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

by John Newman

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.