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.
In both the preceding cases, to ensure stability and to prevent any protective works being pushed forward, the foundations must be below the formation or ground level. To lessen sliding action the foundation should incline at right angles to the face, and should have a batter either for the whole width or for a distance not less than about one-third to one-half of the bottom thickness in the case of low retaining walls. A wall having a steep batter upon each face, and therefore a wide base as compared with its area, causes the centre of gravity to be at a greater distance from the exposed face, and therefore the resistance to overturning is increased; but care must be taken that it does not slide forward. A foundation for such a wall upon clay or tenacious soil is to be preferred, provided there is no upheaval of the ground in front, but it should not be upon a thin stratum, or the latter may slide upon another.