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

by John Newman

With regard to earthworks in or upon sidelong ground, the configuration of the surface conduces to a movement of the hill-slope of a cutting, and the centre of gravity of an embankment is not in the middle, and in a narrow embankment may be outside the central portion, the tendency, therefore, is for the slope of an embankment on the upper or hill-side to assume a steeper inclination than that on the lower, as the earth is tipped against the inclined ground until the point is reached where a perpendicular erected upon the surface of the ground cuts the top of the inside slope. This slope, if protected from weathering, might be allowed to stand at the inclination at which it remains when tipped, and be simply trimmed and covered; but the toe of the slope on the lower or valley side is especially likely to slip, and therefore, every precaution should be taken to prevent movement. A retaining wall is a certain protection, benching the ground, weighting the foot of the slope, dividing it into benches about 10 to 15 feet apart, varying in width from 5 feet and increasing according to the depth; the benches having sufficient inclination to prevent an accumulation of water; counterforting the slopes in dry and mild weather with simple earth counterforts 5 to 12 feet in width at intervals, and in length about twice their height with a reasonable slope, the height being about half that of the embankment; a dry retaining wall may also be erected in temperate climates and in a comparatively unexposed situation: are all precautionary measures that can be adopted. But in the case of a cutting in drift or permeable soil upon rock or an impermeable surface liable to become wet, which not infrequently occurs, the conditions of equilibrium may easily be destroyed, and the slope upon the higher side may not stand at a less inclination than that of the hill. Obviously this slope cannot be obtained either in cuttings or embankments, therefore, provided the drift is of considerable depth and treacherous, means must be taken to prevent movement, and drains should be inserted so as to make the earth capable of standing at a steeper inclination than it naturally assumes; and in order to drain the ground at the seat of an embankment, a trench on the valley side filled with stones or supported by other means acting as a wall and drain can be made. Every precaution should be taken to prevent any accumulation of water in earthworks in sidelong ground, and to gently control them, and to promote this object it is well that lateral outlets be opposite to each other so as to afford through drainage and an unobstructed flow.

As drift soil upon the inclined surface of a rock may be held by friction, the least change may impair it sufficiently to cause movement, therefore it is advisable to have a narrow gullet or heading excavated in short lengths, followed pari passu with protective works, whether a retaining wall or not, so that the earth is weakened as little as possible by exposure and deprivation of its usual support; and there are many places in which, unless the system of excavating in short lengths be adopted, failure and slips must result.

In sidelong ground the formation is frequently partly in cutting and partly in embankment, if so, and the drain is impermeable and can be thoroughly relied upon, it may be advisable to make the formation incline towards the hill, so as to drain it, lessen, and perhaps prevent percolation into the semi-embankment or under its seat, for the soil being loosened by the process of excavation and deposition is in the mass more porous than in a cutting.

The chief aim of all drainage operations for the preservation of earthworks in sidelong ground is to prevent the hill waters guidelessly flowing into, upon, or under them, their accumulation, or any obstruction to their easy discharge; and to attain this object it is necessary to know the location of the sources of the supply, the quantity, and other particulars, vide Chap. IV. A proper system of drains must therefore be made upon the hill-side to gather and gently guide the waters, to lead them by channels across the formation and discharge them outside it, care being taken to protect and line the surface of the drains so that no leakage occurs or any erosion of the face. In countries where the rainfall is excessive or sudden at certain places or entirely, it may be sufficient to protect the slopes so as to allow of a flow over them, and particularly upon the valley slope of an embankment, and in a lesser degree upon the hill-slope of a cutting.

