Earthwork Slips and Subsidences upon Public Works / Their Causes, Prevention, and Reparation

by John Newman

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 1s. to 2s. 6d. 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.