In the design described above, the extension of the toe beyond the face of the wall is so short that there is no danger that the toe will be broken off on account of either shearing or transverse stress. It is usually good policy to place some transverse bars in the base-plate which are perpendicular to the face of the wall, and to have them extend nearly to the point of the toe. No definite calculation can be made of the required number of these bars, unless they are required to withstand transverse bending of the toe.
If there is any danger that the subsoil 'is liable to settle, and thus produce irregular stresses on the base-plate, a large reinforcement in this direction may prove necessary. It is good policy to place at least '-inch bars every 12 inches through the base-plate, for the prevention of cracks; and this amount should be increased as the uncertainty in the stress in the base-plate increases. Although there are no definite stresses in the top of the wall, it is usual to make the thickness of the face-plate at least 6 inches at the top, and also to place a finishing cornice on top of the wall, somewhat as is shown in Fig. 113.
When the subsoil is very unreliable, it is even possible that there might be a tendency for the front and back of the base-plate to sink, and to break the base-plate by tension of the top. This can be re sisted by bars in the upper part of the base-plate which are perpen dicular to the wall.
304. L=Shaped Retaining Walls. Retaining walls of very moderate height may he constructed in L-shaped sections without buttresses, by thickening the walls at the base, and by using suffi cient reinforcement to resist the tr'msverse stresses, which, of course, have their maximum value at the base of the wall (Fig. 114). From
the standpoint of cubic yards of concrete and pounds of steel, such a wall .is not as economical as the buttressed wall, but the forms are very much more simple and are less expensive. A low wall is always made much thicker than mere theoretical computation would call for, and in such a case the additional thickening for the L design might be little or nothing. For high walls—twenty feet or more—the economy utterly disappears. The mechanics of this form of wall is quite different from the form with buttresses. In the case of a buttressed wall, the vertical plate between the buttresses is merely designed to resist the bursting pressure on a slab which has the buttresses as abutments. When there are no abutments, the pressure on each unit vertical strip of the wall must be computed; and the strength at every section (vertically) must be computed on the basis of a cantilever acted on by horizontal forces. This practically means that the moment increases from zero at the top of the wall to a maximum at the base just above the base-plate. Of course the mechanics of the wall taken as a whole, in its pressure on the subsoil, is identical with that of the other form of retaining wall.