Page 79 - Buried Pipe Design

External Loads 55

5. Buried pipe made of ductile materials [steel, or more recently
polyvinyl chloride (PVC) and high-density polyethylene (HDPE)]
performed well.

Understanding this track record is important when developing
design criteria, to emphasize the positive characteristics and avoid
those that resulted in failures.
An earthquake may affect the integrity of a buried pipe in two pos-
sible ways: through wave passage (transient ground deformation) and
through permanent ground deformation.

Wave passage
The passage of seismic waves in the soil generates compressive, ten-
sile, and bending strains in a buried pipe. Extensive research in the
seismic performance of buried pipelines points to the fact that wave
passage alone does not seem to fail arc-welded steel pipe or polyethyl-
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ene pipe. Older buried piping assembled with oxyacetylene welds or
with mechanical joints is more susceptible to transient seismic ground
movements due to wave passage. Techniques do exist to analyze the
4
effects of seismic wave passage on a buried pipe (ASCE). This analy-
sis can be carried out with several levels of complexity.
In the simplest of cases, the pipe strain is set equal to the soil strain
and compared to a strain limit. The value of the strain limit is not
standardized. Compressive strain limits to avoid wrinkling of the pipe
wall on the order of 0.4t/D or 2.42(t/D) 1.6 have been proposed. In the
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1970s Hall and Newmark 12 had proposed strain limits of 1 to 2 per-
cent. Tensile strain limits on the order of 3 to 6 percent have been used
for modern steel pipeline construction.
In a more detailed analysis, the soil is modeled as three-directional
springs around the pipe. The soil strain is applied to the model, and
the axial and bending stresses are computed and compared to an
allowable. In this case, two difficulties remain to be solved: the choice
of a stress equation (and stress intensification factor) and the allow-
able stress. Unintensified stress limits of S y (ISO/DIS 13623) 14 and
intensified elastically calculated stress limits of 2S y have been pro-
posed by Bandyopadhyay. As a shortcut for applying the soil strain to
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the model, the seismic problem may be approached as a thermal
expansion problem: The soil strain is converted to an equivalent tem-
perature rise, which is applied to the pipe. 10
A more detailed analysis would include finite element models of the
pipe and the soil and would subject the model to time-history input
motions. The waves would be applied at several angles of incidence rel-
ative to the buried pipe. Strain or plastic stress criteria have been used
in these cases.