been placed upon
the ground under the whole area of the seat of an embankment at its
greatest depth, a proper fall and bed being given to it.
The toe of the slope on the up-stream side should be protected from any
wash, and the stone layer be carried a few feet beyond the foot of the
slope on the lower side.
The following general principles it is well to remember in designing
culverts.
When a culvert is of uniform section, which is almost invariably the
case, it should have the same inclination throughout.
Avoid, or ease, all bends as much as possible.
Have splayed wing-walls.
Make provision against undermining by scour or percolation of water.
Have smooth and even surfaces so as to reduce the friction and increase
the discharge.
Have an arched invert to a culvert, and a flat bed stone to all small
surface drains, with complete connection to the side walls.
In some countries having a severe climate, or in high mountainous
districts where the soil is rock and a heavy discharge of flood-waters
occurs, instead of placing a culvert and gathering the waters at or
about the level of the toe of an embankment, in deep hillsides or
ravines an unlined tunnel is made under the embankment in the rock, thus
avoiding a masonry or brickwork structure, which could only be set in
the summer months, and preventing the waters touching the seat of an
embankment and promoting a slip.
When the level of a rivulet allows, and the waters are simply surface
discharge, the system has been used of making two open channels, one
upon each side of a steep valley, thus retaining the waters, allowing
the adoption of two short span open culverts and two channels instead of
one large culvert at the deepest place, and saving expense, the original
bed of watercourse being filled with the surplus excavation. The
embankment consisting of broken rock or hard granular soil, any little
percolation of water along the old bed will not deleteriously affect it,
but will find a passage.
CHAPTER V.
APPROXIMATE SAFE MAXIMUM LOAD UPON DIFFERENT EARTHS.—NORMAL
PRESSURE OF THE EARTH.—THE SAFE MAXIMUM LOAD UPON DEPOSITED
EARTH.—APPROXIMATE SAFE MAXIMUM HEIGHT OF AN EMBANKMENT.
There is no limit to the depth of a cutting except a due regard to
economical construction, provided the slopes are sufficiently flat, and
the lateral and upward fluid pressure in the slope, and formation and
quantity of water not too great for the stability of the earth; but, in
the case of embankments, the load upon the ground and the deposited
material in great measure restricts the height, or necessitates an
embankment being gradually spread out, so as to enlarge the bearing area
as the weight is increased.
The following values of the safe maximum compressive load have been
compiled from actual practice, but are, of course, only intended as a
guide to the safe load for foundations excavated in ground not
artificially deposited. The condition of the earth in each case should
be considered, and in works of magnitude it is advisable to make
experiments extending as long as practicable, and for at least a month,
and it is false economy not to carefully ascertain the character,
condition and other circumstances of a foundation destined to support
any part of a structure, a failure of which may result in serious
consequences; and it should be borne in mind that a continuous surface
possesses greater sustaining power than the same area in detached
portions, as the adhesion of the sides is not destroyed; similarly, the
load that a tenacious earth will support upon a small area is somewhat
greater than over a large area, because the lateral surfaces are
relatively larger in proportion to the area, and, therefore, the effect
of cohesion is proportionately greater; but in loose soils it is not so,
for cohesion exists but in name, and the ground around would be upheaved
upon an excessive load being superimposed. In testing the weight any
earth will support, it is not so much the first settlement, provided it
is not excessive, that it is desirous to know, but whether after the
first settlement it ceases or the earth, as it were, reacts and
rebounds, which it may do in firm ground to the extent of one-eighth to
half an inch. If so, the ground is not overloaded.
After ascertaining by experiment the pressure any earth will bear over a
given area, the object should be to make the soil neither drier nor
wetter than that of its natural state when experimenting, and it should
be maintained in that condition. In testing the weight which a soft
earth will support, some days should be allowed for the sinking of the
test platform, and such subsidence should be ascertained periodically by
careful levels. A month is not too long for a reliable and complete
test, as many soft soils continue to yield. In soft clay soils
considerable depression often proceeds for weeks after a load has been
applied, but except in peculiar earths such settlement will ultimately
be imperceptible, and will practically cease. A considerable margin of
stability should in all cases be allowed. Although it may not be
absolutely necessary to experiment when the nature of the ground is well
known, wherever stability is of great importance, the cost of a
practical experiment being so small, there is no sufficient reason why
an actual test of the sustaining power of the soil should not be made in
the majority of instances, for there are many earths whose friction and
cohesiveness can alone be depended upon for resistance to displacement.
In such cases the initial pressure upon the earth should not be much
exceeded.
Description of Earth. Approximate
Safe Maximum
Load in Tons
per Square
Foot.
Bog, morass, quicksand, peat moss, marsh-land, silt 0 to 0·20
Slake and mud, hard peat turf 0 to 0·25
Soft wet pasty or muddy clay, and marsh clay 0·25 to 0·33
Alluvial deposits of moderate depths in river beds, &c. 0·20 to 0·35
NOTE.—When the river bed is rocky and the deposit firm
they may safely support 0·75 ton, but not more.
Diluvial clay beds of rivers 0·35 to 1·00
Alluvial earth, loams and loamy soil (clay and 40 to 70
per cent. of sand), and clay loams (clay and about 30 0·75 to 1·50
per cent. of sand)
Damp clay 1·50 to 2·00
Loose sand in shifting river bed, the safe load increasing 2·50 to 3·00
with depth
Upheaved and intermixed beds of different sound clays 3·00
Silty sand of uniform and firm character in a river bed 3·50 to 4·00
secure from scour, and at depths below 25 feet
Solid clay mixed with very fine sand 4·00
NOTE.—Equal drainage and condition is especially necessary
in the case of clays, as moisture may reduce them from
their greatest to their least bearing capacity. When found
equally and thoroughly mixed with sand and gravel their
supporting power is usually increased.
Sound yellow clay containing only the normal quantity of 4·00 to 6·00
water
Solid blue clay, marl and indurated marl, and firm boulder 5·00 to 8·00
gravel and sand
Soft chalk, impure and argillaceous 1·00 to 1·50
Hard white chalk 2·50 to 4·00
Ordinary superficial sand beds 2·50 to 4·00
Firm sand in estuaries, bays, &c. 4·50 to 5·00
NOTE.—The Dutch engineers consider the safe load upon firm
clean sand as 5½ tons per square foot
Very firm, compact sand foundations at a considerable 6·00 to 7·00
depth, not less than 20 feet, and compact sandy gravel
NOTE.—The sustaining power of sand increases as it
approaches a homogeneous gravelly state.
Firm shale, protected from the weather, and clean gravel 6·00 to 8·00
Compact gravel 7·00 to 9·00
NOTE.—The relative bearing powers of gravel may be thus
described:—
1. Compact gravel. 2. Clean gravel. 3. Sandy gravel. 4.
Clayey or loamy gravel.
Sound, clean, homogeneous Thames gravel has been weighted
with 14 tons per square foot at a depth of only 3 to 5
feet below the surface, and presented no indication of
failure. This gravel was similar to that of a clean pebbly
beach.
In loose non-cohesive earths the load may be increased, when the depth
is considerable, as the soil has been subject to a greater normal
pressure due to the weight of the soil upon it at any depth; but it is
not advisable to consider such increase of bearing power of the soil,
unless at any depth it is found that the normal pressure augments the
bearing power and makes the earth more dense, which may be approximately
ascertained by experiment. In such event the load upon the base can be
increased by the weight of the normal pressure removed. Supposing 5 tons
per square foot was known to be the safe load upon the surface of the
ground, and at any depth it was found that the normal pressure of the
soil was 2 tons; 5 + 2 = 7 tons placed at that depth would equal 5 tons
at the surface. In the worst case, when the loose earth is of great
depth, and it is certain that it cannot be tapped or disturbed at the
depth at which it is decided to place the foundations of a structure,
provided the load is not more than the normal pressure, it is not
probable that it will subside or slip, as no additional weight is
imposed.
In foundation and general work, rocks are usually not loaded with a
greater weight than from 8 to 18 tons per square foot, according to the
character of the rock. As the crushing strength has been principally
ascertained from cubes, and not from prisms, rectangular blocks, or
irregularly shaped pieces, and as the resistance of rocks to transverse
strain or breaking across is considerably less than the compressive
strength, and varies greatly and not always according to the crushing
resistance of the material, from 8 to 20 tons per square foot is a
prudent limit for the safe load, and should not be exceeded unless under
exceptional circumstances; as unequal bearing may greatly intensify the
strain, and irregularity in the texture may reduce the resisting powers
to that of the weakest part. Sandstone rock that can be crumbled in the
hand should not be loaded with more than 1½ to 1¾ ton per square foot,
or it will probably begin to flake and disintegrate. The strength of
sandstone varies very greatly, and in experiments it has been found that
when fine close grained, it supported before being crushed five times
the weight that very coarse gritty sandstone, having a sandy appearance,
would sustain; the respective crushing pressures per square foot being
362 and 67 tons.
