Page 336 - Buried Pipe Design

Steel and Ductile Iron Flexible Pipe Products 307

Figure 6.16 Test 2, silty-sand soil at 84 percent standard Proctor density.

localized buckling started at the sides of the pipe. As the load
increased further, this buckling became more pronounced, and at 68 ft
of cover, plastic hinges began to form. The results of this test are
shown in Fig. 6.16.
Test 3. The test pipe was installed in silty-sand soil compacted to
95 percent standard Proctor density. This type of installation would be
considered excellent and would normally be the very best installation
that could be expected. At about 86 ft of cover, slight local buckling
began at the sides of the pipe. At about 100 ft of cover, the top began to
flatten and started to show signs of localized buckling. At 105 ft of
cover, small local buckles were visible at some seams. At 110 ft of cover,
plastic hinges were definite at the sides of the pipe. Some bulging also
occurred at the bottom of the pipe. (See Fig. 6.17.) The results of this
test are shown in Fig. 6.18.
Overall results. The vertical deflections of the three tests are shown in
Fig. 6.19. This graph shows the importance of soil density in the per-
formance of buried pipes. The response to soil pressure was excellent.
The resulting deflections were reasonable and about what would be
expected. No seams opened or failed during the tests, even at extreme
heights of cover. Because the rib height is properly designed, the rib
acts as an integral part of the pipe wall. This allows the rib to stiffen
the wall and resist buckling.