First.—Weight of concrete, 140 lb. per cu. ft.
Second.—Weight of material from the surface of the ground to a depth of 12 ft. (which was shown by tests made in bore-holes to be the elevation of the ground-water surface), 100 lb. per cu. ft.; and angle of repose, 30 degrees. The distance of 12 ft. below the surface was the depth of the inverts of the sewers, which undoubtedly drained the ground above them, thus accounting for the standing of the ground-water in planes practically parallel with the surface.
Third.—Weight of buildings back of wall neglected, as that of the present type will about equal the cellars filled with material at 100 lb. per cu. ft., and if large buildings are erected in the future they will undoubtedly be carried to rock.
Fourth.—Reaction from superstructure, live and dead load, 20,000 lb. per lin. ft. of wall.
Fifth.—Weight of materials below the 12-ft. line, 124 lb. per cu. ft., ascertained as follows: The material was considered as weighing 165 lb. per cu. ft. in the solid, and having 40% of voids filled with water at 62.5 lb. per cu. ft., the resulting weight being (165 × 60/100) + (62.5 × 40/100) = 124 lb. per cu. ft.
Various angles of repose were used for this material in the investigation, and it was finally decided that 30° was the greatest angle that could be expected, whereas the worst condition that could be anticipated was that the sand and water would act separately and give a pressure as follows:
Hydraulic pressure from liquid weighing 62.5 lb. per cu. ft. plus pressure from sand with angle of repose at 30° and weight as follows:
Weight of 1 cu. ft. in air = 165 × 60/100 = 99 lb.
Weight of water displaced by 1 cu. ft. = 60/100 × 62.5 lb. = 37.5 lb.
Weight in water, therefore = 61.5 lb. per cu. ft.