Reference to authorities on the resistance of stones to crushing,
tension and transverse strain, will give the approximate safe load per
square foot; but in foundations, _i.e._, upon the rock in its natural
location, it should not exceed one-tenth of the ultimate resistance, and
the compressive strength should not alone be taken as a guide to the
safe load, but the resistance of the rock to tensional and transverse
strain should be considered. The value given for crumbling sandstone is
for the softest material that can be called rock, and is merely stated
to show that although some earths may be generally classed as rocks
their bearing power may be limited. The safe load upon an artificial
rubble mound foundation depends upon its character, and firmness and
solidity when deposited, and upon that of the ground on which it is
placed. No general values can be named, although it may be classed as
clean or compact gravel.
In a cutting, by excavation, the normal pressure of the earth, which
varies with the depth and weight of the soil, is removed.
Let
D = the depth in feet of a cutting from the original surface of the
ground,
W = the weight of a cubic foot of earth in decimals of a ton,
P = the normal pressure in tons per square foot at any given depth,
Then P = D × W.
On the other hand, in the case of an embankment the normal load upon the
earth is not affected, but an additional weight is superimposed;
consequently, to prevent slips or subsidence from overweighting, there
is a limit to the height of an embankment, apart from economical
considerations of its deposition. The problem to determine is,
therefore, the limit of the height to which an embankment may be
deposited without exceeding the safe load that the natural ground will
permanently sustain. The safe load upon the earth of which an embankment
is composed is referred to in due course.
Let
S = the safe load in tons per square foot upon the original ground or
earth not artificially deposited.
H = the theoretical limiting height in feet of an embankment.
W = the weight in tons of a cubic foot of the deposited earth.
NOTE.—When the earth is of a mixed character, the safe load should be
that of the weakest soil.
The condition of equilibrium is that the height is not greater than the
safe load divided by the weight of the ground, and is consequently given
by the expression—
H = S/W.
In order to prevent a reference to other books, a table is appended of
the approximate weights of different earths in their ordinary condition,
compiled from the best authorities.
The weights are those of solid rock, therefore, when it is deposited
they will be lighter according to the volume of the interstices.
───────────────────────────────────────┬───────────────────────────────
Name of Earth. │ Weight.
───────────────────────────────────────┼───────────────┬───────────────
│ Decimals of a │ Tons.
│ Ton. │ Cubic Yard.
│ Cubic Foot. │
───────────────────────────────────────┼───────────────┼───────────────
Basalt, solid │ 0·083 │ 2·25
Bath stone, solid │ 0·052 │ 1·40
Chalk, damp to wet, loose to close │0·056 to 0·074 │ 1·50 to 2·00
Clay │0·054 to 0·059 │ 1·45 to 1·60
Flint, solid │ 0·074 │ 2·00
Granite │ 0·078 │ 2·10
Gravel and shingle │0·046 to 0·055 │ 1·25 to 1·50
Limestone. Lias to compact mountain │0·067 to 0·078 │ 1·81 to 2·10
Marl │0·044 to 0·052 │ 1·20 to 1·40
Mud, at surface │ 0·044 │ 1·20
Mud, at about 15 feet in depth │ 0·048 │ 1·30
Peat, hard, and top mould │ 0·036 │ 0·98
Portland stone, solid │ 0·065 │ 1·75
Quartz, solid │ 0·076 │ 2·05
Sand, dry river │ 0·041 │ 1·10
Sand, damp and shaken │ 0·055 │ 1·50
Sandstone, solid │0·063 to 0·072 │ 1·70 to 1·95
Shale │ 0·074 │ 2·00
Slate, solid │ 0·080 │ 2·15
Trap, solid │ 0·078 │ 2·10
───────────────────────────────────────┴───────────────┴───────────────
In determining the safe load upon deposited earth it is well to remember
that:
1. Excavated earth cannot be restored in bulk to its original condition.
2. When the earth is simply deposited from a tip head, it cannot be
immediately consolidated by the act of deposition.
3. Until subsidence may, for all practical purposes, be considered at an
end, no deposited earth can be regarded as stable; but a mere uniform
weathering of the surface, which cannot be prevented when the exterior
is uncovered, will generally not cause instability of the mass.
4. Upon most public works the earth in an embankment is exposed in thin
layers.
5. The earth is loaded, in the great majority of cases, soon after it is
deposited and before it has settled or become consolidated, and is also
subject to vibration from earth waggons, locomotives, &c.
6. The comparative dry or wet state of the deposited earth and its power
of resistance to meteorological deterioration. _Vide_ also Chapter IX.
Taking into consideration these and other deteriorating influences, the
problem to be solved is, what deduction must be made from the safe load
upon unexcavated earth in foundations, in order to know the safe
sustaining power of the same earth when deposited in an embankment? Much
depends upon the liability of the soil to become saturated or in a
damper state than that in which it is known to be stable. For this
reason the height of embankments in clay soils, which are so
deleteriously affected by the weather, is generally made as little as
possible. Of course, temporarily an embankment will stand with a greater
pressure, _i.e._, at a greater height than the safe height, and,
provided no lateral movement took place, an embankment would be stable
until the earth was nearly crushed; but permanent stability and freedom
from slips and subsidence is the object to be attained, and everything
must be subject to local conditions, such as the amount of rainfall, the
situation, the care exercised during deposition, the protection given to
the surface, and the general drainage, and these must always govern the
application of any general rule. It is, therefore, necessary to divide
earths into two kinds, namely:—granular and non-granular; the former is
assumed to have particles, for purposes of earthwork, insoluble in
water, the latter to be liable to be dissolved by aqueous action, or to
be so affected by it as to lessen the stability. It is impracticable to
determine by a formula the permanent safe maximum height of an
embankment; the values named are, however, an approximately reliable
indication of the height, and are based upon the assumption that the
slopes are sufficiently flat to be stable, and that the embankments are
deposited in the ordinary way, the width of the formation or top being
not less than about 15 feet. It is seldom economical to make an
embankment more than 70 to 90 feet in height, except for short lengths,
and where deep cuttings cannot be avoided, and it becomes a question of
tipping spoil banks or main embankments, or the foundations for a
viaduct are known to be of a treacherous and doubtful character. The
heights named have been calculated under the worst conditions, _i.e._,
that any weight upon the formation will be directly communicated to the
base as in the case of a column, and not through the cross-section of an
embankment, and over the whole area of the base; however, there is
always the danger that the central portion may subside, as the weight
upon it is the greatest, and that the slopes may be disturbed and
pressed out, especially in soft or soluble earth likely to be quickly
affected by moisture. The soil is considered to have no reliable
cohesion.
In this country the limits of general practice for the height of
embankments when unaided by retaining walls or other support, but with
the slopes soiled, covered with grass, and only externally drained, has
been as follows:—
Surface soils, about 20 to 30 feet.
Boulder clay from 25 to 30 feet.
Yellow, or ordinary clay, 30 to 40 feet.
Ditto, ditto, in cuttings, 60 to 65 feet.
Chalk, 30 to 40 feet.
Gravel and sand, about 60 feet.
Ditto, in cuttings, 80 feet.
These have, however, been exceeded in many instances, especially upon
the lower side of an embankment upon sidelong ground. Probably the
highest artificial embankments unsupported by walls or any protective
works, are quarry and mining-tips, and ballast-heaps. These have been
cast out in all weathers, and allowed to assume any slope or shape, and
to a height as great as 100 to 120 feet, and have remained stable,
although usually in an exposed situation, but almost free from
vibration.
In Chapter VI. a railway embankment 90 feet in height, in treacherous
clay-soil is instanced, and there are many embankments of various earths
from 70 to 90 feet in height on the centre line, but a railway
embankment exceeding 100 feet in height on the centre line is rare,
although the height from the toe of the slope to the formation level in
sidelong ground may be frequently exceeded. A short and very high
embankment may stand where one of considerable length would slip,
consequent upon variations in the character and condition of the earth,
the bearing capacity of the ground, the state of the weather, the flow
of the surface water, general homogeneousness, the configuration of the
ground, and other deteriorating influences; but draining, careful
deposition, and a judicious adoption of the varying slope system, and
other precautionary works, will conduce to stability. Unless in
exceptional circumstances, it is advisable to avoid very high
embankments, as a slip of earthwork seldom gives much prevenient notice,
and the works of reparation can scarcely be effected by any means than
manual labour. The quantity of earth to be treated may be so large that
before the remedial works can be completed a wet season may arrest
operations and cause additional movement and subsidence of the earth.
GRANULAR EARTHS.
Description of Earth Embankment. Approximate safe
permanent maximum
height in feet, of an
embankment in earth,
conditions as
described.
Feet.
Indurated compact gravel, cementing material 120
imperishable. Clean ballast
Clean gravel, unwashed 110
Sharp compact clean sand 100
Firm, clean, angular, large-grained sand 90
Ordinary nodular sand, slightly loamy 60 to 70
Loamy sand 50 to 55
NOTE.—Clean fragmentary rock of uniform size, carefully tipped,
would stand permanently at any height within reasonable approach to
the safe compressive load; but in deposited earth, as in cuttings,
in addition to the contingency that the natural ground at the base
may not be able to bear the strain, the effect of water pressure
must be considered as it may become of sufficient force to cause the
face of the slopes to become loose and finally separate and slip. At
a height of only 50 feet clear head, the pressure would be about 22
lbs. per square inch; therefore, it is not so much the weight the
deposited earth will bear as the effects of water upon the earth,
and the water pressure that have to be considered.
