The pressure of the pile of brick per square foot of trench area is, from Table 46, 120 × 6 = 720 pounds per square foot. The value of _C_ from Table 44, is about 0.70. Therefore _L_{p}_ is 0.7 × 9 × 720 = 4536 pounds. The equivalent depth of backfill weighing 130 pounds per cubic foot is (4536)⁄130 × 9 = 3.88 foot. The total equivalent depth of backfill is therefore 3.88 + 15 = 18.88 feet. The ratio of depth to width is 18.88⁄9 = 2.98. The coefficient _C_ in the expression _W_ = _CwB_^2 is 2.17. The total load per foot length of sewer is therefore _W_ = 2.17 × 130 × 81 = 22,800 pounds. TABLE 47 WEIGHTS OF SHORT LOADS ON SEWER TRENCHES (Adapted from Specifications of the American Bridge Company for Bridges) ──────────────────────────────┬──────────────────────────────────────── Street railways, heavy │A load of 24 tons on 2 axles on 10 foot │ centers. Street railways, light │A load of 18 tons on 2 axles on 10 foot │ centers. For city streets, heavy │A load of 24 tons on 2 axles 10 feet traffic │ apart and 5 foot gage. For city streets, moderate │A load of 12 tons on 2 axles 10 feet traffic │ apart and 5 foot gage. For city streets, light │A load of 6 tons on 2 axles 10 feet traffic or country roads │ apart and 5 foot gage. │ Road rollers │Total weight 30,000 pounds. Weight on │ front wheel, 12,000 pounds, and on │ each of two rear wheels, 9,000 pounds. │ Width of front wheel, 4 feet and of │ each of two rear wheels 20 inches. │ Distance between front and rear axles │ 11 feet. Gage of rear wheels, 5 feet, │ c. to c. ──────────────────────────────┴────────────────────────────────────────