When the configuration of the country allows, it is advisable to avoid cuttings, and particularly embankments, in or upon drift soil lying upon an inclined surface of rock or impermeable soil, because it can so easily be disturbed, become unstable, and of the difficulty of restoring it to a state of equilibrium; in fact, this may be impossible, as vibration may cause it to move. In such a case the only means of preventing a slip is by erecting a wall at the toe of the slope, and as it is necessary that the foundations be in the rock beneath the surface earth, the simple operation of excavating may impel the drift down the hill. Also should an embankment be required upon drift soil its weight may be sufficient to cause the mass to slide and a diversion of a railway may be imperative, for the cost of a retaining wall, because of its height and the great pressure it would have to sustain, or be built to sustain, would prohibit the adoption of such a remedy. Therefore, an embankment is more to be feared than a cutting in such earth. Making benchings in the rock below the drift to resist movement of an embankment will most probably be impracticable, and the soil may be in such a delicate condition that a shock or very little additional weight may make it slip, and when once it has moved, its stability may not be permanently restored. Such a case may be considered as one of the worst that can be encountered, and certainly the simple removal of detached masses of rock that may slide upon the formation is easier to effect than to treat drift soil upon an inclined rock bed.

It is always well to remember that in sidelong ground a hill may be reposing at the steepest slope of stability, and when the earth is rock it may dip in the direction of a cutting or be imposed upon a perishable stratum, such as porous or inferior shale. Support upon the valley side is then absolutely necessary to restore its normal equilibrium, and every care should be taken to prevent water reaching the embankment or trickling under its base, and to control all water that may permeate through fissures in the rock face, especially should it be loosely bedded. Earthworks in such positions require to be regularly watched.

In forming an embankment upon sidelong ground every means should be adopted to increase the friction between the tipped material and the soil; consequently all turf or herbage should be removed in order that the embankment may be upon the bare earth, care being taken that no loose top mould or turf is deposited, but only the firm soil. It may be advisable to only tip earth excavated from the solid hill-side, and not top soil such as that obtained from side cutting, and unless in rock or firm soil to avoid partly side cutting and embankment.

Should an embankment have to be tipped upon rock, care must be taken to prevent the surface water flowing under its base, and to counteract sliding movement the ground should be benched and a drain made on the higher side; also in a deep cutting the slope or face should be stepped, or have a cess at about mid-height, or where desirable, to lessen the effect of the trickling of the surface water and to prevent a flow in a direct line, which might make the velocity of any leakage waters dangerous, especially after a rapid thaw. On a mid-cess a catchwater drain should be cut to intercept the surface water, which may not percolate but flow upon the slope, and, unless diverted and conducted to an outlet, may saturate or erode the bottom.

Before a cutting in drift soil upon the side of a hill can be drained, it may be necessary to sink a shaft upon the higher side some distance from the slope and construct a complete series of drains under the formation. Also should an embankment in sidelong ground be near to a river, the exposed river-bank face must be protected, and in any case, when a pier of a bridge has to be placed close to the edge of a rock, the face of the earth should be preserved by a substantial Portland cement concrete covering in order to prevent disintegration culminating in a slip or subsidence.

In setting out a line of railway or a canal care should be taken to avoid, if practicable, either cuttings or embankments in soil resting upon inclined rock, as slips are almost sure to occur. However, should it be unavoidable, their depth and height should be reduced to a minimum and, if possible, the formation be raised until the bare rock crops out, and this can frequently be effected within the limits of deviation. Also deep cuttings in clay or earth, having particles soon affected by weather, should be avoided, but they may be necessary; and should any have to be located upon a sharp curve, it is advisable to excavate the projecting spur on the convex side of the curve more than that required to obtain the formation width, as it is peculiarly exposed, and therefore more likely to slip. When cuttings are upon the escarpment of a clay hill the slope upon the valley side need not be so flat as that against the hill. An inclination of 3 or 4 to 1 may be necessary upon the hill-side, whereas a 1½ to 1 slope may be sufficient upon the other. When a slip may involve the destruction or injury of any adjacent buildings, the best plan is to erect a retaining wall at the foot of a slope, not less than about one-fourth of the height of an embankment or the depth of a cutting, ample provision being made for drainage and the expansion of the earth. It is a sure protection and support, and the consequences of a slip may be most serious in such a position. The protection of the slope and a breast wall may, however, be alone necessary upon the valley-side.