129. When a wall has to keep in place a filled-in backing of earth, rock, or gravel, it is called a retaining wall, because it retains the earth in its place, and resists its natural tendency to cave in. Foundation walls also" act as retaining walls, with this difference, that they support a superstructure whose weight is usually found sufficient to overcome the thrust of the earth against the wall. A true retaining wall carries itself, and also must have strength enough to resist the pressure of the earth behind it. Area walls are, to a certain extent, retaining walls; but, as has been already shown, they are usually braced by arches or cross walls from the foundation wall, and hence do not require the same thickness as a retaining wall proper.

130. Nearly every writer on engineering subjects has some theory or formula for computing the best section and thickness of retaining walls. The best authorities admit, however, that so many conditions enter into the design of these walls - the nature of the soil, how far and to what extent the bank has been disturbed, the way the earth or other materials is filled in against the walls, etc. - that they can place but little confidence in theoretical formulas, and prefer to be guided more by rules based on experience.

131. The cross-section generally used by engineers is shown in Fig. 52. The wall may be built plumb on the inside, as shown, or inclined against the bank. The latter method is considered to be the better one, on account of giving more stability, but it is more difficult to build well, and as the beds of the stones must all be inclined, water running down the face of the wall is likely to penetrate the joints.

Retaining Walls 54

Fig. 52.

The best material for retaining walls, and the one generally used, is good hard split or block stone, having a level bed, always laid in cement mortar, and carefully bonded, so that the stones will not slide on the bed joints from the pressure of earth behind them.

The wall should be not less than 12 inches thick on top, and the thickness above each step should be about one-third distance from the top. Thus, in Fig. 52, the wall at c is 12 inches thick, at a1 above the step the distance from the top is 4 feet and the wall is 1 foot 4 inches ( = 4 ft. 3) thick; at a2, the distance from the top is 8 feet and the wall is 2 feet 8 inches thick; at a3, the distance from the top is 12 feet and the thickness of the wall is one-third of 12 feet, or 4 feet; while at the base a4 the wall, being 15 feet high, is 5 feet thick. At b, b are shown the footings, each course 12 inches thick, and each projecting 6 inches beyond the course above.

The stability as well as the appearance of a retaining wall is increased by battering or sloping it outwards. This batter, as shown at c, c, is usually about 1 inch to the foot.

132. When the ground back of the wall slopes towards the wall, a cement gutter d should be formed behind the coping and connected with a drain pipe e, to carry off surface water that would otherwise run down behind the wall and affect its stability. Sometimes an open drain is placed at g, on the section connected with the gutter at the bottom of the wall as shown by the dotted lines //; where the backing of the wall is gravel, or a shelving clay bank with sand on top, this precaution is necessary. The form of drain used is shown at (b) in Fig. 5.

133. When the earth is terraced or banked up above the wall, as shown by the dotted line f-f, additional thickness is required. It is generally agreed that if the embankment is one-third the height of the wall, the wall should have a thickness equal to one-half the distance from the top. Therefore, the wall at a2 would be 4 feet thick; at a3, 6 feet thick; and at a4, 7 feet thick. The stability of a retaining wall is increased by stepping the wall on the back, because the steps bond the wall into the material behind it, and the weight of the soil resting on the steps being added to the weight of the wall, it is not so likely to slide out at the bottom. The footings should be carried well below the base of the wall to insure against heaving from frost or settling. It is also well to plaster the back of the wall and the top of the steps with cement, to a depth of 3 or 4 feet from the top of the wall.