OTHER EARTHS.
Description of Earth Embankment. Approximate safe
permanent maximum
height in feet, of an
embankment in earth,
conditions as
described.
Feet.
Peat moss, marsh earth, consolidated mud, silt, 0 to 7
hard peat turf
Alluvial soil obtained from a river bed 5 to 8
NOTE.—When the river bed is rocky and the deposit
firm, the height may be increased to 15 feet.
Soft wet pasty clay, and marsh clay, moist and 5 to 6
difficult to drain
Diluvial clay soil of river beds, according to
its uniform character, degree of firmness and 6 to 20
hardness
Alluvial soil. Loam and loamy earth. (Clay and 40
to 70 per cent. of sand.) Clay loams (clay and 15 to 30
about 30 per cent. of sand)
Damp clay soil. Equably damp, that can be drained 25 to 30
and will partly drain itself
NOTE.—The earths named will generally have their bearing power
increased by careful deposition in an embankment, for the act of
equal separation of the mass will cause a decrease in the quantity
of water contained in them: and they will be relieved of the
water-pressure to which they were liable in their natural position.
On the contrary, when carelessly tipped they may be deteriorated.
_Vide_ Chapter IX.
In the case of all earths that are readily impaired by water, it is the
degree of permanent uniform dryness or wetness that governs the safe
height of an embankment; for even the surface of the most impervious
clay will be liable to become in a muddy state, although in the mass it
may be dry or in a condition conducive to stability. Almost all
aluminous or calcareous earths hold or retain water in varying quantity,
and as in embankments this may constantly change, no safe permanent
height of an embankment in such soils can be established. The most solid
impermeable clay when separated into small pieces and impregnated with
water to any quantity above that it normally contains, will have its
bearing power and general stability reduced, until, when saturated, it
becomes as mud. Impure and argillaceous chalk, marly clay, marl, and
shale, are all more or less weakened by aqueous action, and their safe
permanent height may be anything not exceeding the crushing strength of
the earth, with a due allowance for the deteriorating influences of
weather, vibration, and internal water-pressure.
In this chapter reference has been made to certain limits of the safe
height of embankments; but were these to be adopted as _the_ maximum
safe height in each earth it would be an indefensible edict, for by the
adoption of the required precautionary measures named in the several
chapters of this book the height can be considerably increased, and what
is more, has been much exceeded with impunity in numerous instances. The
inclination that any earth assumes, after being exposed to the weather a
sufficient time for it to be affected by it, will afford a good
indication of the safe load it will support. As the slope of repose
becomes flatter, the bearing power of the soil in an embankment will be
reduced, all the deteriorating influences being identical.
For the best information “On the actual lateral pressure of earthwork,”
_vide_ the paper by Sir B. Baker, M. Council Inst. C.E., Minutes of
Proceedings of the Institution of Civil Engineers, London, vol. lxv.,
part iii.
CHAPTER VI.
SLOPES, GENERAL CONSIDERATIONS.—TABLE SHOWING THE USUAL RANGE OF
SLOPES.—TABLE OF COEFFICIENTS OF FRICTION.—NOTES ON THE COHESION OF
EARTH.—FORM OF A SLOPE.—SOME CONDITIONS GOVERNING THE NECESSARY
INCLINATION.—WIDENING EARTHWORKS WITHIN THE ORIGINAL FENCES.
With regard to the inclination and the form of a slope, and the
prevention of slips in earthwork, in any slip the surface must be
affected; therefore, to determine the slope of permanent stability of
any earth, whether in a cutting or an embankment, is of great
importance. An unnecessarily flat slope is not only a monetary waste but
may be a cause of instability, for it exposes a larger surface to
deterioration by the weather. By what means is a correct decision to be
attained? Experience has shown that certain earths under known
conditions will repose at particular inclinations; however, to
empirically assume that any earth will always stand under any
circumstances is clearly imprudent and untenable. As a guide, a table of
slopes for earths is valuable, but consideration of the consequences of
movement, and the distinctive features of each case should govern a
decision.
Should the earth be treacherous and require a flat slope, it may be
advisable to reduce the quantity of excavation by the erection of a
retaining wall, not only to effect a saving in expenditure, but also to
permanently support the soil, and prevent instability.
An inspection of neighbouring quarries and the waste tips, sand or
gravel pits, hills and cliffs, cuttings and embankments for roads, river
banks, &c., will afford an indication of any local peculiarities caused
by the climate or otherwise; and a comparison of the natural slopes
assumed when the soil has to withstand vibration, and when not so
disturbed, will enable a judgment to be formed of its deleterious effect
upon the earth; but above all, an examination of the cuttings and
embankments passing through or upon the same geological formation.
Reliable evidence can be so obtained upon which the slope of permanent
repose can be determined according to the dictates of science and
experience, but the probable consequences of instability will demand a
due regard to provision against contingent deteriorating influences
which may almost destroy cohesion and render it necessary to rely solely
upon frictional resistance, the remaining resisting power against
movement of the earth, except in solid rock, which may stand with an
overhanging or vertical face, whereas mud and quicksand may not be in a
state of rest even when horizontal.
The following table of slopes for different earths has been carefully
compiled to indicate the probable permanent slope, but it should not be
separately considered from the other chapters in this book, as
circumstances modify even a considerable range of inclinations; for
instance, earths that will be stable at a certain slope in temperate
climates will require a much flatter slope in tropical or very cold
countries. Those named are more especially for artificially deposited or
embanked earths subject to vibration, and, therefore, in cuttings the
slope might be steeper, although not so in aluminous soils or those in
which the particles become decomposed upon exposure. In Chapter II.,
under the head of each kind of earth, the approximate slopes of repose
are more exactly named, as they would occupy too much space in a table.
When not mentioned in Chapter II. the slope is given.
GENERAL NOTE.—The slopes must be flatter according to the amount of
water in the same soil or to which it may be subject, the depth of a
cutting and height of an embankment, and the presence of all other
disturbing influences. _Vide_ Chapter V. for the safe maximum depth of
cuttings or height of embankments.
TABLE SHOWING THE GENERAL RANGE OF SLOPES.
Description of Earth. Inclination.
Peat moss, marsh earth, consolidated mud, silt, Horizontal to 4 to 1
hard peat turf, when loaded
Alluvial soil 2 to 1 TO 3 to 1
Ditto. When wet about 4 to 1
Soft wet pasty clay in superficial beds 3 to 1 TO 4 to 1
Diluvial firm clay of river beds 2 to 1 TO 3 to 1
Alluvial soil. Loam and loamy earth. (Clay and 40 1½ to 1 TO 2 to 1
to 70 per cent. of sand)
Ditto. Clay loams. (Clay and about 30 per cent. of
sand)
Ditto. When wet and crumbling 3 to 1 TO 4 to 1
Damp clay soil 3 to 1
Upheaved and intermixed beds of different sound 2 to 1 TO 3 to 1
clays
Solid clay mixed with very fine sand 2 to 1
Sound yellow clay containing only the normal
quantity of water, the surfaces covered after 2 to 1 TO 3 to 1
deposition
Brown laminated clay, quickly affected by
weather:—
Surface covered 6 to 1
Surface exposed 12 to 1
Solid blue and firm clays, marl and indurated 1½ to 1 TO 2 to 1
marl, and boulders imbedded in sand and gravel
Soft chalk, impure and argillaceous 1½ to 1 TO 2 to 1
Ditto. In cuttings ½ to 1 TO 1 to 1
Hard white chalk, having greater density than the 1 to 1 TO 1¼ to 1
preceding.
Ditto. In cuttings Nearly vertical TO ½
to 1
Ashes 1 to 1
Very fine dry sand 1½ to 1 TO 2 to 1
Firm sand, surface not completely protected 1¼ to 1 TO 1½ to 1
Firm sand in embankments, surface protected by 2 to 1 (least) TO 3
fascine mattresses, as in Holland; and exposed to 1
to moderate sea
Ditto, on land side. 1½ to 1 (least)
Firm shale, surfaces covered ½ to 1 TO 1 to 1
NOTE.—When the shale is greasy and becomes
unguentous upon being exposed to the weather, it
must be considered as a clay.
Clean gravel and dry shingle 1¼ to 1 (dry) TO 1½
to 1 (wet)
Ordinary clean coarse sea beach, 3 to 1 at top,
graduating according to the depth, size, range,
set of the tide, and exposure, to 5 to 1 TO 8 to 1
at moderate depths, and at the base to 20 to 1 TO
30 to 1, the curve of the slope being parabolic if
the waves alone disturb it. The angle of repose
will be less as the size of the particles become
smaller, and between high water and a few feet
below low-water mark will seldom be less, if not
sheltered, than 4 to 1 to 6 to 1; but coarse firm
sand that has become consolidated will often stand
at a steeper inclination than a mass of rolling
stones, however hard they may be.
Compact gravel 1 to 1
Ordinary road metalling, moderate height 1 to 1 (clean), TO
1¼ to 1 (stacked, as
excavated from
road.)
