Page 163 - Trenchless Technology Piping Installation and Inspection

Pipe and Pipe Installation Considerations 129

4.3.4 Behavior of Rigid Pipes
As noted earlier, rigid pipes are designed to transmit loads through
their material strength. Since rigid pipes do not deflect appreciably,
their design does not consider the horizontal passive resistance of the
side soils. Therefore, the allowable load on rigid pipes depends only on
the strength of the pipe and the strength of the soil below the pipe. This
is usually represented by the following relationship (Howard, 1996):
pipe strength × soil strength
h
Load on the pipe = (4.1)
safety factor
Typically, the strength of the material of a rigid pipe is determined
in the laboratory by the three-edge bearing test (Fig. 4.6). The three-
edge bearing strength is load per unit length required to cause either
crushing or critical cracking of the pipe (Moser and Folkman, 2008).
Strength of soil refers to the amount of support on the bottom half
of the pipe and depends on the properties of soil and the contact area.
Figure 4.7 illustrates two extremes of contact area available for pressure

Rigid
steel
member

Bearing
strips

FIGURE 4.6 Three-edge bearing test. (ACPA, 2000.)

Point Distributed
load load
FIGURE 4.7 Load distribution on rigid pipes.