It is a significant fact that a retaining wall may apparently withstand the pressure against it for a period of several years, and may then slowly and gradually fail. This is sometimes due to the action of frost on the soil behind the wall. The water accumulates behind the wall in the early winter, and, if it is unable to drain away, may freeze, expand, and exert a pressure on the wall which forces it out. One great precautionary feature in the construction of retaining walls is to place drain-pipes through the wall at sufficient intervals so that water cannot accumulate and remain behind the wall. The gradual failure of walls may also be due to the undermining and weakening of the subsoil, which makes it unable to resist the concentrated pressure on the toe of the wall. Faulty construction and the violation of the ordinary rules of good masonry work - the latter being sometimes done with the idea that anything is good enough for a retaining wall - are also responsible for some failures, since they prevent the body of the wall from acting as a unit in resisting a tendency to overturn.

The tendency to slide outward at the bottom, and even the tendency to overturn, may be materially resisted by making the lower course with the joints inclined toward the rear. This method of construction is all the more logical, since it makes the joints nearly perpendicular to the line of pressure. In fact, the line of pressure is really a curved line which is more nearly vertical toward the top of the wall, and more and more inclined to the horizontal toward the bottom of the wall. The recognition of this principle has sometimes resulted in designing retaining walls on the principle illustrated in Fig. G9, which is somewhat similar to a section of an arch set on end. Such curved outlines, of course, are more expensive, and are sometimes inconvenient, and for that reason are but seldom adopted.

Fig. 69. Retaining Wall with Curved Cross Section.

Fig. 69. Retaining Wall with Curved Cross-Section.

A detail which is frequently adopted in the design of retaining walls, is to use what is virtually a batter to the rear face of the wall, but to accomplish this by a series of steps on the rear of the wall. This not only permits the use of rectangular blocks of stone and the employment of vertical joints, but also adds considerably to the stability of the wall, since the vertical pressure of the earth on the horizontal steps adds considerably to the resistance to overturning. In

Fig. 70 is shown a design for a retaining wall made to support a railway embankment in a location where the natural surface was so steep that the embankment would not readily obtain sufficient support. Although this use of a retaining wall is somewhat special, the general outline of the design not only conforms to the standards on that railroad, but represents good practice and is an illustration of many of the points referred to above. It should be noted that in this case the total width of the base of the wall is nearly one-half the height.