The Golden State: Where & How to Live, Secure, Visit, Enjoy and Thrive in California
How To Design And Install Plumbing
The sewer proper usually is poured in sections. The length of the section which may be poured is determined by the stability of the soil. The bottom half of the sewer is framed first and concrete is then poured into it. A 4x4 timber is pressed into the soft concrete to form a depression, thus providing a more substantial bond for the top half of the sewer, which is poured later, Fig. 3.
After the lower half of the sewer sets, the top portion or crown of the sewer is framed. The concrete is forced or puddled into the space between the ceiling of the tunnel and the framing or rough shoring which forms the interior of the sewer. After the concrete sets, the interior walls of the sewer are waterproofed with cement mortar, brushed or troweled to a smooth finish.
When quicksand is encountered, a much more complex problem arises.
The walls of the tunnel must be held up by forcing compressed air into the excavation. The process is slow because of adverse conditions. Men must work under pressures which they ordinarily are unaccustomed to and they must be in splendid physical condition to endure the work for any length of time. Under these circumstances a series of locks or compartments is constructed at the bottom of the shaft to make entry and exit into the tunnels possible. The increased pressure must be taken on by the tunnel worker gradually and upon leaving the tunnel he must accustom himself to the normal atmospheric pressure in the same manner. In this way he averts sudden illness often referred to as the "bends" or "caisson" disease. Tunnel workers generally work in three shifts under continuous operation. The first shift tunnels, the second shift frames the sewer, and the third shift pours the concrete into the forms. These men are well paid because their work is unusually hazardous.
Intercepting sewers are thousands of feet in length. In larger cities the sewer may be required to extend a distance of many miles before it reaches its terminal. It would be unreasonable to suppose that a sewer could be placed an adequate distance from the surface of the street (10 feet) at its farthest point, be graded even slightly (one inch in 50 feet of run) and then be a reasonable depth below the ground at its terminal. A sewer 50,000 feet in length (approximately ten miles) under these conditions would be 1000 inches or 83 feet plus the depth at its farthest point under the level of the street. Sewers of greater length would be deeper in the ground, rendering them practically useless. To overcome this difficulty the lengths of the sewer are shortened, and the terminal of each division consists of a well, or lifting station, provided with a sewage ejector or pump to elevate the sewage to a higher level so it can flow by gravity to the next lifting station and finally be discharged by the forces of gravity into the sewage plant, which is placed at some low point in the city. Fig. 4 illustrates this procedure.
