Chapter I., &c. Some of the most important operations to prevent slips

in an embankment are to thoroughly drain its seat, prevent a flow upon the surface of the original ground, percolation of drainage waters into its lower part and filtration of rain-water at the crown, and to generally protect the surface.

The stability of an embankment is not regulated by the cohesiveness of the soil, as a sand or gravel embankment, or that formed of any material whose particles are not deleteriously affected, will stand with a sufficiently flat slope if protected against erosion, and be more stable than a clay or any embankment in which the particles are soluble and soon impaired by water, however great their original tenacity. If any part of an embankment has become saturated, the internal water must be tapped and drained; as the lower surface portion is almost certain to be the wetter, the drains should be made at the base, trenches filled with open porous material upon the slopes, and, perhaps, it may be necessary to sink a few shallow wells. The slopes and formation should be covered so that when the excess of moisture is extracted from the mass it cannot be replaced. Porous earth counterforts can be adopted, 6 to 10 feet in width, placed at intervals depending upon the nature and condition of the soil and height of an embankment, or a continuous bank of similar material at the toe to support the embankment during the temporary weakening from drainage operations. One of the worst cases that may have to be treated in embankments is in clay or marl soils, when the central portion has been first tipped in a wet state in the winter months, and after an interval the embankment completed to its required width in a frozen condition, or nearly so, or one in which the earth becomes frozen when deposited.

In Russia it has been found that rain-water percolating into such an embankment cannot drain away, but accumulates and finally bursts the slopes, and that water will exude from frozen soil when it thaws, a considerable time elapsing before all the frozen earth has thawed; water is thus, as it were, taken into the mass, which in all retentive soils will be difficult to drain without turning over the earth; and it will cause slimy surfaces and general instability. It will always be an expensive operation to make such an embankment secure, and it cannot be made as firm as one properly deposited. After subsidence has ceased, an approved remedy in such a case is to cover the formation with an impermeable layer, and to raise the embankment to the rail level with sand, which is ultimately alone used for repairs, the slopes being carefully trimmed and sown.

Although not considered in the usual acceptation of the word as a slip, the trickling of the surface soil is mentioned as it is a movement which may, if allowed to proceed, cause a slip, and frequently necessitates attention because of the soil becoming in a liquid condition and flowing upon land beyond that acquired for any works, and also because it obstructs and chokes drains. It may be expected when a thaw occurs after severe frost, or heavy rain succeeds drought, or subsequent to a rapid change of weather in any earth of a clayey or calcareous nature, as, for instance, in clay marl and argillaceous chalk cuttings or embankments, and if the configuration of the ground should be favourable to its passage to land outside that purchased, such issue must be prevented by protecting the slopes by means of a covering, by draining, or by the consolidation of the surface of the earth, which latter operation may be difficult, or by the erection of a small mound near the fence line.

In ground containing salt or other solvents in appreciable quantity, care should be taken that water does not reach it; if it should, the soil immediately becomes damaged and subsides; also it is found that the earth from which soda nitrate is manufactured in North Chili must be kept perfectly dry to be secure as a foundation. All soils of a salifiable character should be considered as likely to subside and slip.

In the salt-producing districts in England it is found that when the brine, which is about 25 per cent. of the mass, is pumped up to the surface to be made into white salt, the land will subside, as in effect it is pumping up the underground supporting stratum or rock salt bed; and when a river is contiguous or copious springs, the rock-salt will be supplied with water to make it brine. The experience of those who have had to maintain embankments in these districts indicates that so long as subsidence is uniform embankments can be raised and maintained; but when water penetrates into an old pit previously comparatively dry, unequal and dangerous slips and subsidences may be expected, and on so large a scale as to require much expenditure for restoration. If the settlement is uniform, the easiest way is to simply raise an embankment; the rate of sinking varies considerably, averaging, say, from 2 to 5 feet per annum, and depending upon the amount of brine extracted and the percolation of water, &c. It is obvious that embankments upon such land require constant attention to prevent serious slips.

At the edge of a cliff or hill where loose rock exists and is joined by clayey soil, but is sufficiently stable not to slip in a mass, it may be necessary to have a cover shed over a railway or road to prevent detached pieces of rock falling upon the surface, the slopes from being injured, and larger masses sloping down. Covering a slope in such a case is useless, but an open deep trench, specially constructed to catch pieces of rock, may suffice.

