1 March 1997
5-3. SUPPORTING STRENGTH OF SEWERS. A sewer's ability to resist external earth and
superimposed loads depends not only on the pipe's inherent structural capability, but also on the
method of installing the pipe in the trench, i.e., class of bedding, type of backfill materials and soil
compaction effort, etc.
a. Rigid conduit. Pipe strength in general will be determined by the three-edge bearing test
or TEBT (termed crushing strength in various pipe specifications) and is expressed in kilograms
per lineal meter (pounds per lineal foot). However, since this does not represent the actual field
loading conditions, a relationship must be established between calculated load, laboratory test
strength and field support strength. The definitions and terminology listed below will be used to
develop this relationship. The total load must not exceed the safe supporting strength.
- Field support strength is the maximum load in kilograms per lineal meter (pounds per lineal
foot) which the pipe will support when installed under specified trench bedding and backfill
- The load factor is the ratio of the field support strength to the TEBT, and will be selected
from figure 5-1 depending on the class of bedding used.
- Safe supporting strength is the field support strength divided by a factor of safety, equal to
1.5 for rigid conduits.
- An additional parameter is the working strength, which is the three-edge bearing strength
divided by the factor of safety.
For piping not tested and rated by the TEBT method, other strength criteria will be applied as
follows. Reinforced concrete pipe strength will be based on D-loads at the 0.25-mm (0.01-inch)
crack load and/or ultimate load as described in the Concrete Pipe Handbook published by the
American Concrete Pipe Association. For ductile iron pipe, ANSI A21.50 will be used to calculate
the required pipe thickness classification in relation to field loadings. See paragraph 8-1 for
additional information. The strength of cast iron soil pipe, which normally will be used for building
connections only, should be evaluated as outlined in the Cast Iron Soil Pipe & Fittings Handbook
published by the Cast Iron Soil Pipe Institute.