Large concrete blocks. Sheltered position and
carefully deposited. ½ to 1
Harbour side
Large concrete blocks. Exposed site and carefully 1 to 1
deposited
Rubble mound. Sheltered position 1¼ to 1 TO 1½ to 1
(harbour side.)
Rubble mound. Exposed to sea 2½ to 1 (sea slope.)
NOTE.—If an exposed coast, the rubble may require from 4 to 1 to 7
to 1 slope, depending upon its size, the currents, depth, and
“fetch” of the sea, and solidity of the mass.
The usual slopes adopted for cuttings and embankments may be said to
range from 1 to 1 for firm earth, having particles not seriously
affected by water or weather, to 4 to 1, and the most frequent, 1 to 1
TO 1½ to 1 in cuttings and 1½ to 1 in embankments.
With respect to the chief organ of stability in earths other than rock,
namely, the frictional resistance; friction during motion is generally
considered to be less than the force necessary to overcome it when at
rest, and undoubtedly this is the case when the surfaces are similar,
and are smooth and hard and not easily impressed, as iron, granite,
concrete, and metals generally; but when they are comparatively soft and
incapable of resisting indentation at any pressure that they may have to
bear, the difference between the coefficient of friction during motion
and that at the commencement of motion or of repose will not be so
marked, for other resistances may come into action not due _solely_ to
surface friction of the mass. A surface may become indented or roughened
thus offering opposition to motion not existing at the commencement of
movement, and particularly so in any earth of a mixed character
possessing hard particles, such as boulders, or sand in clay. On the
other hand, in the case of hard rock, solid clay, or other homogeneous
earth, the difference between friction during motion and that of
friction at rest may be reliably determined.
In soils of a granular or gritty nature small particles become detached
during motion, and by pressure occupy or become wedged into any cavities
upon the surfaces, and therefore offer resistance which is not _alone_
due to friction of a mass upon a like mass. From this cause friction
during motion may seemingly even become greater than during rest, but
with material consisting of rounded particles that will not wedge, the
friction upon a sliding surface may be lessened by reason of the grains
revolving.
In deducing a slope of repose for earth, the lowest value of frictional
resistance, whether during motion or at rest, should be taken, and
always as if the surfaces were wet. The coefficients of friction, F,
during motion usually range between 0·25 to 1·10, and the slope of
repose, S to 1, is consequently found by the expression—
S = 1/F,
therefore, S would equal to 1/0·25 to 1/1·10 = 4 to 1 TO 0·91 to 1, say,
1 to 1.
This is the required inclination to prevent movement provided no
pressure is exerted upon the surface, and not taking into consideration
the disturbing and weakening effects of vibration and all other
deteriorating influences, such as the variable degree of moisture of the
soil, the irregularity of its character, the destruction of the
continuity of the surface by trenches or drains in a slope, the effect
of gravity to detach a mass, the process of excavation or deposition,
and the expansion and contraction of soils of an argillaceous nature.
When any earth becomes suddenly water-charged or deteriorated by any of
the agencies previously and subsequently mentioned, movement may be
expected, and it should be remembered that in the same soil the
resisting powers to disintegration frequently vary, consequent upon
inequality in the quantity of moisture, the roughness, evenness,
smoothness, compactness, looseness, the degree of fineness of the earth,
and also the manner in which strain is applied.
Friction upon a dry surface is almost invariably greater than that upon
a wetted surface, and is so beyond all question upon any plane
lubricated with an unguent. The disturbing and enfeebling effect of
water may be judged from a careful analysis of many reliable experiments
to ascertain the frictional resistance in the case of the same material
in a dry and in a wet state on an unplaned surface of cast iron and on
timber piles. It shows the following results in addition to those given
in the table of coefficients of friction of earth upon earth in the next
page.
The surface friction of masonry or brickwork upon dry clay is reduced by
from 25 to 30 per cent. when the clay is wet.
The frictional resistance of an unplaned surface of cast iron upon wet
sand is about 16 per cent. less than the resistance upon the same
material when dry. In the case of timber piles, it is about 12 per cent.
less, and about 40 per cent. less in sandy clay and gravelly clay soil.
In sandy gravel the difference in the resistances is small, being only
from 5 to 10 per cent. less when the earth is wet.
Sand has about 20 per cent. more friction than sandy gravel, both
materials being in a wet state; and below a depth of from 10 to 15 feet,
the frictional resistance increases little in gravelly sand and gravelly
soils.
In lieu of a sufficient example under similar conditions and
circumstances, the most reliable method to ascertain the slope of repose
of any earth is that of S, the slope of repose, to 1, = 1/F, any
cohesion of the soil being disregarded and considered as a margin of
stability liable to be much impaired; and, therefore, except in a mass,
it is prudent to look upon it as non-existent in the whole of a slope;
and when motion has commenced, as even a means of accelerating movement
by causing lumps to become displaced instead of mere particles.
The following coefficients of friction during motion are here given in
confirmation of the frictional resistance of an earth being an indicator
of its slope of repose.
They have been tabulated from different authorities, are the average
results of practical experiments, and have been compared with the
inclination of slopes actually assumed under the ordinary conditions of
work.
───────────────────────────────────────┬────────────┬──────────────────
Description of Earth or Material. │Coefficient │ Corresponding
Earth upon Earth. │of Friction │ Slope.
│ during │ S TO 1 = 1/F.
│ Motion—F. │
───────────────────────────────────────┼────────────┼──────────────────
Damp vegetable or loamy earth │0·50 to 0·67│2 to 1 TO 1½ to 1
Clean dry shingle or ballast, nearly │0·75 to 0·80│1⅓ to 1 TO 1¼ to 1
without sand │ │
Clean wet shingle or ballast, nearly │ 0·67 │ 1¼ to 1
without sand │ │
Shingle or ballast with ordinary │ 0·80 │ 1¼ to 1
quantity of sand │ │
NOTE.—The presence of sand in gravel │ │
increases the frictional resistance │ │
because it makes it gritty. │ │
Excavated hard road metalling, cast │ 0·80 │ 1¼ to 1
into a mound │ │
Fine dry sand │ 0·75 │ 1⅓ to 1
Hard clay, slightly damp │ 0·67 │ 1½ to 1
Hard clay, damp to wet │0·40 to 0·50│2½ to 1 TO 2 to 1
Damp lias, yellow, and most upheaved │0·25 to 0·33│ 4 to 1 TO 3 to 1
clays in superficial beds │ │
Wet rubble on wet rubble, ordinary size│0·67 to 0·80│1½ to 1 TO 1¼ to 1
and character │ │
Do., do., when the surfaces are unclean│ 0·67 │ 1½ to 1
Do., do., Large to very large size. │0·80 to 1·0 │1¼ to 1 TO 1 to 1
Surfaces clean │ │
NOTE.—All rubble carefully deposited. │ │
Rough-faced granite on gravel and sand,│ 0·54 │ 1·85 to 1
both dry │ │
Rough-faced granite on gravel and sand,│ 0·48 │ 2·09 to 1
both wet │ │
Rough-faced granite on sand, both dry. │ 0·70 │ 1½ to 1
Rough-faced granite on sand, both wet │ 0·53 │ 1·88 to 1
│ │
CONSTRUCTED MATERIAL ON EARTH. │ │
│ │
Rubble masonry, or brickwork on clay, │ 0·50 │ 2 to 1
dry │ │
Rubble masonry, or brickwork on clay, │ 0·33 │ 3 to 1
wet │ │
│ │
NOTE.—_Vide_ Chapter II. │ │
───────────────────────────────────────┴────────────┴──────────────────
Friction is the chief cause of stability in granular soils and those
readily affected by moisture which have for practical purposes no
immutable cohesion. In few earths are both cohesion and friction of
considerable and reliable value, one or the other quickly becoming
impaired or destroyed. Movement is caused by such various means that
each earth must be separately considered, and also the circumstances
under which it is placed. The particles of the earth may be dissolved by
water and become in a muddy state, or they may be considered insoluble
as in clean sand and gravel, although in compact sand or gravel the
cementing material may crack and weather. Provided it was certain any
earth would always remain as originally formed, a condition which it is
impossible to guarantee in work, the cohesion and frictional resistance
being known, the correct slope could be mathematically determined, but
as all earths are subject to varying deteriorating influences, such a
deduction is only valuable as a guide for reasonable inference. Cohesion
may be more quickly impaired by certain action than friction, and _vice
versâ_. Probably, of all soils, that to be most distrusted is one that
expands and contracts, such as clay earth, which although possessing
considerable cohesion may become upon drying a mere congregation of
disconnected lumps ready to move upon the return of wet weather, thus
its considerable power of cohesion may practically be one of the chief
causes of a slip. On the contrary, clean sand, although devoid of
cohesion, will not crack or have a greasy surface, but the particles may
be washed away.
