Page 334 - Buried Pipe Design

Steel and Ductile Iron Flexible Pipe Products 305

Height of cover is calculated from measured vertical soil pressure
3
using a soil unit weight of 120 lb/ft as follows:
2
vertical soil pressure (lb/ft )
Height of cover (ft)
120 lb/ft 3
In each test, vertical loading was increased until plastic hinging
was observed. At that point, the load was held constant. The pipe
did not collapse or continue to deflect under that load. An increase
in load was required for the deflection to continue. Therefore, even
after plastic hinging, the pipe-soil system is still under stable
equilibrium.
Test 1. The test pipe was installed in silty-sand soil compacted to
76 percent standard Proctor density. This type of installation would
be considered a poor installation and would normally not be recom-
mended. At about 35 ft of cover, the top began to flatten, and signs
of localized buckling began to appear at the sides of the pipe (see
Fig. 6.13). As the load was increased, the localized buckling became
more pronounced, and at about 40 ft of cover, plastic hinges began to
form. (See Fig. 6.14.) The results of this test are shown in the graph
of Fig. 6.15.

Figure 6.13 Inside the pipe at 35 ft of cover.