Page 123 - Buried Pipe Design

98 Chapter Three

deflection and the time required to reach it are largely controlled by
the soil density. However, it is interesting to note in Fig. 3.14 for tests
at different temperatures with the same soil density:

1. The equilibrium deflection is slightly larger for higher tempera-
tures because the effective pipe stiffness is lower.
2. The time for equilibrium to be reached is shorter for higher tem-
peratures since the soil-pipe system can interact at a faster rate in
achieving equilibrium.

The above described long-term tests were carried out in a soil cell.
The imposed load on a pipe in a soil cell is almost instantaneous
because the loading plane is only about 30 in above the pipe. This pro-
vides a significant advantage over tests in either trench or embank-
ment conditions. In both the trench and the embankment, it takes
substantial time for the full load to reach the pipe—months and years
have been reported. When long-term tests are carried out in trenches
and embankments, the change in deflections with time is due to
increasing loads and soil consolidation. Figure 3.15 shows long-term
deflection curves for PVC pipe buried in an embankment.
During September 1975, an embankment installation reaching a
depth of cover of 22 ft was constructed over four test pipe sections that

Percent Deflection

Time (Hours)
Figure 3.15 Deflection versus time for 10-in-diameter PVC sewer pipe (22-ft-
deep embankment).