To ascertain that any earth is uniformly affected throughout the mass,
and to prevent or provide against deteriorating influences is the chief
aim. It is useless to declare any earth possesses considerable cohesion
when the power can be quickly dissipated by ordinary atmospheric action
and even become a cause of movement, and to rely for permanent stability
upon such property. In ordinary earths, not rock, it will generally be
found that cohesion is small or insignificant in soil having a
coefficient of friction of some moment, and the reverse. In most earths
friction, although it is affected in a greater degree by vibration, has
to be relied upon, and not cohesion, as the latter is variable and may
exist almost unimpaired in a lump, which nevertheless may become
detached because of fissures. The coefficients of friction of different
earths are also better known than the cohesion; but how easily even
friction is impaired may be gathered from the sudden manner in which
bridge cylinders will sink after having hung for days by surface
friction, or been held by the transitory expansion of clay. Mr. Wilfrid
Airy, B.A., in a series of experiments on the cohesion of earth, found
that in strong brick loam it is about 168 lbs. per square foot, in
compact clay and gravel it may reach 800 lbs. per square foot of
section, and in clean sand it is practically nihil.
In rock the slope of repose depends whether the earth is unstratified or
stratified; and if stratified, upon the dip of the strata and their
resistance to the effects of the weather. A vertical face may be stable
in unstratified or stratified rock, provided in the latter case it does
not dip towards a cutting; or the requisite slope may range from ¼ to 1
to such an inclination that the cohesion and friction, which vary
greatly, are sufficient to prevent movement. In sidelong ground should
the rock dip parallel to the surface the hill slope may require to be
flat, whereas the valley slope may stand vertically.
Although, perhaps, in many instances the slopes of cuttings and
embankments have been arbitrarily fixed, it may be said, on the whole,
no very serious interruptions to traffic have been caused from the _sole
want of sufficient initial flatness of a slope_, as that will soon
become known. In determining the inclination it is not the angle at
which the earth will stand at the time of excavation or deposition, and
for a few months after that is required to be ascertained, but that
which will _permanently_ suffice to prevent movement. It is well known
that almost all freshly cut soil stands nearly vertically for a small
depth for a few days in ordinary weather but then begins to crumble and
finally break away.
What then will be the slope of permanent stability? This chiefly depends
upon the degree of exposure, the effects of the weather, water, and
vibration upon the soil, and the depth or height of a cutting or
embankment.
Each earth requires to be duly considered; for instance, gravel and sand
are pieces of rock, however small, and for earthwork purposes the
particles may be regarded as insoluble in water; nevertheless in the
case of sand, should it be charged with water, it may be necessary to
treat it as a fluid, the same as mud, although water does not change the
particles; however, slips in cuttings and embankments in sandy or
gravelly soils are not usually caused by their becoming gradually
saturated throughout their mass, but by a flow of water which creates
water seams: the stability, therefore, is dependent upon equal
percolation and drainage and protection of the surface; and the slope
that should be given to a sand cutting is also governed by the quantity
of water it will have to hold, and whether the sand is pure or loamy.
The depth of a cutting in sand has a considerable influence upon the
slope of stability, for frequently sand is in a dry state in the upper
portion of a cutting, but beneath it is wet, and partakes more of a
silty character, and therefore may stand at a steeper inclination in the
upper part, but require a flat slope in the lower portion. The same
conditions are found in all soils, and the sides of a cutting vary, one
may be comparatively compact and free from water, the other in a wet
state and disintegrated. They will not permanently repose at the same
angle, the slope varying according to the degree of dryness, size, and
uniformity of the particles. In all earths in which cohesion is liable
to be quickly destroyed, a straight slope is the best as having an even
surface, which prevents the formation of depressions and causes the
water to drain away, and also offers the least surface to the weather.
In all earth such as shingle, gravel, or sand, consisting of pieces of
rock or rubble, or having round particles, it is important to remember
that any surface disturbance may cause serious movement, particularly
should it commence at the bottom of a slope, as then the revolving
action may not cease until a flatter inclination is produced by material
rolling from the top and not reposing until it nearly reaches the base.
The quantity of moisture is the chief governing condition of the slope
of repose in clay soils, and it should not be forgotten that this may
vary considerably. Clay when only slightly moist may stand at a 1 to 1
or 1½ to 1 slope, but as it gradually becomes in a wet state will
require a flatter inclination, and may not be at rest until the slope is
at least 3 to 1, and it is not safe to rely upon a steeper slope than 3
to 1 in the case of almost any surface clay beds liable to become
charged with water, and even 4 to 1, should the excavation be on the
side of a clay hill and near houses, or the ground be loaded; but clay
having some powers of cohesion which usually are greater as the clay is
harder, is not so quickly disintegrated as in the case of more porous
soil, and the form of the slope it assumes is not a straight line; that
most usually approached by sand or gravel or a granular soil consisting
of particles of the same character, although the lower part of the slope
may be flattened consequent upon the erosion of the finer particles
which crumble and become deposited at the base.
As a rule, the greater the cohesion of the soil the more curved is its
natural slope, the greatest pressure being at the base where the
inclination is flatter, and is steeper towards the top, as the ground
may be held together by cohesion at a vertical face. The harder and
looser the particles the straighter will be the slope, and if the ground
gradually increases in firmness it will usually be nearly straight; but
if the contrary condition exists, the natural slope will be flatter
towards the base although nearly vertical for a few feet from the top.
As a proof how quickly clay becomes less stable and loses its cohesive
power with the usual quantity of moisture in it, when first tipped it
may assume a slope of 1 to 1 TO 1½ to 1, but upon exposure to the
weather, which causes the lumps to waste away and the clay to swell from
moisture and other agencies, the firmest clay in cuttings and
embankments may be said to be unstable until a slope of at least 1½ to 1
is reached in moderate depths, and 2 to 1 TO 3 to 1 in high embankments
or deep cuttings. In all earths the chief cause of movement of earth is
water, and the main questions to be decided are so far as regards the
inclination of the slope.
1. Should the surfaces of a cutting be drained and protected and be
excavated to a comparatively steep slope; or,
2. Should they be left undrained and uncovered and be excavated to a
flat slope. Provided a cutting can be readily drained and covered and
there is no probability of any sudden or permanent increase of moisture,
perhaps the first method is the more economical; but much depends upon
the quantity of water held by the earth in its normal state, whether it
is of the same character throughout, and the depth of a cutting. Should
the beds be upheaved or intermixed, then a flat slope is necessary and
no covering except a wall may make it stable at a steep slope, and, for
instance, should clay be always in a semi-saturated condition, 3 to 1 is
the least slope at which it will permanently stand, and it will usually
require a more moderate inclination. A medium course to adopt is that of
varying the inclination of the slopes, the steepest, of course, being at
the top and the flattest towards the toe; this is in accordance with the
laws of pressure and a mathematical investigation of the theoretically
correct slope, which nearly corresponds with the actual slope a high
embankment will assume when allowed to weather and settle: for by
varying the inclination of the slope the latter becomes practically a
curved line and approximates to that of the curve of equilibrium. In
almost all slips the surface from which the fallen mass has become
detached is curved, the upper part being concave and the lower slightly
convex, the outline being caused from the upper portion falling, the
lower receiving it and being pressed outwards; however, it may happen
that the lower part of a slope has remained intact, and only the upper
slipped and become deposited upon it.
The varying slope system has recently been adopted by Mr. Francis Fox,
M. Inst. C.E., upon the Scarborough and Whitby Railway, where an
embankment about 90 feet in height in treacherous clay had slopes of 1½
to 1 for the upper 30 feet in height, 2 to 1 for the middle 30 feet, and
3 to 1 for the bottom 30 feet. Formation width 28 feet. A calculation of
the insistent weight per square foot, without a train, at stated heights
gives the following results; taking the weight of the earth at 0·055 of
a ton a cubic foot, or 1½ ton a cubic yard, and assuming the worst case,
that of the earth for the width of the formation, viz., 28 feet, to act
simply as a column 1 foot square and the load as not being distributed
over the area of the entire base at any point.
At the base of the upper 30 feet, 1½ to 1 slopes, it would be about
1·65 ton per square foot.
At the base of the middle 30 feet, 2 to 1 slopes, it would be about
3·30 tons per square foot.
At the base of the lower 30 feet, 3 to 1 slopes, it would be about
4·95 tons per square foot.
If the weight of 1 foot lineal of the embankment is taken and considered
as equally distributed over the whole area of the base at the 30 feet
divisions, the strain per square foot would be as follows:
At the base of the upper 30 feet, 1½ to 1 slopes, about 1 ton per
square foot.
At the base of the middle 30 feet, 2 to 1 slopes, about 1¾ ton per
square foot.
At the base of the lower 30 feet, 3 to 1 slopes, about 2¼ tons per
square foot.
NOTE.—The actual strain is probably approximate to a mean between
the two values, and is nearer the latter than the former.
There is no reason why the varying slope system should not be adopted in
embankments of clay or soil having considerable powers of cohesion, as
the expense of trimming the slopes is very little more than making them
to a straight line. To trim a slope to an elliptical, parabolic, or
cycloidal curve would be a needless refinement requiring a template;
moreover, the surface is better when it consists of straight lines,
provided that at the junction of any two inclinations the point of
meeting is sufficiently rounded to prevent a lodgment of water. In
cuttings in non-weathering tenacious soil the upper part might be left
at a steep slope for 5 or 6 feet from the surface of the ground.
