10-inch pipe carry out the house sewage, and the large sewer discharges the storm water, and both operate without producing any nuisance. Fig. 170 shows a design for an outfall of this sort in Binghamton. The combined sewer is 4 feet in diameter, and the outfall pipe is 12 inches. To secure a freedom from flow out of the big sewer except in time of storm, a concrete dam is built in the big sewer, 6 inches high, to force the sewage to drop through Fig. 169 the opening into the smaller pipe. This dam is not usually necessary, however, unless the grade is very high. This plan is often adopted to avoid the necessity of building a large sewer in a deep trench or down a steep bank. Fig. 171 shows a conventional design recommended by Moore, and Fig. 172 shows the actual construction of the Niagara Falls outlet. The main trunk sewer is 38 X 48-inch brick, and at the disposal point a shaft 5 × 7 feet is sunk below the sewer to a point level with the bottom of the cliff, exposed in the gorge a depth of about 50 feet. A sump hole allows the Fig. 171 falling water to strike without causing damage. From the face of the cliff a tunnel is driven 5 X 6 feet in section to the bottom of the shaft, on a 10 per cent grade. This tunnel is smoothed up on the bottom with concrete and brick into a semicircular channel, but the arch is left unlined. The mouth of the tunnel is protected by a heavy masonry arch and retaining wall. Then down the slope at an angle from the horizontal of 38 degrees is laid a 3-foot circular wood pipe. This pipe is anchored at the bottom of a mass of 70 cubic yards of stone masonry thoroughly tied together with iron straps and collars. water cushion is provided at the lower end of the pipe. A Figs. 173 and 174 show a more elaborate construction at the mouth of the Aramingo Canal Sewer in Philadelphia, the invert at the outfall being three feet below low water. Piles were driven in bents in a trench from which the soft mud had been dredged and which was then refilled with gravel and cobbles. On top of the cobbles was a cement mattress made of two sheets of burlap with 1 Eng. Rec., Vol. 44, p. 614. |