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

by John Newman

3. Overhead arches and iron struts at intervals to resist and reduce the pressure upon a wall.

In such a situation, should a tunnel or covered way be not required, any subsidence of the earth may cause the destruction of valuable property, and the erection of a retaining wall be imperatively necessary from a due consideration of prudential construction, and altogether regardless of the character or condition of the earth or its liability to become water-charged. In order to prevent movement of the earth different forms of support may be required. In some cases, owing to excessive pressure, a counterforted wall with an invert under the permanent way may be essential, or the invert may be flat and be arched on plan, thus supporting the toe of a retaining wall between the intervals, in combination with a vertical or inclined pilaster and a front counterfort system, and in addition flat overhead struts of iron or other material between the walls acting as an auxiliary support above the required traffic space may have to be adopted where the walls have to sustain considerable thrust, the thickness of the retaining wall being thus reduced; or a simple retaining wall may be sufficient. A non-jointed material as Portland cement concrete of equal character is to be preferred to a mass consisting of hard materials yet incapable of possessing a joint equal to their strength or durability.

With regard to the protection of the toe or lower portion, of a slope by means of a retaining wall, it may be the only effectual support in one case, and not succeed in an exposed country without a complete system of open or closed drains, and then its adoption may be superfluous. In the following few paragraphs an endeavour is made to indicate some situations in which retaining walls have or have not been completely successful when erected for such purpose.

First, it is most important that ample provision be made for draining the back of the walls, for if the egression of the surface waters be obstructed, they must accumulate and cause hydrostatic pressure, soften the lower portion of the earth, thereby failing to partly effect one of the objects of their erection, and cease to protect the surface; the probable result being that the wall is pushed forward, broken up, or overturned. The drainage must be regulated by the quantity and velocity of the flow, and ample weep-holes should be provided to prevent an accumulation of water at the back. In damp soil there should be one to about every three or four superficial yards, and an outlet at each wet place, or a wall may not stand. The wall should be backed with a filtering medium such as coarse gravel, hard ashes, ballast, and no retentive earth should be used. In clay earths a retaining wall should always have a dry porous backing, as it not only reduces any pressure due to a head of water, but also allows of the earth swelling without affecting the stability of the wall, as it probably would if the clay rested against the back of the wall.

Retaining or breast walls are particularly useful in loose soil having no cohesion, i.e., those of a sandy character or consisting of very small grains, and which upon saturation by water or by the action of its flow become in an unstable state or one of actual movement; in such, a case not only is it requisite to protect the surface of a slope, but support at the toe is indispensable. In cohesive soil, such as clay or clay marls, surface protection, combined with systematic and thorough drainage, may be all that is required to make a slope stable, and the erection of an impervious or solid high wall be unnecessary, always provided the earth cannot slide from being superimposed upon an inclined stratum. In fact, retaining walls in retentive soil have been found to induce a slip, because they neither drain the earth nor prevent the additional impregnation of water, and they have consequently been destroyed.

In countries where floods or very heavy and sudden rainfall, frost and snow quickly succeed, masonry or dry stone retaining walls at the foot of a slope soon become impaired, and require constant supervision and careful maintenance, and cannot be considered as economically or generally effectual; and should the earth settle, as they are comparatively solid they will not follow any subsidence of the surface; therefore cracks occur and water accumulates in the hollows, the slope has no longer uniform support, a localization of the egression of water is caused, owing to the fissures in the wall inducing a flow, and the wall becomes a cause of a slip instead of a protection against movement. Should such a wall bulge after it has been restored to the condition of being a continuous support, its forward movement may be arrested by the erection of counterforts with an inclined face in front from 10 to 30 feet apart according as weak places exist, having joints at right angles to the batter; and this is, perhaps, the cheapest and quickest remedy, but the slope must be carefully drained and the number of weep-holes be increased.

In order not to obstruct, but induce the through drainage of water, retaining walls to prevent slips have been erected, consisting of arches turned upon piers, the intervening space being dry walling which allows a free flow of water, the idea being to afford the necessary support without interfering with the drainage. In some cases, as it tends to condense the soil, weighting the toe of the slope may prevent movement, and on the side of a hill weight and mass, apart from slope protection or drainage, must be provided to arrest a slip or prevent further movement, either by means of a continuous wall, or by the addition of frequent counterforts, 5 or 6 feet in width, to an existing wall, if space permits, the back and front having a considerable batter: the centre of gravity of such a mass being low and the base large, overturning is improbable. Forward movement can be also guarded against by deep foundations, and as the earth will rest on the inside they can have a flatly inclined base sloping towards the cutting, as it will tend to prevent overturning, the depth of the foundations at the face being two or three times that at the back. Such a structure can hardly be called a wall, being really an extensive and massive concrete toe.

Perhaps generally the most economical and secure way of preventing movement is to erect a low Portland cement concrete wall at the toe of a slope with a considerable batter; or to cut a trench at formation level outside the line of the slope, and to shore it by means of old sleepers strutted at intervals when better and durable material cannot be readily obtained.

The chief object of a protection at the toe in clay soils is to prevent the bottom of a slope being softened by a lodgment of water, or fissured by heat or drought; this may be effected without a wall; but in the case of sand or fine granular earth cuttings liable to become quicksand, support at the base is absolutely necessary, and the best way to prevent movement may be to erect a breast retaining wall at the toe a few feet in height, say 3 to 5 feet, the backing being of dry porous material such as ashes, coarse gravel, or broken stone, increasing according to the depth or height of the earthwork and quantity of water, the thickness being more than sufficient to contain the drainage waters, and not less than 1 foot 6 inches, in order that it may act as a filter and drain the slope, &c., so as to lessen the percolation of water. Sheet-piling and a backing of rubble acting as a drain 2 or 3 feet square, or a strutted timber duct, both made out of old sleepers, have been used with this object in quicksand where water rapidly percolated to and accumulated at the base. Instability of the toe may also be prevented by covering it with layers of gravel, but one of the reasons that may cause a gravel counterfort to fail in loose porous soil is that sand or mould may be washed through the interstices in the stones forming the gravel; hence a more impermeable covering is to be recommended in such a case.

A toe of rammed coarse gravel or broken stone may supply the required support in the case of coarse and fine sand, when the latter is alone movable. Experiments have shown that sand rammed in layers of about 5 inches and earth mould 2 inches in thickness give the best results. The less the weight is increased by ramming the more solid the original earth. Brushwood or fascine work weighted with gravel, stone or broken bricks, and stone pitching can also be used for sandy soils, but a filtering layer must cover the sand, or the latter will be eroded. Counterforts of well-rammed natural earth may be sufficient provided the soil is firm and is made compact and uniform in texture. In otherwise stable soil, when it is known that a sand vein is the cause of a slip because of water percolating through it, it should be raked out as deeply as practicable if not too large, and the space be filled with stones, coarse gravel, or dry material forming an open drain for the water to issue without a flow of sand, thereby preventing any accumulation. When a stratum of shifting sand overlies beds of conglomerate and gravelly or clayey sand it will be necessary to support the lower portion of its slope by a wall. The wettest places in a slope should be noted and the surface be turfed or covered, and should the depth of the unstable sand exceed 5 or 6 feet, narrow benchings can be made 2 or 3 feet in width at every 5 or 6 feet of vertical height to divide the flow of water and prevent it following a continuous course. It also sometimes happens that two treacherous earths have a stratum of stable soil between them such as rock; in that event it is advisable to leave a cess between the bottom of the slope of the upper stratum of unstable soil and the underlying rock, so as to allow of weighting or other works to resist movement of the toe.