82. A retaining wall is one that is built to hold up a bank of earth, which is afterward deposited behind it. Retaining walls differ from foundation walls, in that the latter support a superstructure whose weight is generally sufficient to overcome the thrust of the earth against the wall. A retaining wall, on the other hand, depends upon its own stability to resist the earth pressure:
True retaining walls are seldom designed by the architect, as the only place for which he would be likely to plan such walls is for the support of terraces, etc.
Area walls, it is true, generally serve as retaining walls, but as they are usually braced by arches or cross walls from the building wall, they do not require the same thickness as a retaining wall proper. Several theoretical formulae have been evolved by writers on engineering subjects for computing the necessary thickness and most economical section of retaining walls, but so many variable conditions enter into the designing of such walls, such as the character and cohesion of the soil, the amount the bank has been disturbed, the manner in which the material is filled in against the wall, etc., that little confidence is placed in these theoretical formulae by practical engineers, and they appear to be guided more by empirical rules, derived from experience.
The cross section that appears to be most generally approved for retaining walls, particularly in engineering work, is shown in Fig. 44.
The wall may either be built plumb, as shown, or inclined toward the bank. The latter method is generally considered as securing greater stability, although it is open to the objection that the water which runs down the face of the wall is apt to penetrate into the inclined joints.
The thickness of the wall at the top should be not less than 18 inches, and the thickness, a, just above each step should be from one-third to two-fifths of the height from the top of the wall to that point.
If the earth is banked above the top of the wall, as shown by the dotted line, Fig. 44, the thickness of the wall should be increased. A thickness equal to one-half of the height will generally answer for a height of embankment equal to one-third that of the wall.
The outer face of the wall is generally battered, or sloped outward, about 1 inch to the foot.
Stepping the wall on the back increases the stability by bonding the wall into the material behind and having its weight increased by the weight of the soil resting upon the steps.
If built upon ground that is affected by frost or surface water, the footings should be carried sufficiently below the surface of the ground at the base of the wall to insure against heaving or settling.
If the ground back of the wall slopes toward the wall a cement gutter should be formed behind the coping and connected with a drain pipe to carry off the surface water. The back of the wall and tops of steps should be plastered with cement to the depth of at least 3 or 4 feet.