Page 55 - Buried Pipe Design

External Loads 33

embedded directly in such soils. Nevertheless, such shifting by adja-
cent soil can and will affect a pipeline. Normally these movements are
relatively small but may be large enough to adversely affect the pipe
performance.
To mitigate such adverse effects for rigid pipe, short lengths are
used with flexible joints. In the case of flexible pipe, the pipe’s natural
flexibility tends to allow the pipe to conform to these movements with-
out structural distress. In this case, both longitudinal flexibility and
diametrical flexibility are important.
Tidal water may also cause ground movement. These movements
may be designed for as described above.

Wheel Loading (Live Loads)
Boussinesq solution
Here, live loads mean static or quasi-static surface loads. Buried con-
duits may be subjected to such applied loads produced by ground
transportation traffic. The French mathematician Boussinesq calcu-
lated the distribution of stresses in a semi-infinite elastic medium due
to a point load applied at its surface. This solution assumes an elastic,
homogeneous, isotropic medium, which soil certainly is not. However,
experiments have shown that the classical Boussinesq solution, when
properly applied, gives reasonably good results for soil.
Figure 2.18 compares the percent of a surface load that is felt by a
buried pipe as a function of depth of burial as calculated by the
Boussinesq equation and as found from measurements.
Hall and Newmark 12 integrated the Boussinesq solution to obtain
load coefficients. The integration developed by Hall for C s is used for
calculating concentrated loads (such as a truck wheel) and is given in
the following form:
C s PF′
W sc (2.13)
L
where W sc load per length on the pipe, lb/ft
P concentrated loads, lb
F′ impact factor (see Table 2.5)
L effective length of conduit (3 ft is typically used), ft
C s load coefficient which is a function of B c /(2H) and L/(2H),
where H height of fill from top of pipe to ground
surface, ft; and B c diameter of pipe, ft (see Table 2.6)

17
The integration developed by Newmark for C s is used for calculating
distributed loads and is given in the form
(2.14)
W sd C s pF′B c