Page 164 - Trenchless Technology Piping Installation and Inspection

130 Cha pte r F o u r

distribution below a rigid pipe (Howard, 1996). The point load is the
worst possible support case, which gives the strength equal to the
three-edge bearing test. For distributed load case, as for trenchless
installation, the strength of the pipe may be enhanced due to uniform
distribution, of stress as shown in Fig. 4.7. Research has shown that
the load required to cause failure in installation conditions is typi-
cally greater than the three-edge bearing strength owing to distribu-
tion of forces (Moser and Folkman, 2008). To ensure appropriate
distribution it is vitally important to achieve design densities in the
haunch area of the pipe.
The ratio of field strength of a rigid pipe to the three-edge bearing
strength is called a bedding factor and is given by

Bedding factor = field strength (4.2)
three-edge bearring strength
Bedding factors are specified by different manufacturers for dif-
ferent types of pipes based on the placement methods and materials
used.

4.3.5 Behavior of Flexible Pipes
Unlike rigid pipes, flexible pipes are designed to transmit part of the
load to the side soils. As the load on the pipe increases, the vertical
diameter of the pipe decreases and the horizontal diameter increases.
The increase in horizontal diameter mobilizes the lateral resistance of
the soils as shown in Fig. 4.8. Change in vertical or horizontal dimension
of a pipe is usually represented as a percent change and is given by

Percent deflection = change in diameter × 100 (4.3)
pipe diametter
Consider the flexible steel pipe, which is a perfect circle when it is
laid on top of the bedding and no soil load has been placed. Steel is a
linearly elastic (not viscoelastic) material. After backfilling, though,
the steel pipe deflects. When first deflection takes place, two things
happen. First, soil arching reduces the soil load on the steel pipe. So
the load the pipe is resisting has decreased. Second, the material in
the haunch zone has been further compacted by the expansion of the
horizontal diameter. In other words, using Eq. (4.4), the numerator
has decreased because of soil arching and the denominator has
increased because the soil’s stiffness has increased due to compaction
from the pipe’s horizontal expansion.

Pipe deflection = load on the pipe (4.4)
+
pipe stiffness + soil stiffness
Nevertheless, the load on the pipe has not been reduced suffi-
ciently yet. The pipe further deforms to the shape given by the
second deflection in Fig. 4.8. The load on the pipe reduces further