For reservoirs the line of pressure O P is always at right angles to the back surface of the wall, so that we can simplify formula (50) and use for rain water: p = 311/4. L2 (53)

For salt water: p = 32. L2 (54)

Where p = the amount of pressure, in pounds, on one running foot in length of wall, and at one-third the height of water, measured from the bottom, and p taken normal to back surface of wall, Where L = the depth of water in feet.

Where there is a superimposed weight on the backing of a retaining-wall proceed as follows: In Figure 59 draw the angle C A D = x, the angle of friction of the material. Then take the amount of load, in pounds, coming on B C and one running foot of it in thickness (at right angles to B C), divide this by the area, in feet, of the triangle ABC and add the quotient to w, the weight of the backing per cubic foot, then proceed as before inserting the sum w1 in place of w in formula; (47) to (51) and in

Table XI, when calculating p; or w1 = w + 2.z/B.L (55)

Reservoir

Walls.

If Backing is

Fig. 59.

Where w1 = the amount, in pounds, to be used in all the formulae (47) to (51) and in Table XI, in place of w.

Where w = weight of soil or backing, in pounds, per cubic foot.

Where z = the total superimposed load on backing, in pounds, per running foot in length of wall.

Where B = length, in feet, of B C, as found in Figure 59.

Where L = the height of wall, in feet.

Where there is a superimposed load on the backing, the central line of pressure p should be assumed as striking the back surface of wall higher than one-third its height, the point selected, being at a height X from base; where X is found as per formula (56)

X = L. (w1,-2/3w) (56)

2w1-w

Where X = the height, in feet, from base, at which pressure is applied, when there is a superimposed load on the backing.

Where L = the height, in feet, of wall.

Where w = the weight, in pounds, per cubic foot of backing.

Where wt = is found from formula (55)

When calculating the pressure against cellar walls, only the actual weight of the material of walls, floors and roof should be assumed as coming on the wall, and no addition should be made for wind nor for load on floors, as these cannot always be relied on to be on hand. The additional compression due to them should, however, be added afterwards.1

The graphical method of calculating re-taining-walls is much easier than the analytical, being less liable to cause errors, and is recom-mended for office use, though the analytical method might often serve as a check for detecting errors, when undertaking important work.