It is evident that the depth of a cutting or the height of an embankment
affects the stability of the slopes, but some soils are so weak that an
embankment of even little height will not be at rest until the toe of
the slope is supported. If the slopes were not pressed out the earth in
the central portion of an embankment would stand at any weight less than
that which would crush it when in its weakest condition; therefore, as
the load increases downwards it is a logical deduction that the slope
should be flatter as it approaches the base. In slips the form generally
assumed in soils having considerable cohesive power nearly approaches
that of a parabolic curve, which shows that a straight slope is not the
correct one in tenacious soil, and theory confirms it.
In the case of cuttings of considerable depth, apart from the question
of the best form of slope, in order to lessen the velocity of the
surface water and the extent of a slip, and cause supported weight upon
the slopes, they can be divided by broad terraces or benchings, about 6
feet in width, and at vertical distances of 15 to 20 feet, upon which
can be impermeable catchwater drains. It is important not to allow a
flow in a straight line or nearly so, as the velocity of the water may
erode the slope. If the nature of the earth and its resistance to
erosion will allow, the benches or steps should be abrupt, in order to
cause the greatest resistance and deviation from a direct discharge. The
slopes of the catchwater drains may require to be covered in a flood
district or one having a heavy rain or snowfall.—_Vide_ Chapter IV.
In excavating a cutting in soil likely to slip, care should be taken
that the surfaces are not strained by lumps being left upon them which
are only retained by reason of the cohesion of the earth, as they will
cause weak places less able to bear any pressure brought upon the slopes
through the sides being deprived of continuity of support. Therefore, in
such cases, the slopes should always be rough trimmed as quickly as
possible after the gullet has been excavated. Also when the slope of a
cutting is furrowed so that its surface consists of separate and
unsupported masses of earth clinging to it, continuity of support is
destroyed and the earth is more exposed to meteorological dissipation;
and in non-cohesive soils such as sandy or gravelly earth, in which
especially a straight and uncut surface is of importance, movement is
incited. Therefore, when it is found necessary to insert open trenches
in a slope, they should be at right angles to its foot, and the inserted
material should be well packed so as to support the sides and to
interfere as little as possible with the slope. Even in clay soils or
any having considerable cohesion, trenches _diagonally or transversely_
cut in the surface are generally inexpedient as disturbing and
destroying the continuity of support of the surface and increasing its
exposure to the action of the weather; and although they may be
temporarily effectual as drains, such division of the inclined face
cannot but induce a disunited condition which will at once be apparent
should the trench become choked and it miscarry as a drain; and a
temporary failure may so destroy the existing delicate equilibrium as to
cause movement. Such trenches should be regarded with suspicion, as any
stability caused by their draining or conducting away water may be
effected at the cost of continuity of support, the deterioration of the
resistance of the soil to weather, and the impairment of its frictional
and cohesive properties. If placed upon a slope at right angles to the
formation the preceding objections are removed.
It may become necessary to excavate or trim a slope to a steeper
inclination than that which would otherwise be adopted and is considered
to be its angle of repose, in order to widen a railway between the
fences, or enlarge a station. Then, pre-eminently the question of the
effects of a slip demand attention. The great majority of railway
stations are located prior to the commencement of, or as the works
progress, but additional accommodation which, for reasons of economy
must proceed _pari passu_ with the development of a district, is usually
required some time after a railway has been opened for public traffic,
and it may be imperatively necessary to confine the works within the
boundaries of the land originally purchased. Consideration of the
several chapters in this book will recall to the mind the chief points
to be regarded. Fortunately, stations are seldom placed in deep cuttings
or upon high embankments; but frequently when a station is opened,
houses will be erected around it, thus causing any movement of the
ground to be of serious moment and dangerous. Assuming the railway must
be widened within the original fences, the position of the toe of the
slope is fixed, and also that of the top of the slope. The questions
then in the case of a cutting are principally:—
1. Will the slope be too precipitous for the earth to stand at one
inclination?
2. Can it be made sufficiently steep for 5 or 6 feet from the top to
obtain an inclination for the lower portion at which it will have
permanent stability?
3. Can the earth be made to repose if the face is evenly protected under
circumstances 1 and 2?
4. Is it necessary to erect a retaining wall to a distance a few feet
from the original surface of the ground?
5. If a wall be necessary, what should be its lowest height consistent
with the stability of the unsupported inclined earth above it?
Provided there is no appreciable superincumbent weight to be borne
nearer than 10 feet from the top of the slope, and that the foundations
of a building are at a considerable depth in the ground, and the surface
and back drainage waters properly controlled, no retaining wall may be
required, assuming the original inclination under ordinary conditions to
be proved to be the permanent slope of stability; but should the
distance of the face of a building from the toe of the slope after
widening be insufficient to allow of the original slope being adopted, a
retaining wall will be necessary, its height being chiefly governed by
the proximity of any building, and the necessity of nearly maintaining
the originally established steepest slope of repose in the case of any
unsupported earth. The advantage of a sufficient cess in such a case is
that it makes provision against deterioration of the surface, and causes
an imaginary slope of the same inclination as the original slope to be
contained within the space between the face of the building and that of
the retaining wall at formation level. In sandy or loose soil if any
buildings or wells near the site show signs of cracking, the excavation
should at once be stopped to see what preventive measures are requisite,
and pumping water out of a trench may be dangerous. Any retaining wall
in such a position should be erected in short lengths, so that the earth
and foundations are exposed to the weather as little as practicable.
CHAPTER VII.
NOTES UPON THE PRESERVATION OF THE FOOT OF A SLOPE.—VARIOUS METHODS OF
COVERING AND SUPPORTING A SLOPE.—PROTECTION FROM SNOW-DRIFTS.—THE
FORMATION WIDTH OF CUTTINGS AND EMBANKMENTS.—THE DELETERIOUS EFFECTS
OF VIBRATION.
The protection of the toe of a slope is of importance, as it is usually
the most vulnerable and the weakest part. When the earth is of the same
character, the quantity of water is usually greater at the lowest level
than above it, and the stability of the soil in its vicinity is
therefore lessened. In clay soils this softening action at the base may
cause a slip and probably can only be prevented by reducing the
percolation by surface drainage, which has been referred to in Chapter
IV.; the matter under consideration being the counteraction of movement
in the toe by other means than draining, although combined with it, such
as by—
1. An impervious retaining wall with a pervious backing of ashes, gravel
or sand, and ample weep-holes, at the foot of a slope, which, by
preference, should be of Portland cement concrete, a more homogeneous
material than brickwork or masonry, as it has no joints, and is
particularly to be preferred for retaining walls in clay soils, as it
approaches in a greater degree the condition of air-tightness and that
of equal resistance.
2. By a pervious wall or counterfort of gravel, burnt ballast, hard
chalk, rubble, strutted timber framework, or a covering of other firm
material.
In both the preceding cases, to ensure stability and to prevent any
protective works being pushed forward, the foundations must be below the
formation or ground level. To lessen sliding action the foundation
should incline at right angles to the face, and should have a batter
either for the whole width or for a distance not less than about
one-third to one-half of the bottom thickness in the case of low
retaining walls. A wall having a steep batter upon each face, and
therefore a wide base as compared with its area, causes the centre of
gravity to be at a greater distance from the exposed face, and therefore
the resistance to overturning is increased; but care must be taken that
it does not slide forward. A foundation for such a wall upon clay or
tenacious soil is to be preferred, provided there is no upheaval of the
ground in front, but it should not be upon a thin stratum, or the latter
may slide upon another.
In towns, or where land is of considerable value, the two sides of a
cutting can be made to support each other, and one of the following
principal methods may be adopted.
1. An invert under the line.
2. A heavy flat platform, arched on plan or solid throughout, under the
permanent way or formation, to prevent a forward movement of the toe.
3. Overhead arches and iron struts at intervals to resist and reduce the
pressure upon a wall.
In such a situation, should a tunnel or covered way be not required, any
subsidence of the earth may cause the destruction of valuable property,
and the erection of a retaining wall be imperatively necessary from a
due consideration of prudential construction, and altogether regardless
of the character or condition of the earth or its liability to become
water-charged. In order to prevent movement of the earth different forms
of support may be required. In some cases, owing to excessive pressure,
a counterforted wall with an invert under the permanent way may be
essential, or the invert may be flat and be arched on plan, thus
supporting the toe of a retaining wall between the intervals, in
combination with a vertical or inclined pilaster and a front counterfort
system, and in addition flat overhead struts of iron or other material
between the walls acting as an auxiliary support above the required
traffic space may have to be adopted where the walls have to sustain
considerable thrust, the thickness of the retaining wall being thus
reduced; or a simple retaining wall may be sufficient. A non-jointed
material as Portland cement concrete of equal character is to be
preferred to a mass consisting of hard materials yet incapable of
possessing a joint equal to their strength or durability.
With regard to the protection of the toe or lower portion, of a slope by
means of a retaining wall, it may be the only effectual support in one
case, and not succeed in an exposed country without a complete system of
open or closed drains, and then its adoption may be superfluous. In the
following few paragraphs an endeavour is made to indicate some
situations in which retaining walls have or have not been completely
successful when erected for such purpose.
