Page 71 - Buried Pipe Design

External Loads 47

Longitudinal fractures. Longitudinal fractures occur if vertical pressure
P exceeds the ring strength. Generally, the worst location of the sur-
face load is directly above the pipe, as shown in Fig. 2.22. Minimum soil
cover H is based on punch-through of a pyramid or cone. Longitudinal
fractures occur at 12 and 6 o’clock and 9 and 3 o’clock. This is not col-
lapse of the pipe. Many gravity flow pipes serve even when cracked.
The soil envelope holds the ring in nearly circular shape. But for some
rigid pipes, such as pressure pipes, longitudinal cracks are unaccept-
able. Occasionally one longitudinal hairline crack occurs—at 12 o’clock,
or possibly at 6 o’clock if the pipe is on a rigid bedding. If the embed-
ment is compacted select soil, a crack at 12 o’clock might be caused
either by a surface wheel load or by a conscientious installer who com-
pacts the first layer above the pipe directly against the pipe. It is pru-
dent to compact sidefills; however, one should leave the first layer
uncompacted over the pipe within one pipe diameter. For many buried
rigid pipes, longitudinal cracks are not the performance limit. Good
embedment holds the pipe in shape such that the pipe is in ring com-
pression—not flexure. It performs in the same way as brick sewers with
no mortar. Brick sewers function structurally, but are not leakproof.
The vertical pressure is P P l P d where the live load pressure P l
is found by the pyramid/cone theory. For minimum cover analysis,
dead load pressure P d is negligible. The live load pressure P l is a func-
tion of height of cover H. Minimum cover can be found from equating
P cr P l where critical pressure P cr is a function of class of bedding and
class of pipe. Values are published for each class.

Broken bells. If a pipe section acts as a beam, the performance limit
may be signaled a broken bell. Under heavy live load and minimum
soil cover, rigid pipes require support under the haunches. If soil is not
deliberately placed under the haunches, a void remains. See Fig. 2.26.
If the angle of repose of the embedment is ′ 40°, the void is wider
than one-half the outside diameter [0.643(OD)]. Live load on the pipe
could cause the top of the pipe to move downward either by cracking
the pipe or by pressing the pipe into the bedding. Under the haunches,
loose soil at its angle of repose offers little resistance. As a pipe section
deflects downward, it becomes a simply supported beam with reactions
at the ends of the pipe section. See Fig. 2.27. It is this reaction Q that
fractures the bell. Clay pipes and nonreinforced concrete pipes are vul-
nerable because of low tensile strength. The maximum tensile stress is
in the bell near the spring line. Once it is cracked, a shard forms
roughly one diameter in length, as shown in Fig. 2.27. An approximate
analysis is done by equating the Q that can be withstood by the bell to
the Q reaction caused by the surface load on the pipe section acting as
a beam.