Sewer pipes are laid in trenches at least three feet deep, to insure against the action of frosts.

Construction.—The level of the trenches in which sewers are laid should be accurate, and a hard bed must be secured, or prepared, for the pipes to lie on. If the ground is sandy and soft, a solid bed of concrete should be laid, and the places where the joints are should be hollowed out, and the latter embedded in cement.

Joints.—The joints of the various lengths must be gas-tight, and are made as follows: into the hub (the enlargement on one end of the pipe) the spigot end of the next length is inserted, and in the space left between the two a small piece, or gasket, of oakum is rammed in; the remaining space is filled in with a mixture of the best Portland cement and clean, sharp sand. The office of the oakum is to prevent the cement from getting on the inside of the pipe. The joint is then wiped around with additional cement.

Fall.—In order that there should be a steady and certain flow of the contents of the sewer, the size and fall of the latter must be suitable; that is, the pipes must be laid with a steady, gradual inclination or fall toward the exit. This fall must be even, without sudden changes, and not too great or too small.

Fig. 15.

A BRICK SEWER.

The following has been determined to be about the right fall for the sizes stated:

4-inch pipe1 foot in 40 feet
6 " "1 " " 60 "
9 " "1 " " 90 "
12 " "1 " " 120 "

Flow.—The velocity of the flow in sewers depends on the volume of their contents, the size of the pipes, and the fall. The velocity should not be less than 120 feet in a minute, or the sewer will not be self-cleansing.