Page 173 - Trenchless Technology Piping Installation and Inspection

Pipe and Pipe Installation Considerations 139

FIGURE 4.17 Ring defl ection of a fl exible pipe. (Moser and Folkman, 2008.)

FIGURE 4.18 Reversal of curvature due to overdefl ection in fl exible pipes.
(Moser and Folkman, 2008.)

Longitudinal Tensile Failure
In general, rigid or flexible pipe products are not designed to with-
stand longitudinal tension. Longitudinal failure in pipes can occur due
to several factors that are given below (Moser and Folkman, 2008):

• Thermal expansion or contraction
• Longitudinal bending
• Poisson’s ratio (due to internal pressure)

The tendency of a pipe to move in the axial direction is resisted by
the frictional forces of soil surrounding the pipe. This creates longitu-
dinal stresses that can cause a circular break at weaker cross-sections
(cross-sections weakened by corrosion or cracking). Figure 4.19 illus-
trates longitudinal tensile failure in pipes. Figure 4.20 illustrates the
effect on pipe due to the loads.