The extent of a slip will to some extent govern the remedy. Simple open stone-filled drains, 2 to 4 feet in width, and 1 foot to 2 feet in depth, extending from the base to the top, may be sufficient for shallow cuttings or embankments, such as 10 to 15 feet, and larger and deeper trenches above those depths or heights, and complete drainage of and around the slipped earth, and division of it by means of drains and pipes.
In countries where there is an excessive rainfall in a short time, it has been found necessary to catch as much of the surface flood waters and torrential streams as possible, and to reduce their velocity before passing through an embankment or down a cutting, and to provide a pond or “tumbling bay” at the base of a waterfall for such purpose, or to erect dams, when the force is not too great, so as to arrest and lessen the velocity of the flow. Without such precautions, flood waters will erode the earth, and the beds become gradually deeper; and walls at the toe, culverts, and dry stone filling across the whole width of an embankment, and stone covering upon the slopes where water issues or flows may be required. Catchment reservoirs have also proved of use in controlling the surface waters before they reach a cutting or embankment, and in permitting them to be controlled.
Many of the chief causes of slips in embankments are enumerated in 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:—