The matters considered in the two preceding sections (§ 1026 and 1027) probably explain why many retaining walls founded upon a compressible soil have tilted forward. The tilting of such walls can be prevented by placing the footing so that the center of pressure on the soil shall be inside or back of the center of the footing, thus making the pressure under the heel of the footing a maximum and producing a tendency of the top of the wall to crowd against the back-filling. The earth has a greater passive resistance than an active thrust; and hence . the crowding inward of the wall is not likely to do any harm, and in any case is preferable to a ten dency to tilt outward.
Unfortunately, in the present state of our knowledge of the theory of earth pressure, it is not possible to determine certainly the center of pressure; and hence all that can be done is to determine it as accurately as possible, and then provide a liberal factor of safety by making the footing large enough to reduce the uniform pressure on the soil to a reasonably safe value. The center of the footing should be placed as near the center of pressure as possible, but pre ferably on the outside,—for the reason stated in the preceding paragraph.
Sometimes it is impossible to extend the footing in front of the face of the wall on account of conflicting property interests, in which case it is necessary (1) to use piles or their equivalent under the toe of the wall, or (2) to build relieving arches (0 1034) against the back of the wall, or (3) to build the back of the wall on a batter so flat as to throw the center of pressure at a considerable distance from the toe of the wall. It is hardly possible to secure the last solution with out building a hollow wall or using a reinforced-concrete counter forted wall (I 1051).
Next to a settlement of the foundation, water behind the wall is the most frequent cause of the failure of retaining walls. The water not only adds to the weight of the backing material, but also softens the material and changes the angle of repose so as to greatly increase its lateral thrust. With clayey soil,
or any material resting upon a stratum of clay, this action becomes of the greatest importance. Further, the freezing of.undrained back filling and the consequent expansion is a potent cause of the failure of retaining walls.
To guard against the possibility of the backing's becoming saturated with water, holes, called weepers, or weep holes, are left through the wall. When retaining walls were built of stone-block masonry, the usual rule was to allow one weep-hole, two or three inches wide and the depth of a course of masonry, for each four or five square yards of front of the wall. When the wall is constructed of concrete, a 3- or 4-inch tile should be built into the wall at intervals along the base of the wall 'according to the climate and the reten tiveness of the back-filling—in the north Central States not usually more than 10 or 15 feet apart.
When the backing is clean sand, the weep-holes will allow all the water to escape; but if the backing is retentive of water, a vertical layer of broken stone or coarse gravel or cinders is sometimes placed next to the wall to act as a drain. Sometimes vertical lines of tiles with open joints or of perforated wrought-iron pipe are in serted behind the wall to conduct the water to the weepers. Some times both the porous back-filling and the vertical drains are used together.
When the backing is likely to be reduced to quicksand or mud by saturation with water, and when this liability can not be removed by efficient drainage, one way of making provision to resist the additional pressure which may arise from such saturation is to calculate the requisite thickness of the wall as if the earth were a fluid (see the third paragraph of 5 1007). A puddle-wall is sometimes built against the back of dock-walls to keep out the water.