First, it is most important that ample provision be made for draining
the back of the walls, for if the egression of the surface waters be
obstructed, they must accumulate and cause hydrostatic pressure, soften
the lower portion of the earth, thereby failing to partly effect one of
the objects of their erection, and cease to protect the surface; the
probable result being that the wall is pushed forward, broken up, or
overturned. The drainage must be regulated by the quantity and velocity
of the flow, and ample weep-holes should be provided to prevent an
accumulation of water at the back. In damp soil there should be one to
about every three or four superficial yards, and an outlet at each wet
place, or a wall may not stand. The wall should be backed with a
filtering medium such as coarse gravel, hard ashes, ballast, and no
retentive earth should be used. In clay earths a retaining wall should
always have a dry porous backing, as it not only reduces any pressure
due to a head of water, but also allows of the earth swelling without
affecting the stability of the wall, as it probably would if the clay
rested against the back of the wall.
Retaining or breast walls are particularly useful in loose soil having
no cohesion, _i.e._, those of a sandy character or consisting of very
small grains, and which upon saturation by water or by the action of its
flow become in an unstable state or one of actual movement; in such, a
case not only is it requisite to protect the surface of a slope, but
support at the toe is indispensable. In cohesive soil, such as clay or
clay marls, surface protection, combined with systematic and thorough
drainage, may be all that is required to make a slope stable, and the
erection of an impervious or solid high wall be unnecessary, always
provided the earth cannot slide from being superimposed upon an inclined
stratum. In fact, retaining walls in retentive soil have been found to
induce a slip, because they neither drain the earth nor prevent the
additional impregnation of water, and they have consequently been
destroyed.
In countries where floods or very heavy and sudden rainfall, frost and
snow quickly succeed, masonry or dry stone retaining walls at the foot
of a slope soon become impaired, and require constant supervision and
careful maintenance, and cannot be considered as economically or
generally effectual; and should the earth settle, as they are
comparatively solid they will not follow any subsidence of the surface;
therefore cracks occur and water accumulates in the hollows, the slope
has no longer uniform support, a localization of the egression of water
is caused, owing to the fissures in the wall inducing a flow, and the
wall becomes a cause of a slip instead of a protection against movement.
Should such a wall bulge after it has been restored to the condition of
being a continuous support, its forward movement may be arrested by the
erection of counterforts with an inclined face in front from 10 to 30
feet apart according as weak places exist, having joints at right angles
to the batter; and this is, perhaps, the cheapest and quickest remedy,
but the slope must be carefully drained and the number of weep-holes be
increased.
In order not to obstruct, but induce the through drainage of water,
retaining walls to prevent slips have been erected, consisting of arches
turned upon piers, the intervening space being dry walling which allows
a free flow of water, the idea being to afford the necessary support
without interfering with the drainage. In some cases, as it tends to
condense the soil, weighting the toe of the slope may prevent movement,
and on the side of a hill weight and mass, apart from slope protection
or drainage, must be provided to arrest a slip or prevent further
movement, either by means of a continuous wall, or by the addition of
frequent counterforts, 5 or 6 feet in width, to an existing wall, if
space permits, the back and front having a considerable batter: the
centre of gravity of such a mass being low and the base large,
overturning is improbable. Forward movement can be also guarded against
by deep foundations, and as the earth will rest on the inside they can
have a flatly inclined base sloping towards the cutting, as it will tend
to prevent overturning, the depth of the foundations at the face being
two or three times that at the back. Such a structure can hardly be
called a wall, being really an extensive and massive concrete toe.
Perhaps generally the most economical and secure way of preventing
movement is to erect a low Portland cement concrete wall at the toe of a
slope with a considerable batter; or to cut a trench at formation level
outside the line of the slope, and to shore it by means of old sleepers
strutted at intervals when better and durable material cannot be readily
obtained.
The chief object of a protection at the toe in clay soils is to prevent
the bottom of a slope being softened by a lodgment of water, or fissured
by heat or drought; this may be effected without a wall; but in the case
of sand or fine granular earth cuttings liable to become quicksand,
support at the base is absolutely necessary, and the best way to prevent
movement may be to erect a breast retaining wall at the toe a few feet
in height, say 3 to 5 feet, the backing being of dry porous material
such as ashes, coarse gravel, or broken stone, increasing according to
the depth or height of the earthwork and quantity of water, the
thickness being more than sufficient to contain the drainage waters, and
not less than 1 foot 6 inches, in order that it may act as a filter and
drain the slope, &c., so as to lessen the percolation of water.
Sheet-piling and a backing of rubble acting as a drain 2 or 3 feet
square, or a strutted timber duct, both made out of old sleepers, have
been used with this object in quicksand where water rapidly percolated
to and accumulated at the base. Instability of the toe may also be
prevented by covering it with layers of gravel, but one of the reasons
that may cause a gravel counterfort to fail in loose porous soil is that
sand or mould may be washed through the interstices in the stones
forming the gravel; hence a more impermeable covering is to be
recommended in such a case.
A toe of rammed coarse gravel or broken stone may supply the required
support in the case of coarse and fine sand, when the latter is alone
movable. Experiments have shown that sand rammed in layers of about 5
inches and earth mould 2 inches in thickness give the best results. The
less the weight is increased by ramming the more solid the original
earth. Brushwood or fascine work weighted with gravel, stone or broken
bricks, and stone pitching can also be used for sandy soils, but a
filtering layer must cover the sand, or the latter will be eroded.
Counterforts of well-rammed natural earth may be sufficient provided the
soil is firm and is made compact and uniform in texture. In otherwise
stable soil, when it is known that a sand vein is the cause of a slip
because of water percolating through it, it should be raked out as
deeply as practicable if not too large, and the space be filled with
stones, coarse gravel, or dry material forming an open drain for the
water to issue without a flow of sand, thereby preventing any
accumulation. When a stratum of shifting sand overlies beds of
conglomerate and gravelly or clayey sand it will be necessary to support
the lower portion of its slope by a wall. The wettest places in a slope
should be noted and the surface be turfed or covered, and should the
depth of the unstable sand exceed 5 or 6 feet, narrow benchings can be
made 2 or 3 feet in width at every 5 or 6 feet of vertical height to
divide the flow of water and prevent it following a continuous course.
It also sometimes happens that two treacherous earths have a stratum of
stable soil between them such as rock; in that event it is advisable to
leave a cess between the bottom of the slope of the upper stratum of
unstable soil and the underlying rock, so as to allow of weighting or
other works to resist movement of the toe.
With respect to the adoption of open surface rubble drains upon the
slopes, in Chapter IV., page 78, special reference is made to them as
drains, and in Chapter VI., pages 111, 112, an objection is examined;
but they are generally effective if cut at right angles to the formation
and not diagonally or transversely. When a slip is repaired by replacing
the slipped earth when dried and rammed between such drains, it should
be determined whether it would be better to cover the slope or increase
the number of drainage channels. A simpler remedy is the insertion of a
dry rubble lining on the surface of the original ground upon which a
slip has occurred, and the restoration of the slope by means of firm
earth or rammed and dried slipped soil; but the system of rammed earth
counterforts, although it has succeeded when effected during the dry
season, the earth being wetted sufficiently to make it cohesive by
ramming, will not succeed in wet or winter weather, as the soil cannot
then be consolidated by ramming operations, for it will be overcharged
with water from the effects of rain, frost, or snow. Fair or summer
weather is required to ensure success.
Should it be considered advisable, in addition to the erection of rammed
earth counterforts to a height of from one-third to one-half of that of
an embankment, a porous layer or wall of broken stone can be inserted
between the slipped material, which is not touched, so that the waters
can filter away through it and the slope be restored; and the
counterfort have a channel at its base, in order to prevent a lodgment
of water, to drain the slipped earth and the embankment, and to prevent
the counterfort becoming saturated. This method may be insufficient in
any but comparatively firm soil, and it may be necessary to make a
trench at the toe of the slope having its base 2 or 3 feet below the
seat of the slip or firm ground, it being filled with rock chippings,
shingle, gravel, burnt ballast, broken bricks, ashes, coarse clean sand,
or other material having equable frictional stability and particles that
may for earthwork purposes be considered as insoluble in water, and
therefore that form stable masses and yet act as drains; and to drain
the slopes by trenches varying in width according to the position and
depth of cutting, the depth of a trench may be little or have to be such
that it extends to the solid ground through the slip, or about 50 to 75
per cent. of its depth, which must be determined in each case, may be
sufficient. Such dry stone counterforts may be required at distances
from centre to centre of 15 to 66 feet, and are sometimes placed in the
same right line and even through an embankment, and are connected with a
drain sufficiently deep to prevent any lodgment of water at the centre
of the base of the embankment or near the foot of a slope.
With regard to covering a slope of a cutting or embankment, its
principal use is to lessen deterioration from meteorological action, to
keep the ground underneath in an equable condition, to reduce
percolation and make it uniform, to diminish the danger arising from
cracks and fissures caused by heat, evaporation, or drought, to prevent
the erosion of the surface by a flow of water or the production of
unctuous surfaces, and yet not interfere with effectual drainage.
