CONSTRIICTION OF RETAINING WALLS stated in Section 126, the most common cause of failure of retaining walls is defective foundations. Careful attention must always be given to the sufficiency of the foundation, footings being arranged so that excessive pressure does not come upon the soil upon which the structure rests.
On compressible soils it is important to equalize the pressures so that settlement under the toe of the wall may not cause the wall to tip forward. In constructing gravity walls this is accomplished by using a footing under the main wall which extends sufficiently beyond the base of the wall to cause the pressures to be equalized over the foundation soil, and bring the resultant near the middle of the foundation. Reinforced concrete walls must be given sufficient base to prevent excessive pressures on the foundation soil.
The extension of the front base cantilever may often be used as a means of securing good distribution of pressures upon the founda tion; when this is not feasible, widening the base at the back of the wall may answer the same purpose.
When the soil is compressible, there is always some settlement, and this is greatest where the load is greatest. In many instances, there fore, it may be advisable to extend the footing sufficiently to bring the center of pressure back of the middle of the foundation so as to make the pressure greater at the heel than at the t-oe of the wall, and produce a tendency to tilt backward.
When soft materials are encountered, or when the pressures cannot be safely distributed over the foundation soil, a pile foundation or some other means of securing firm support for the wall must be em ployed. Methods of constructing such foundations, and the loads which may be borne by soils are discussed in Chapter XII.
The depth of foundations should be sufficient to prevent freezing in the soil under the footing of the walls, or of the earth in front of the wall at the depth of the bottom of the footing. This usually requires that the footing extend from 3 to 5 feet below the surface of the ground, depending upon local and climatic conditions.
133. Drainage and of retaining walls have frequently occurred because of the lack of proper drainage, hence pro vision should always be made for the ready escape of water from the earth behind the wall. If the water is held in and the back-filling becomes saturated, the weight of the material is increased and the angle of friction decreased, thus producing a much heavier pressure against the wall. Freezing of wet material behind the wall may also produce dangerous pressures against the back of it.
To provide for drainage, weep-holes are commonly left through the base of the wall at intervals of 10 or 15 feet. Li concrete walls, these are usually made by the use of drain tile about 3 inches in diameter. In stone masonry walls, the stones are set so as to leave an opening 2 or 3 inches wide through the course of masonry at the base of the wall.
When the back-filling is of retentive material through which water will not readily pass, a layer of cinders, gravel, or some other porous material should be placed against the back of the wall t-o per mit the water to reach the drains without difficulty. It is always important that water be not held in the back-filling.
The manner of placing the back-filling may sometimes have an important effect upon the pressures against the wall. The layers in which the filling is placed should slope away frotn the wall. With some materials, there is a tendency for the earth to slide along the surfaces between the layers in compacting and settling into place, which may materially increase the pressure if inclined toward the wall.
134. Gravity Walls.—In constructing gravity walls it is common to give the hack of the wall a batter by stepping off the surface, thus widening the base and making a smaller projection of footing neces sary. In walls of stone masonry, the steps are usually the height of one or more courses while in plain concrete walls the steps are usually of uniform height of 2 to 4 feet, to simplify the form work, and for convenience in placing the concrete. Fig. 70 shows a typical section for a wall of this kind, as used for carrying a railway embankment.
It is common to batter the back of a masonry wall at the top for 3 or 4 feet (see Fig. 70) to prevent injury if the backing becomes frozen near the surface and is lifted by the expansion. This is known as frost batter, and is commonly 2 or 3 inches to the foot.
Concrete is quite largely replacing stone masonry in the construc tion of retaining walls. For high walls, reinforced concrete is econom ical and usually employed, while for walls less than 20 or 25 feet high, gravity walls may often be less expensive than reinforced walls. A larger quantity of concrete is required for the gravity wall, but concrete of less rich character may be employed and no steel is needed. For reinforced walls, about 1 to 6 concrete is usually used for the body of the work, while 1 to 9 concrete may commonly be used for gravity walls; footings being made of 1 to 11 or 1 to 12 mixtures. The cost of forms does not vary greatly for the two types of wall.