1 March 1997
STRUCTURAL DESIGN OF SEWERS
5-1. GENERAL. The structural design of a sewer requires that the supporting strength of the
pipe as installed, must equal or exceed the external loading multiplied by a factor of safety. The
following criteria for structural design of sewers are based on the assumption that sewers will be
laid in open trenches entirely below natural ground surface and backfilled with suitable materials,
that the sides of the trench will be nearly vertical below the top of the pipe and will have slopes
no flatter than one horizontal to two vertical above the pipe, and that the trench width at the top
of the pipe will be relatively narrow. In general, the trench width will be limited to the maximum
allowed or recommended by the pipe manufacturer. Special cases involving sewer installation in
unsatisfactory soil, rock, embankments or fills, sewers requiring jacking, boring or tunneling, and
pipe placed above ground, are too rare to warrant lengthy consideration in this manual.
Paragraph 5-4 contains a general discussion of such situations. Sewers installed in cold regions
or in seismic zones will require special design considerations.
5-2. LOADS ON SEWERS. There are three kinds of external loads to which a sewer laid in a
trench may be subjected. They are (l) loads due to trench filling materials, (2) uniformly
distributed surface loads, such as stockpiled materials or loose fill, and (3) concentrated surface
loads, such as those from truck wheels.
a. Trench fill loads. The Marston formula will be used for calculating loads on rigid conduits
Wt = CtwBt2
= vertical load on conduit in kilograms per lineal meter (pounds per lineal foot)
= trench load coefficient for buried conduits
= unit weight of trench fill materials in kilograms per cubic meter (pounds per
cubic foot), and
= horizontal width of trench at top of pipe in meters (feet)
For calculation of loads on flexible conduits the prism formula will be used as follows:
= height of fill from top of pipe to ground surface in meters (feet), and
= horizontal width or outside diameter of pipe in meters (feet).
The unit weight of soil backfill normally varies from a minimum of 1600 to a maximum of 2160
kilograms per cubic meter (100 to 135 pounds per cubic foot). In the absence of soil density
measurements, the weight per cubic meter (cubic foot) of various materials may be taken as