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.