Provided there is no appreciable superincumbent weight to be borne nearer than 10 feet from the top of the slope, and that the foundations of a building are at a considerable depth in the ground, and the surface and back drainage waters properly controlled, no retaining wall may be required, assuming the original inclination under ordinary conditions to be proved to be the permanent slope of stability; but should the distance of the face of a building from the toe of the slope after widening be insufficient to allow of the original slope being adopted, a retaining wall will be necessary, its height being chiefly governed by the proximity of any building, and the necessity of nearly maintaining the originally established steepest slope of repose in the case of any unsupported earth. The advantage of a sufficient cess in such a case is that it makes provision against deterioration of the surface, and causes an imaginary slope of the same inclination as the original slope to be contained within the space between the face of the building and that of the retaining wall at formation level. In sandy or loose soil if any buildings or wells near the site show signs of cracking, the excavation should at once be stopped to see what preventive measures are requisite, and pumping water out of a trench may be dangerous. Any retaining wall in such a position should be erected in short lengths, so that the earth and foundations are exposed to the weather as little as practicable. CHAPTER VII. NOTES UPON THE PRESERVATION OF THE FOOT OF A SLOPE.—VARIOUS METHODS OF COVERING AND SUPPORTING A SLOPE.—PROTECTION FROM SNOW-DRIFTS.—THE FORMATION WIDTH OF CUTTINGS AND EMBANKMENTS.—THE DELETERIOUS EFFECTS OF VIBRATION. The protection of the toe of a slope is of importance, as it is usually the most vulnerable and the weakest part. When the earth is of the same character, the quantity of water is usually greater at the lowest level than above it, and the stability of the soil in its vicinity is therefore lessened. In clay soils this softening action at the base may cause a slip and probably can only be prevented by reducing the percolation by surface drainage, which has been referred to in Chapter IV.; the matter under consideration being the counteraction of movement in the toe by other means than draining, although combined with it, such as by— 1. An impervious retaining wall with a pervious backing of ashes, gravel or sand, and ample weep-holes, at the foot of a slope, which, by preference, should be of Portland cement concrete, a more homogeneous material than brickwork or masonry, as it has no joints, and is particularly to be preferred for retaining walls in clay soils, as it approaches in a greater degree the condition of air-tightness and that of equal resistance. 2. By a pervious wall or counterfort of gravel, burnt ballast, hard chalk, rubble, strutted timber framework, or a covering of other firm material.