Page 127 - Buried Pipe Design

102 Chapter Three

and was totally stable after 20 months. For the silty sand, the critical den-
sity is about 92 percent standard Proctor density. Thus, the pipe that was
installed at 88 percent had an increase in density when the soil became
saturated and became more dense. The two pipes that were installed in
dumped gravel showed a small increase in deflection. This shows that the
water serves as a lubricant for the gravel particles and allows some densi-
fication. The pipe that was installed in silty sand compacted to 93 percent
standard Proctor density had almost no increase in deflection. This is a
direct indication that pipe should be installed in soils compacted to densi-
ties higher than critical if deflection control is pertinent.
A new stable or equilibrium deflection level was reached at about 32
months. The water table continued to fluctuate on an annual basis for
the remaining test period. These subsequent water table movements
had no measurable influence on the deflection readings.
This test location became a victim to progress. Thirty months after
installation, a subdivision moved into the area. The access pipe was
removed and homes now stand over the pipes which are 22 ft down.
Extensive research has established that any buried flexible pipe
(i.e., steel, fiberglass, or plastic) will continue to deflect as long as the
surrounding soil consolidates. Thus, as previously stated, the creep
properties of pipe materials have little effect on the long-term deflec-
tion behavior of flexible pipe when buried in soil, and in most cases, a
deflection lag factor D L of 1.5 conservatively accounts for long-term
effects due to time-dependent load increases and due to consolidation
of soil in the pipe zone. Alternatively, design can be based upon the
anticipated prism load and a D L of 1.0.

PVC versus steel. Time-versus-deflection curves for pipe under con-
stant load in a soil test cell are given in Fig. 3.17. The two pipes are
from totally different materials (steel and PVC) but have exactly the
2
same pipe stiffness (F/ y 6.7EI/r 46 lb/in ). Both pipes are
3
installed in the same soil (silty sand) compacted to the same soil den-
sity (85 percent standard Proctor density). For these constant-load
tests, equilibrium is achieved in about 25 h. This shows that the basic
material properties of the pipe have little to do with overall perfor-
mance of the pipe. For instance, PVC creeps at a much higher rate
than does steel, but this difference in creep properties has no effect on
performance. Also, the modulus of elasticity of steel is 75 times that of
PVC. The two most important properties that have the principal influ-
ence on the performance of a buried pipe are first and foremost, soil
density, secondarily, pipe stiffness.
In very simple terms, the soil stiffness is primarily a function of soil
density, and the soil stiffness and the pipe stiffness work together in
supporting the imposed loads. Thus, the two contribute directly to the
overall pipe performance.