Page 141 - Buried Pipe Design

Design of Gravity Flow Pipes 115

vacuum) to be essentially constant around the pipe. This condition is
not met when a very large pipe is placed in shallow burial below the
water table. In this case, the hydrostatic pressure can vary substantially
from the top to the bottom of the pipe.

Overdeflection. Deflection is a design parameter for flexible pipes and
semirigid or semiflexible pipes. It is rarely, if ever, considered in the
design of rigid pipe installations.
Flexible pipe products will have a deflection design limit (Fig. 3.24).
This design limit is not a performance limit, but is often based on a
performance limit with a safety factor. For example, PVC pipes will
not start a reversal of curvature until about 30 percent deflection. (See
Fig. 3.25.) Thus a design deflection of 7.5 percent is based on a safety
factor of 4.
Not all design deflections are based on reversal of curvature. For
cement-lined steel and ductile iron pipe, the design deflections are
based on deflection limits (performance limits) which produce sub-
stantial cracking in the cement lining. Other products have deflection
limits to limit bending stresses or strains. The design engineer must
be aware of each product’s limitations for design calculations and to
assess adequate safety.
The semirigid and semiflexible products depend on their deflection
capability to carry the imposed soil load—just as all flexible products

Figure 3.24 Ring deflection in a
flexible pipe.

Figure 3.25 Reversal of curva-
ture due to overdeflection.