Important questions to determine when a slip has occurred are:—

1. Should the whole or part of the earth that has slipped be removed?

2. How are the voids to be filled that have been caused by the slip?

3. Can the disturbed material be again used?

4. What protective measures should be adopted?

A thorough examination of the site of a slip and the slipped earth is absolutely necessary before the most effectual and economical means of restoration can be determined, for weakness or the presence of a disturbing agent in the upper or lower portion of a cutting or embankment may be the cause of movement. An embankment may be solid in the mass and only portions may slip and subside, but then may become unstable and require different treatment, or local restoration may alone be necessary, and the slipped earth to be removed be small in quantity. Extensive slips of the whole of an embankment usually occur from springs in its seat or the existence of a flow of water upon the ground under the base, producing a greasy surface. Should it happen that an embankment of pervious soil is tipped upon impervious ground having a depression resembling a basin, water will accumulate until it reaches the level of discharge, and a serious slip may result. In such a case the slipped earth must be removed and the water tapped and permanently drained. The upper portion of an embankment may slip and the lower be stable; if so, it is not so serious as when movement commences at the toe and the slope bulges outwards and the embankment subsides; in the former case, provided the lower portion is not affected or its drainage obstructed, it may not be necessary to remove the slipped earth, but it is advisable to drain it, and any localization or lodgment of water between the slipped mass and the firm part of an embankment must be prevented, or the toe of the slope will be made in an unstable condition.

In cuttings in order to keep open the formation the whole of the slipped earth may have to be excavated, but in embankments, so long as the soil does not extend outside the fencing, its entire removal becomes optional, and is unnecessary provided further movement be prevented, and the soil drained; but in most treacherous earths, although a slip may be arrested, it will generally be a place requiring constant watching, and be one of doubtful stability. Earthwork slips require to be remedied as soon as possible after they occur, not only to repair them and obviate an interruption of traffic, but in order that the unslipped portions may not be deleteriously affected and movement be induced.

When the earth is very soft, silty, and difficult to drain, the only course may be to remove the slipped material, although it may not be necessary to excavate all of it, as it may form a reservoir for the accumulation of water, and is certain to be liable to disturbance from the effects of weather; but in firmer soil a portion of the slipped earth may be excavated, and be rammed in layers inclining at right angles, or nearly so, to the surface of the slope, and a drain can be inserted at about the bottom of the line of the slip to prevent any water that may percolate from the unslipped mass flowing into the rammed earth or any counterfort so constructed; but counterforts may afford insufficient support in very treacherous soil, and it may be necessary to remove either the whole or a considerable part of a slope and replace it with the best available material watered and rammed: however, the simple ramming of the earth and depositing it in inclined layers may not be sufficient to ensure stability, and should it be found that the slipped material is very soft and cannot be readily drained, it must be excavated, and solid and firm earth put in its place.

When the base of a slip is level with the bottom of any side ditch that may have existed before movement occurred, the drain should be below the level of the ditch, or a flow of water may be induced at the seat which will probably cause further unsettlement; and if a slip extends below the bottom of a cutting it is necessary to remove the slipped earth as far as the solid ground, and to fill the void with dry material of sufficient weight to prevent the surface being uplifted, and to cause solidity in order to avoid any movement of the toe of a slope.

When the slipped soil is clay or shale it can be burnt _in situ_ down to the solid ground, or upon an incombustible bed, and be converted into a kind of brick rubbish and then be restored to its original place; but this may be an expensive method, and it may be cheaper to procure firm earth, nevertheless, should no other material be available it may be the only economical means of repairing a slip. Before deciding whether clay or shale shall be burned _in situ_ it may be advisable to test the amount of ballast that can be made by, say, 1 ton or more of coal, and the cost including every item of expenditure: 10 or 12 cubic yards of ballast may be obtained for every ton of coal burnt, but this quantity may be so reduced that the cost of burning may prohibit the use of such a method for replacing the slipped earth when made into firm soil. It much depends upon the quantity of water in the material, and also upon the nature of the earth; for instance, burning becomes more difficult as the amount of silica in the clay becomes greater, and the ballast is not so good as the quantity of lime increases in the clay; therefore pure clay makes the best burnt ballast. Should it be decided to burn the slipped earth, it is necessary that it be placed upon firm ground, and that it rests upon an open layer of stone or material that will not kindle in order to obtain the necessary draught. The thickness of the layers must be regulated by the degree of wetness of the soil, from 1 to 2 feet being required for thorough burning, and should layers of a less thickness than 1 foot be required, the process of making the slipped earth into burnt ballast will usually be too expensive, but of course much depends upon the price of the coal upon the site. The burnt ballast may cost anything from 1_s._ to 2_s._ 6_d._ a cubic yard; when the latter price is reached, it may be cheaper to procure sound earth. If the slipped earth approaches the condition of carbonaceous shale, black or dark brown in colour, it may kindle easily; the more argillaceous shales will require a little coal to convert them to burnt ballast, the quantity increasing as they gradually become of a clayey character. Blue clay, when thoroughly burnt, generally makes better ballast than most other clays, but as a drain the ballast is not equal to clean gravel.

In considering whether it is only necessary to simply replace the dried 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.