A retaining wall is a wall built for the purpose of "retaining" or holding up earth or water. In engineering practice such walls attain frequently large proportions, being used in the construction of railroads, docks, waterworks, etc.
The form of cross-section varies considerably according to cir cumstances, and often according to the fancy of the designer. The more usual forms are shown in Figs. 22 to 25. The triangular section is the one which is theoretically the most economical, and the nearer that practical consideration will allow of its being conformed to the better. • All other Lungs being equal, the greater the face batter the greater will be the stability of the wall; but considerations connected with the functions of the wall limit the full application of this con dition, and walls are usually constructed with only a moderate batter on the face, the diminution towards the top being obtained by a back batter worked out in a series of offsets. Walls so designed contain no more material and present greater resistance to overturning than walls with vertical backs.
Dry stone retaining walls are best suited for roads on account of their self-draining properties and their cheapness. If these dry walla are properly filled in behind with stones and chips, they are, if well constructed, -seldom injured or overthrown by pressure from behind. the stone is stratified with a flat cleavage, the construction of retain ing and parapet walls is much facilitated. If the stone has no natural cleavage, great care is necessary to obtaill a proper bond. If walls built of such stone are of coursed rubble, care is required that the masons do not sacrifice the strength of the walls to the face appearance. The practice of building walls with square or rectangular-faced stones, tailing of behind, laid in rows, one course upon the other, the rear portions of the walls being of chips and rough stones, set anyhow, cannot be condemned too strongly. Such a construction, which is very common, has little transverse and no longitudinal strength.
. Little or no earth should be used for back filling if stone is avail able. Where earth filling is used, it should only be thrown in and left to settle itself; on no account should it be wetted and rammed.
Thickness of Walls. Retaining walls require a certain thickness to enable them to resist being overthrown by the thrust of the material which they sustain. The amount of this thrust depends upon the height of the mass to be supported and upon the quality of the material.
Surcharged Walls. A retaining wall is said to be surcharged when the bank it retains slopes backwards to a higher level than the top of the wall; the slope of the bank may be either equal to or less, but cannot be greater, than the angle of repose of the earth of the bank.
Proportions of Retaining Walls. In determining the pro portions of retaining walls experience, rather than theory, must be our guide. The proportions will depend upon the character of the material to be retained. If the material be stratified rock with inter posed beds of clay, earth, or sand, and if the strata incline toward the wall, it may require to be of far greater thickness than any ordi nary retaining wall; because when the thin seams of earth become softened by infiltrating rain, they act as lubricants, like soap or tallow, to facilitate the sliding of the rock strata; and thus bring an enormous pressure against the wall. Or the rock may be set in motion by the action of frost on the clay seams. Even if there be no rock, still if the strata of soil dip toward the wall, there will always be danger of a similar result; and additional precautions must be adopted, especially when the strata reach to a much greater height than the wall.
The foundation of retaining walls should be particularly secure; the majority of failures which have occurred in such walls have been due to defective foundations.
Failure of Retaining Walls. Retaining walls generally fail (1) by overturning or by sliding, or (2) by bulging out of the body of the masonry. Sliding may be prevented by inclining the courses inward. An objection to this inclination of the joints in dry walls is that rainwater, falling on the battered face, is thereby carried inwards to the earth backing, which thus becomes soft and settles. This objection may be overcome by using mortar in the face joints to the depth of a foot, or by making the face of the wall nearly vertical.
Protection of Retaining Walls. The top of the walls should be protected with a coping of large heavy stones laid as headers. Where springs occur behind or below the wall, they must be carried away by piping or otherwise got rid of.
The back of the wall should be left as rough as possible, so as to increase the friction of the earth against it.
Weep Holes. In masonry walls, weep holes must be left at frequent intervals, in very wet localities as close as 4 feet, so as to permit the free escape of any water which may find its way to the back of the wall. These holes should be about 2 inches wide ani should be backed with some permeable material, such as gravel, broken stone, etc.
Formula for Calculating Thickness of Retaining Walls.
E = weight of earthwork per cubic yard. W=weight of wall per cubic yard.
H = height of wall.
1' = thickness of wall at top.