Page 126 - Buried Pipe Design

Design of Gravity Flow Pipes 101

are probably within the expected experimental error, and no change in
the pipe’s capacity to resist deflection occurred over this time period.
These pipes were each subjected to a 60 percent deflection test pre-
scribed in D 3034, F 789, F 798, F 784, etc., to determine ductility.
Each sample sustained that deflection level without cracking.
Additional molehole data. After the above pipes were excavated, four
new PVC pipes were installed in the same location in an embankment
installation with 22 ft of cover. This installation was completed on
October 20, 1989. Deflections were monitored for the next 7 years. The
2 years following this installation were fairly dry years, and ground-
water did not rise into the pipe zone during these years. The winter of
1991–1992 was fairly wet. During the mountain snowmelt period in the
spring of 1992, after the pipes had been in the ground for 30 months,
groundwater rose above the pipe and saturated the pipe zone soils.
In-ground vertical deflection data are plotted in Fig. 3.16. It can be
seen that the deflection became almost stable after about 10 months

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Vertical Deflection (Percent)

Time (Months)
Figure 3.16 Embankment data for pipes installed 22 ft deep. Note the increase
when groundwater rose above the top of pipes 30 months after installation.