DETAILS OF CONSTRUCTION.
It is universally admitted that a large majority—by some put at nine out of ten, and by others at ninety nine out of a hundred—of failures of retaining walls are due to de fects in the foundation. The general method of securing a good foundation has already been considered in Part III, and has been referred to incidentally in § 930.
The most frequent cause of the failure of retaining walls is the unequal settlement of the foundation. Since the height is much greater than its thickness, a comparatively small inequality of settlement at the two edges of the foundation produces a relatively large lateral displacement of the top of the wall, which is at least unsightly and which may change the conditions under which the stability was determined; and therefore the utmost care must be taken to secure a nearly uniform distribution of pressure on the foundation, and consequeptly a nearly equal settlement. Of course, if the soil is incompressible, or if the retaining wall rests upon a sub stantial pile foundation, a uniform distribution of the pressure on the foundation is unimportant; but otherwise it is very important.
The projection of the footing should be determined pri marily with reference to the bearing power of the soil. When the foundation is compressible, the width of the footing may properly be considerably wider than the base of the wall proper. Apparently the failure to appreciate this principle has caused retaining walls to be made heavier than was necessary. Because the top of a retaining wall tips forward does not prove that the body of the' wall is too light, since the tilting may be due to the footing's being too narrow. If a wall tips forward, an examination should be made to determine whether the movement is due to overturning at the top of the footing or to an unequal settlement of the soil under the footing. If the former,'the body of the wall is not heavy enough; but if the latter, the footing does not project far enough in front.
It is not uncommon to find cases where a wall without a footing upon a compressible soil has tipped forward; and a second wall, designed in the light of the experience with the first one, has been made heavier but also without a footing. If, instead of enlarging the body of the first wall, part of the masonry had been placed in a foot ing, its stability would have been increased without additional masonry and perhaps with less. By the above process of reasoning,
walls without footings and having a width at the bottom equal to 45 per cent of their height have been declared to be too light; while walls having a width of 25 per cent on top of an ample footing have stood successfully in similar soil. Of course, the weight of the wall is useful in resisting overturning and sliding; but it is as useful for this purpose in the footing as in the body of the wall, and far more economical of material. Sometimes, on account of the high price of land, it is desirable to place the front of the wall on, or at least near, the property line, in which case the footing can not project in front. Under these circumstances, an eccentric footing (§702) wide enough to reduce the pressure on the soil to a reasonable amount must be constructed, or land ties (§ 1033) or relieving arches (§1034) must be employed.
Retaining walls founded upon a compressible soil have tipped forward, apparently partly at least because of an error in the earth pressure formula used. The most common case in practice is a wall to retain earth having a level top surface; and for these con ditions, the formulas ordinarily employed assume that the earth pressure is horizontal. This assumption fails to take account bf the vertical component of the earth pressure, which comparatively recent experiments (see particularly .*1q02, §1003-4, and §1008) have shown to exist; and consequently the ordinary method of solution makes the pressure on the soil, particularly under the toe of the wall, less than it really is. In other words, it is usually assumed, in effect at least, that the total pressure on the foundation of a re taining wall is only the weight of the wall; while in reality it includes also a considerable part of the weight of the retained earth—not only of the earth vertically above the footing, but also part of the earth beyond the vertical through the heel of the footing. Of course, if the equivalent uniform pressure is underestimated by a certain per cent, the maximum pressure is under-estimated by considerably more than that per cent, possibly more than twice as much.