Summary: | Permanent ground deformations (PGD) are a major cause o f damage to buried natural gas
pipeline systems, particularly in areas of ongoing ground deformation. A new testing
apparatus was designed and constructed specifically to meet the requirements of full-scale
physical modeling research on typical polyethylene natural gas pipeline
configurations subjected to lateral ground displacements. The apparatus essentially
comprised a large soil testing chamber in which pipeline configurations could be
monitored with an instrumentation array measuring pullout resistance, pipeline
displacement, and local strain within the pipe specimens. Specialized methods were
developed for mounting strain gauges to polyethylene to obtain reliable readings, and to
reduce imposed drag forces.
A series of tests performed on straight pipeline sections indicated that peak axial pullout
resistance predicted using published design equations tended to over-predict pullout
resistance of polyethylene pipes in uncompacted sand, and to underestimate pullout
resistance in dense sand. Significant limitations were found in the applicability of current
design equations, developed from tests on rigid steel pipelines, to the prediction of axial
pullout resistance of relatively "flexible" polyethylene pipelines. Several soil-pipeline
interaction mechanisms have been suggested that seem to account for the observed
differences in behaviour between polyethylene and steel pipelines.
Axial tests on trunk lines with tee connections to smaller size branch pipes indicated that
branched configurations experience a complex interaction with the soil. The branch pipe
initially deflected and elongated in close proximity to the tee connection, then with
increasing displacement, pipe deformation transitioned to axial pullout of the branch. The contribution to pullout resistance from the branches was similar in both uncompacted
and dense sand, and also was not significantly affected by the diameter of the trunk line.
The anchoring effect of the branch pipe generates significant strain concentrations in both
trunk line and branch pipes. In relatively large diameter (115 mm) trunk lines, the
highest vulnerability to such strain concentrations was in the smaller diameter (15 mm)
branched pipe. In small diameter (60 mm) trunk lines these strain concentrations were
observed to induce yielding of the trunk line. === Applied Science, Faculty of === Civil Engineering, Department of === Graduate
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