It is known that percolation is decreased by vegetation, and that it is
less through turf than bare ground, but varies greatly according to the
kind of covering, the season of the year, the regular or irregular
distribution of the rainfall, and other causes. If the surface has to be
covered with vegetation it should be close, uniform, and vigorous
throughout the whole year, and nothing fulfils these conditions as well
as grass or turf. The protecting value of turfing earthwork may be
judged from an inspection of fortifications in which steep slopes, deep
ditches, and perfect maintenance are a necessity. As a consequence of
the binding and shielding of the earth by the roots of grasses and
plants, and their protection of the surface, the slopes stand at a
steeper inclination than they otherwise would, and the system of
clinching the exposed faces of the parapets of earthwork with a layer of
sods laid header and stretcher is found to be a great support.
In earth possessing considerable cohesion, such as the clays, provided
the covering is uniform, grass having comparatively deep roots is not a
particular advantage, but the closeness of the growth of the plant and
the unimpaired maintenance of a complete cover is of importance, and the
earth should be disturbed as little as possible so as to prevent any
particles comprising the soil becoming disintegrated and dissolved. When
it is considered advisable to adopt a covering of grass, plants, shrubs,
or trees, it should be remembered that some grasses, such as the
couch-grass, are injurious to other plants. To equally cover a slope
with sods of turf they should be cut to a regular form and depth of
about 6 inches, and be firmly pressed into their places so as to produce
an even smooth surface. A steep slope or any less than 1 to 1, if of
considerable depth, will not give a sufficiently flat surface for grass
to properly grow upon it.
In sandy and gravelly ground, as the particles may be considered
insoluble in water and as possessing no cohesion, grass, if it will
grow, having deep roots is to be preferred, even if passing through any
earth that has been placed upon a slope, in order to counteract the want
of cohesion by a greater hold in the soil and any peeling of the turf or
tufts forming or becoming detached, rolling down and leaving an unbared
surface. Bent-grass will usually grow on most sandy soils, and a shrub
known on the north-east coast of England as the sea buckthorn also
rapidly vegetates, but marine grasses must be used in ground impregnated
with brackish or sea water. To prevent light sand from being scattered
by the wind or washed away a species of rush (Bot. _Ammophila
arundinacea_) has been used in Holland and England, and is found to
flourish in dry soils, and as it has spreading roots which often grow to
a length of 20 feet or more it binds the grains of sand. Experience in
Holland has shown that grass upon sea embankments does not grow well or
flourish at a steeper slope than about 6 to 1.
In countries where wind storms occur, or the soil is of so light a
character that the passage of a train at considerable speed raises the
earth and moves it as dust, a covering of fine grass indigenous to the
country has been found to be a protection and necessary to economical
maintenance. Planting the white basket willow, or withy, has been
recommended as a means of preventing slips in loose soil, as the roots
form a network and bind the earth and cover the surface with shrub
growth. In damp ground the cuttings need not be planted as deeply as in
dry earth, but of course they will not flourish in all soils. Osiers
have also been adopted with good results in damp places.
Embankments of blown or drift sand, easily moved by the wind, have also
been protected and maintained by branches of trees laid horizontally.
They were placed in regular courses alternately with a stratum of sand,
the ends projecting over and down the slopes, which were sown with
indigenous plants that it is known will flourish upon it. Great
difficulty had been experienced in maintaining the embankment until its
surface was so shielded. Sandy dunes have also been prevented from
slipping and being eroded and destroyed by the sea and wind by reducing
them to the same line, level, and slope, by filling all openings or
depressions, or, if curved, by trimming them to a regular exterior and
the largest radius, and by a covering of fascine work in the exposed
places, and in any less exposed situation by planting them with grasses
that will grow upon such soil, the object being to remove all local
obstruction to wind or wave force and cause equable resistance and
protection. In France, the drifting of blown sea-sand into cuttings,
which not only may interfere with the free traffic but also be a
destructive agent to the rolling stock, has been prevented by planting
pine trees upon the cess.
In embankments in loose soil and across valleys, especially if they are
narrow and deep, repeated gusts of wind which have their maximum effect
upon a flat surface at right angles to it, will produce a hammering
action upon the windward side, and when the velocity is very great, upon
the leeward portion there may even be a partial vacuum created by the
wind rushing over the top of an embankment. An actual maximum momentary
pressure of wind of 80 lbs. per square foot was registered at the
Liverpool Observatory in 1869, equal to a hammering action of 0·55 lb.
per square inch. It is here referred to in order to show that in exposed
valleys the surface of all embankments of light soil should be covered
so that the particles cannot be blown away at the top or from the
slopes, for a force of 80 lbs. per square foot nearly equals the weight
of a cubic foot of dry open sand which cannot be taken as weighing much
more than 90 to 100 lbs.
If seed be sown upon a slope it is evident that it should be of uniform
kind in order to create a general equable condition, and the growth of
all rank vegetation should be prevented, for an unevenly protected or
covered surface will promote a localization of disturbing agencies, and
especially in treacherous ground when it has been covered, neither the
surface nor the turf should be allowed to be broken.
In ordinary cases where slips may be expected, soiling with mould or top
dressing, and sowing the slopes may be sufficient, and turfing in others
of small extent; but in retentive soluble soil, a filtering layer under
the turf may be required, so as to prevent spreading, particularly near
the toe of a slope; however, in mountainous or hilly districts,
experience seems to indicate that it is advisable to allow earthwork to
become consolidated by the natural effect of rain and atmosphere before
finally trimming or covering the slopes: on the contrary it may be
advisable, especially in treacherous ground, to trim and cover with
mould, sow, turf, plant, or cover the slopes as soon as possible after
they are excavated or deposited, in order to protect soluble soil from
rain, frost, or snowstorms consequent upon the wet season being near. Of
course, when experience shows that slips are improbable, the slopes can
remain bare and simply be trimmed, and a covering of grass be left to
time to effect, and this is now the frequent practice. Should settlement
be expected or be unavoidable, the face protection can be so made that
it will not be disturbed or broken. In such a case a short, straight
slope with a flat inclined berm or cess, the length of the slope being
divided into three or four short continuous straight portions, has been
adopted with success. Should sowing or planting the slopes be considered
as too slow in producing a covering, experiments can be made by mixing
available soils, or with one earth; and by exposing the mixture or earth
to the deteriorating weather influences it will have to resist, the best
protective covering to be readily obtained can be discovered.
Respecting the planting of saplings or shrubs, laws have been enacted to
compel the planting of saplings and trees upon certain lands as a
protection against landslips, and it seems to be generally acknowledged
that trees, particularly when in plantations, are a protection, as they
not only absorb moisture and bind the earth with their roots, but also
lessen any flow of water down the steep sides of a cutting or
embankment; and instances have been recorded in which trees have been
shown to preserve the alluvial banks of rivers, as when they were felled
the sides were eroded, or weakened, probably by the increase of moisture
and exposure, with the result that the channel became widened. The
systematic planting of live slips of poplar or willow has been found to
effectually protect soft banks of rivers, washed by the stream, against
weathering and erosion. Also in treacherous clay marly-soils, in which
slips of earth were numerous, acacia trees have afforded a good
protection, as their widely spreading roots conduced to hold the soil
together, and their foliage and branches gently regulated and lessened
the effects of rain and prevented quick infiltration into the earth.
However, care must be taken that the roots do not open or strain the
surface of the ground by force of the wind or otherwise, and increase
and localize percolation; but they tend to prevent cracking and
fissuring of the surface in clay and argillaceous earth, and form a
protection against the effects of the sun’s rays and drought.
Quick-growing trees should be selected having large and deep roots and
abundant foliage, especially in non-cohesive earths. Acacia and birch
trees appear to give satisfactory results. Unless in exceptional
situations, such as to protect a cutting from drifts of snow, or where
from local experience they are proved to act upon the earth as a
holdfast, it is questionable whether the indiscriminate and
non-systematic planting of saplings or bushes is not more likely to aid
disruption than promote stability, and, as a rule, other and less
expensive means of protecting a slope are to be preferred unless a
uniform covering by trees or shrubs is practicable, whether over a small
or large unstable area. Isolated trees should not be allowed within the
fences of a cutting, although if just outside the toe of the slope of an
embankment the roots may serve as a buttress, and therefore be
beneficial. Although in some dyke embankments in North Europe no trees
or plants are allowed to grow upon them, so that any deterioration of
the mass maybe clearly and quickly apparent; in Holland the defensive
covering varies according to the character of the earth, fascines,
wattling, sodding, a gravel coating, benching at the top of a slope and
planting it with reeds to protect an embankment from erosion and wash
caused by traffic; pitching, plank facing, and sheet planking at the
toe, when the ground is very soft, have all been used with success. On
silty land willows generally grow rapidly, and when planted a little
distance from the toe of the land slope in enclosure embankments they
are found to protect the ground. In some situations it may be necessary
to protect an embankment against boring by burrowing animals or
crustacea. Clay and clay loams are soils especially liable to be
burrowed. Usually as the quantity of sand increases, the boring
decreases; a coating of hard ashes may afford the required protection:
however, in the case of crustacea, local experience alone can indicate
the best protection, probably nothing less than stone pitching may
suffice, but as a general rule in this country, no precautions are
necessary; in warmer regions it may be otherwise.