slope the trench 1/4 inch per foot. This is the grade at
which sewage flows freely through a pipe and provides
proper scouring action to keep the sewage flowing.
When a pipeline is to be laid in stable soil, such as
hard clay or shale, the trench should be excavated
below the pipe grade. If bell-and-spigot pipe is to be
used, excavation must be made for the bells. See that
enough undisturbed earth remains at the bottom of the
trench, so the pipe, both joints and hubs, rests on and is
fully supported by undisturbed earth. In areas where
the temperature drops below freezing, the trench must
be excavated deep enough for the pipeline to be below
the frost line. Pipes that cross under roads or areas of
vehicular traffic must be buried in trenches at least 4
feet deep and may require some type of metallic
sleeving. Refer to the specifications of the job for
details on sleeving pipe.
The sides of excavations, 4 feet or more in depth or
in which the soil is so unstable that it is not safe at
greater depths, should be supported by substantial and
adequate sheeting, sheetpiling, bracing, shoring, and
so forth, or the sides should be sloped to the angle of
repose. Surface areas adjacent to the sides should be
well-drained. Trenches in partly saturated, filled, or
unstable soils must be suitably braced.
SANITARY DRAINAGE PIPING
Among the pipe materials installed underground
by Utilitiesmen are cast-iron soil pipe, vitrified clay
pipe, concrete pipe, and plastic pipe.
Cast-Iron Soil Pipe (CISP)
Cast-iron soil pipe and fittings are composed of
gray, cast iron that is made of compact, close-grained
pig iron, scrap iron and steel, metallurgical coke, and
limestone. Cast-iron soil pipe is used in and under
buildings, protruding from 2 to 10 feet from the
building. (The National Standard Plumbing Code-
Illustrated recommends at least 3 feet.) Here it
connects into a concrete, plastic, or clay house sewer
line. Cast-iron soil pipe is also used under roads or
other places of heavy traffic.
When the soil is unstable, it is better to use cast-
iron soil pipe; however, cast-iron soil pipe should not
be used in soil containing cinders or ashes; the reason
is that the soil may contain sulfuric acids, which cause
the pipe to corrode and to deteriorate rapidly.
When the soil contains cinders and ashes, in-
stead of using cast-iron soil pipe, use vitrified
clay or plastic pipe.
The cast-iron soil pipe used in plumbing
installations comes in 5-foot and 10-foot lengths.
Sizes of cast-iron soil pipe are 2, 3, 4, 6, 8, 10, 12,
and 15 inches nominal inside diameter.
available as single hub or double hub in design, as
shown in figure 3-2. Note that single-hub pipe has a
hub at one end and a spigot at the other. The double-
hub pipe has a hub at both ends. Hubs, or bells, of
cast-iron soil pipe are enlarged sleeve-like fittings.
They are cast as a part of the pipe and are used to
make a water- and pressure-tight joint with oakum
and lead. Cast-iron soil pipe is generally available
in two weights: standard or service (SV) and extra
heavy (XH). The extra heavy pipe is used where
superior strength is required, for example, under
roadways, where the pipe may vibrate or settle
slightly, and tall stacks. Standard or service weight
pipe is adequate for most Navy base construction.
MEASURING. Cast-iron soil pipe sections are
generally 5 and 10 feet in length, but strictly speaking,
this is not true. The reference to a 5-foot length of pipe
applies to the laying length, not the overall
dimensions. For clarity, first note that cast-iron soil
pipe in 2-, 3-, 4-, and 6-inch (inside) diameter sizes
are in common use. The length of the bell for the 3-
inch-diameter pipe is 2 3/4 inches; and for the 4- and
6-inch-diameter sizes, the length is 3 inches. Now
note that while the laying length of a 4-inch-diameter
Figure 3-2.Single-hub and double-hub cast-iron soil pipe.