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

by John Newman

To prevent movement of the earth at the back of a wall pushing or fracturing it and being the cause of a slip, it may be well to briefly state a few points to which attention should be especially directed.

In adopting two systems of design of a wall, it is advisable that the change of form be gradual and not made abruptly, or cracks may occur from unequal settlement or load upon the foundations, and also when a concrete bed is used under a wall a lime concrete layer cannot be firmly joined to one composed of more solid and unyielding material, such as Portland cement concrete. Similar walls erected upon different soils are not equally strained, as one earth may scarcely change from the effects of air and water, and another may vary daily according to the weather.

To counteract any overturning tendency, a wall should have a large bearing area under the face portion, all projections and heavy copings should be avoided, or anything that increases the load near the face at the top. Weight and width of base and considerable depth of foundations are most important, for well designed walls when placed upon stable ground have been pushed forward simply from an insufficiently deep hold in the ground; and the load upon the earth must be within its safe bearing power. A wall should be designed so that its centre of gravity is as far from the face as practicable, and when it is constructed of stone or brick it is necessary that the joints be as strong as possible, and be capable of resisting varying strains. Should lime mortar be used it is probable it will be partly washed out, and Portland cement mortar should always be preferred upon the face, or water may percolate into the wall and even pass through it to the earth and cause slips and subsidences.

Counterforts erected upon the outer face of retaining walls are much to be preferred to any placed at the back, for support at the face is what is required: in the latter case there is always a chance of their becoming separated from the main wall, although when so located at the back they are considered to lessen the lateral pressure of the earth by dividing it, but any reduction caused by friction of the soil against the sides may vary so greatly, according to the condition and subsidence of the earth, that it is prudent to disregard it, and it is well to remember that the pressure may become intensified or uneven upon the main wall by deflection and concentration caused by the counterforts, the result being the creation of weak and unduly strained places; it may therefore be better to uniformly increase the thickness of the wall. Counterforts of triangular shape on plan have been adopted in order to lessen the lateral thrust by directing it to the sides of the triangles, their bases being against the wall. It is doubtful whether the thrust of the earth so acts under the usual varying conditions of work. The necessity of mooring vessels alongside a dock or quay-wall prevents the adoption of face counterforts except to a very limited extent; however, although the form of a ship permits of only a slight batter for the upper half, the lower portion may be inclined for a few feet from the bottom. As the adoption of a batter on the face increases the area of the base of a wall and its frictional surface upon the ground, and tends to lessen overturning movement and undue strain upon the face, a dock, quay, or retaining wall having a batter is to be preferred to one with a vertical face; in fact, it acts as a triangular front counterfort, with the great advantage of being an inherent part of a wall.

In foundations of a sandy or silty character, a dock or quay-wall having wells in the cross section to within a few feet of its base filled with rammed light dry material, the width of the bottom nearly approaching that of the depth of water, has been adopted in preference to a solid wall of less thickness, as affording a wider base, greater resistance to overturning, and a reduction of the weight upon the foundations.

A slip of the earth backing is frequently caused from insufficient drainage and a consequent accumulation of water behind a wall, producing such pressure that the friction of the weight of the wall upon the foundations is impaired and the support in front insufficient to resist a forward movement, therefore the back drainage should receive due attention. When a dock-wall has been thrust forward by the earth it may be a serious movement, likely to increase and culminate in the destruction of the wall, or be merely a slight lateral settlement. Such slips are usually of considerable length, and have an apex where the greatest forward motion has occurred. The chief remedies consist in securing the foundations, supporting the face, lightening or removing the pressure of the backing, draining the backwaters and preventing their reaching the wall. Provided a wall is intact and not damaged, but solidly and horizontally pushed forward, it need not be taken down, but if vertical subsidence has taken place, or a settlement, at the face only, it is generally of serious importance, and it may be necessary to remove and replace it.

When a wall bulges slightly from the pressure of the earth at the back and then ceases to move, the ground behind may have become in a state of permanent equilibrium, and the lateral thrust, which before movement was too great, may be so reduced that in the altered position it may remain stable, provided the void is filled with light porous backing, and the drainage of the earth receives due attention. Counterforts of piles reaching to the level of the ground in front of the face have been adopted to arrest and prevent a forward movement, but when the ground is much disturbed and in a loose state, it is unadvisable to drive piles because of vibration which may induce further motion. Instead of timber pile counterforts, trenches might be cut at intervals along the toe and be filled with quick-setting cement concrete, the filling to follow the excavation as closely as possible, and no unsupported earth to be allowed to remain.

As improper backing often induces a slip and subsidence of the earth and the failure of a wall, it is of importance that it should be carefully executed, for many well-designed walls have failed or bulged from being badly and hurriedly backed with unsuitable and soft retentive material in a deteriorated condition, the result being movement of the earth. Care should be taken that the material of which the backing is composed will always maintain the slope of repose upon which the calculations have been based that determined the dimensions of the wall; therefore, it should always have an angle of repose which is not much affected by moisture. It is important to remember that according to the nature of the backing so will be the pressure upon a wall, depending principally upon the coefficient of friction of the earth upon similar earth, its cohesion, its friction upon the surface of the back of a wall, the inclination of the layers, the general character of the soil and the effect of moisture upon it; and no vegetable, decayed, or “made” earth should be used for such a purpose, nor material which gives a varying thrust according to the state of dampness or dryness, and the lightest dry, firm, and stable earth should be preferred. When rubble backing is adopted and is carefully packed, the lateral thrust may be greatly reduced, as in some degree, instead of backing a dry wall may be considered to have been built at the back of the face wall. Clean ashes are an excellent material for filling damp places in a wall, because they not only absorb moisture, but are light, and stand permanently at a slope of about 1 to 1 TO 1¼ to 1, but they can seldom be obtained in sufficient quantity to be used in considerable masses; also broken bricks and burnt ballast, although heavier, make good backing.

Earth that is much affected by air or water, or that expands, contracts, or fissures, such as clay, is not good for filling, and dry masonry retaining walls should not be hacked with earth but with rubble, or in countries with a heavy rainfall the wall will most probably be forced forward.

Care should be taken to have plenty of weep-holes in a wall so as to obviate any accumulation of water, and it is well if the back has a rough face in order to increase friction and prevent cavities in the backing down which water may percolate. During construction a few weep-holes in a dock or quay-wall should always be provided, until the water is to be let in to a dock, when it is best to fill them with strong Portland cement concrete as water would proceed along them from the face and accumulate at the back of a wall; the permanent drainage being affected by other means.