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Response of buried polyethylene natural gas pipelines subjected to lateral ground displacement Anderson, Christopher D.

Abstract

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.

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