12.0 │ 2.97 │ 3.28 │ 3.63 │ 4.19 13.0 │ 2.99 │ 3.31 │ 3.67 │ 4.25 14.0 │ 3.00 │ 3.33 │ 3.70 │ 4.30 15.0 │ 3.01 │ 3.34 │ 3.72 │ 4.34 ∞ │ 3.03 │ 3.38 │ 3.79 │ 4.50 ──────────────┴──────────────┴─────────────┴─────────────┴───────────── TABLE 43 APPROXIMATE WEIGHTS OF DITCH FILLING MATERIAL TO BE USED IN THE EXPRESSION _W_ = _CwB_^2[69] ───────────────────────────────────┬─────────────────────────────────── Ditch Filling │ Pounds per Cubic Foot ───────────────────────────────────┼─────────────────────────────────── Partly compacted top soil (damp) │ 90 Saturated top soil │ 110 Partly compacted damp yellow clay │ 100 Saturated yellow clay │ 130 Dry sand │ 100 Wet sand │ 120 ───────────────────────────────────┴─────────────────────────────────── Where surface loads are to be carried on the sewer trench the proper proportion of the load to be carried by the sewer is determined by the expression _L__{_p_} = _CL_, in which _L__{_p_} is the equivalent backfill load per unit length of the trench, _L_ is the surface load per unit length of the trench, and _C_ is a coefficient in which allowance is made for the character of the backfilling, the ratio of depth to width of trench, and the character of the load, whether long or short. A long load is a load extending along the length of the trench such as a pile of building material. A short load is one extending across the trench and for only a short distance along it, such as that caused by a street car or road roller crossing the trench. Values of _C_ are given in Table 44 for long loads, and in Table 45 for short loads. Values of long and short loads occasionally met in practice are given in Tables 46 and 47 respectively. TABLE 44 RATIO OF LOAD ON PIPE TO LONG LOAD ON TRENCH[70]