Page 218 - Buried Pipe Design

192 Chapter Four

excess liquid that has been diverted from the main flow to prevent
overpressures, or as supplies of fluid to be added in the case of negative
pressures.

External loads
External earth loads and live loads induce stresses in pipe walls.
Methods for calculating these loads were discussed in Chap. 2, and
design procedures for external loads were discussed in Chap. 3.
These loads and their effects should be considered in pressure
pipe installation design. Often stresses due to external loads are
secondary in nature, but can be the primary controlling factor in
design.

Rigid pipes. Stresses due to external loads on rigid pipes are usually
not directly considered. Strength for rigid pipe is determined in terms
of a three-edge test load (see Chap. 3).

Flexible pipes. Stresses in the wall of a flexible pipe produced by exter-
nal loads can be easily calculated if the vertical load and resulting
deflection are known. Methods for calculating the deflection are given
in Chap. 3. These stresses can be considered to be made up of the fol-
lowing components:
P v D
Ring compression stress: c (4.10)
2t

and bending stresses:
y t
b D f E (4.11)
D D
where P v vertical soil pressure
D pipe outside diameter
t pipe wall thickness
E Young’s modulus
y vertical deflection
D f shape factor
The shape factor D is a function of pipe stiffness, as indicated by
f
Table 4.3. Generally, the lower the stiffness, the higher the D fac-
f
tor. Other parameters such as pipe zone soil stiffness and com-
paction techniques have an influence on this factor, but the values
listed in Table 4.3 are recommended design values for proper instal-
lations.