Page 45 - Buried Pipe Design

External Loads 23

soil support at the sides of the pipe. At the same time, the ring deflec-
tion relieves the pipe of the major portion of the vertical soil load which
is picked up by the surrounding soil in an arching action over the pipe.
The effective strength of the flexible pipe-soil system is remarkably
high. For example, tests at Utah State University indicate that a rigid
pipe with a three-edge bearing strength of 3300 lb/ft buried in class C
bedding will fail by wall fracture with a soil load of about 5000 lb/ft.
However, under identical soil conditions and loading, a PVC sewer pipe
deflects only 5 percent. This is far below the deflection that would cause
damage to the PVC pipe wall. Thus the rigid pipe has failed, but the
flexible pipe performed successfully and still has a factor of safety
with respect to failure of 4 or greater. Of course, in flat-plate or
three-edge loading, the rigid pipe will support much more than the
flexible pipe. This anomaly tends to mislead some engineers because
they relate low flat-plate supporting strength with in-soil load
capacity—something one can do for rigid pipes but cannot do for
flexible pipes.

Marston load theory. For the special case when the sidefill and pipe
have the same stiffness, the amount of load V that is proportioned to
the pipe can be found merely on a width basis. This means that if the
pipe and the soil at the sides of the pipe have the same stiffness, the
load V will be uniformly distributed as shown in Fig. 2.7. By simple
proportion the load becomes
2
W d B c C d B d B c
W c
B d B d
or

(2.10)
W c C d B c B d

Pipe stiffness versus soil compressibility. Measurements made by
Marston and Spangler revealed that the load on a flexible pipe is sub-
stantially less than that on a rigid pipe (see Fig. 2.3). The magnitude
of this difference in loads may be a little shocking. The following anal-
ogy will help us to understand what happens in the ground as a flex-
ible pipe deflects. Suppose a weight is placed on a spring. We realize
the spring will deform, resisting deflection because of its spring stiff-
ness. When load versus deflection is plotted, we find that this rela-
tionship is linear up to the elastic limit of the spring (Fig. 2.8). When
a load is placed on a flexible pipe, the pipe also deflects and resists