CEMP-E
EI 11C201
1 March 1997
Figure 5-1. Load factors and class of bedding
b. Flexible conduit. The capability to resist pipe deflection and buckling under loads is the
primary criterion used in the structural design of flexible conduit. When loaded the pipe walls will
deflect, thereby creating a passive soil support at the sides of the conduit. This pipe-soil system
is essential in providing a high effective strength, often enabling it to out perform rigid pipe under
identical loading and soil conditions. While the three-edge bearing strength is an appropriate
measure of load carrying capacity for rigid conduits, it is not applicable for describing flexible pipe
stiffness. Because a flexible conduit must successfully interact with the surrounding soil to
support its load, the method of backfill placement, types of materials used, soil compaction, etc.,
are more critical than trench width or bedding. Since the theories describing flexible pipe
behavior, stiffness and deflections under load are lengthy, and the formulas cumbersome to use,
they will not be presented in this manual. The same is true with less flexible plastic pipes such
as acrylinitrile butadiene-styrene and profile wall PVC (ribbed or corrugated). The methods and
procedures adopted in the Handbook of PVC Pipe Design and Construction by the Uni-Bell PVC
Pipe Association, and WEF Manual of Practice FD-5 will be used in design. The project
specifications will be prepared to reflect the stringent installation and construction requirements
5-5
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