RETAINING WALLS. These, as their name implies, are walls built to retain earth, sand, or other incoherent substances in positions and forms which without their aid they could not maintain. .
These substances, if left to themselves, will not stand with vertical sides, but will fall down till they assume a certain slope. The angle which this slope makes with the hori zontal is called the "angle of repose." This angle varies according to the nature of the material; for example, that of moist soil is about 45°, while fine sand assumes an angle of about 30°.
In fig.. 1. E represents a section of a mass of earth, which it is desired to retain by means of the wall ABCD.
If we draw BG from B at the angle of repose, it is evident, from what has been said, that the prism ABG is kept in position by means of the retaining wall; and if the earth began to give way, it would do so by slipping on some line, BF. The wedge-shaped piece, ABF, which has the greatest tendency to separate itself from the rest of the mass, is called the "prism of greatest pressure;" and the retaining wall ABCD must be nude of sufficient weight and thickness to prop it up and resist its tendency to slide. The line BF is found to bisect the angle ABG.
In estimating the requisite thickness of the wall, it must be taken into account that the wall may give way in various manners; it may be overturned, or it may slide as a whole along its base DB, or the upper parts may give way, while the base From these data, mathematical formulae have been worked out, which determine the thickness requisite for different situations and materials, such as that given by M. Pon celet for ordinary materials, and within ordinary limits: x = .285(H h).
Where H, the height of the wall, and la, the additional height of the bank above the top of the wall, being given, a!, the thickness of the wall, can be found.
These formulae, however, are not of much practical value, on account of the varying nature of the data on which they are founded, and of the excess of strength requisite in all such constructions, to allow for causes of failure, which cannot be foreseen or pro vided for in the calculations. Practical experience is found to be the only safe guide in all such considerations.
Figs. 2, 3, and 4 represent sections of forms of retaining walls in common use. Figs.
2 and 3 are used in retaining earthworks, while fig. 4 is a common form of dock-wall.
In that shown by fig. 2, the thickness at the top is made i from 2 to 3 ft.; the back is vertical, and the front is sloped ant 1 ft. for every 8 ft. in height; so that the thickness ia creases with the height, in the same manner as the pressure of the earth, which it is required to resist.
The foundation is made of large stones, extending beyond the sides of the wall, so as to distribute the pressure on as huge a surface as possible. It is also sunk for 2 or 8ft. below the adjoining surface, so as to resist its tendency to slip on Its base.
At its back are placed counterforts, C, which are built up with the wall, and are about 3 ft. long by 2,1 ft. wide, placed from 8 to 10 ft. apart. These counterforts stiffen the wall like ribs; they put its center of gravity further back, and so resist the tendency to heeling or overturning; they also act advantageously in dividing the earth. and so diminishing the length of the mass, which can act together against the wall. This form of wall is very simple in construction.
The form of wall shown in fig. 3 is that which requires the least material; it also, on account of its thinness, dries and consolidates rapidly, but is not so easily built as that shown iu fig. 2, The dock-wall shown in fig. 4 is made much heavier than the simple pressure of the earth behind it would require; for it has many strains to bear of au exceptional charac ter due to its situation; such are the machinery and goods deposited on the quays and the possible accident of the dock being suddenly emptied of water, while the earth behind the wail is full of water.
In the construction of a retaining wall, a great desideratum is, that the earth behind it he well drained; for if water be allowed to accumulate behind the wall, the earth gets into a semi-fluid state, in which it gives a very much increased pressure on the wall. For this purpose, holes are left through the wall called . weeping holes:" these holes are about 9 in. high and 2 in. wide, and are generally placed about 1 for every 36 sq. ft. of wall. Also stones without mortar are frequently built up behind the wall, so forming an open stratum, into which the water drains, and is thence carried off through the weeping-holes.