Page 325 - Buried Pipe Design

296 Chapter Six

Compression of the soil has a major effect on performance limits.
Compression is determined by the average vertical soil pressure P
v
and the soil modulus E .Soil modulus E is increased by increasing
the soil density. From the tests, the greater the soil density (greater E ),
the greater the ultimate ring compression stress f in the pipe wall.
c
Quantitative results are shown in Fig. 6.4. The difference between
the values of f in dense and loose soil is roughly a 3:1 ratio. Why
c
should the strength of the pipe be greater in dense soil if the pipe is
exactly the same? Even though the value f is called an ultimate
c
ring compression stress, it actually is a measure of strength of the
soil-pipe system—not just the pipe. The contribution of the soil as a
supportive structure increases the system strength if the soil is
dense and relatively rigid. On the other hand, if the soil is loose and
highly compressible, it will develop a pressure concentration on the
pipe as the soil compresses down under vertical pressure. Moreover,
soil compression causes ring deflection which further weakens the
system by adding flexural stress into the conduit wall and by
increasing the wall thrust by increasing the horizontal diameter. If
the pipe compresses down exactly as much as the soil, then the ver-
tical pressure on the pipe is the same as the vertical pressure P in
v
the soil. If the soil is dense, then soil compression is small and the
cross sectional area of the pipe may be reduced more than the cross-
sectional area of the soil. So the pipe will relieve itself of vertical
soil pressure. This is tantamount to arching action inasmuch as the
soil is forced to bridge or arch over the pipe. Of major significance
is the critical void ratio of the soil. If a soil is compacted such that
it is denser than critical void ratio, then the pressure concentra-
tions on these corrugated steel pipes are only about 20 to 40 percent
of the pressure concentrations if the soil is looser than the critical
void ratio.
Particularly noteworthy is the great difference in the general
slope of the load-deflection plots for pipes buried in loose soil in con-
tradistinction to pipes buried in dense soil. The horizontal deflection
data are about the same as the vertical deflection data; however, it
has been found that vertical deflection data can be measured with
greater precision and, for most analyses, are considerably more
meaningful.
Some plots of general results are indicated in Fig. 6.4, which
shows the ultimate ring compression stress as a function of the
ring flexibility. Because the material is steel with a constant mod-
2
ulus, the ring flexibility can be reduced to (D/r) or because the
radius of gyration is essentially constant for any depth of corruga-
tion, this reduces to just pipe diameter D for specific corrugation
configurations.