b. Design of Rigid Conduit. To design a rigid conduit, the computed
loads (dead and live) are modified to account for bedding conditions and to
relate maximum allowable load to the three-edge bearing test load D. (See
Figure 18d.) See ASTM C76, Reinforced Concrete Culvert, Storm Drain, and
Sewer Pipe, for test standards for D load.
Bedding conditions for pipes in trenches may be accounted for by use
of a load factor, L+f,. Determine L+f, from Figure 18e (Reference 14).
Determine D from the following equation:
D+0.01, = (P+DL, + P+LL,) ))))
D+0.01, = Allowable load in lb/ft of length of conduit
per foot of inside diameter for a crack width of 0.01"
L+f, = load factor
= safety factor (usually 1.25)
With the specified D load, the supplier is able to provide adequate pipe.
The soil pressure against the sides of a pipe in an embankment
significantly influence the resistance of the pipe to vertical load. The
load factor for such cases considers not only pipe bedding, but also pipe
shape, lateral earth pressure, and the ratio of total lateral pressure to
total vertical pressure. For further guidance see Reference 11.
3. FLEXIBLE STEEL PIPE. Corrugated or thin wall smooth steel pipes are
sufficiently flexible to develop horizontal restraining pressures
approximately equal to vertical pressures if backfill is well compacted.
Vertical exterior pressure acting at the top of the pipe may range from
pressures exceeding overburden pressure in highly compressible soils to
much less than the overburden pressure in granular soils because of the
effect of "arching", in which a portion of the overburden pressure is
supported by the surrounding soil.
(1) Dead Load. For flexible pipe, the dead load pressure is simply
the height of the column of soil above the conduit times the unit weight of
P+DL, = [Upsilon] [multiplied by] H
(2) Live Load.
Computed by Boussinesq equations for rigid pipes.
(3) Pressure Transfer Coefficient. The dead load and live load
pressures are modified by pressure transfer coefficient, C+p,, to yield
apparent pressure, P, to be used in design.
P = C+p,(P+LL, + P+DL,)
See Figure 20 (Reference 18, Response of Corrugated Steel Pipe
to External Soil Pressures, by Watkins and Moser) for the values of C+p,.