Strain assessment of polyethylene pipes in dense sand subjected to axial displacements

Buried polyethylene pipes used in gas distribution systems can experience excessive wall strains when exposed to ground movements that can affect the performance of the pipes in service. This paper presents full-scale laboratory tests performed to investigate the responses of medium-density polyethylene (MDPE) gas-distribution pipes in dense sand when subjected to axial ground movements. Pipes buried in the sand in a large test box were pulled at the rates of 0.5 mm/min, 1 mm/min, and 2 mm/min to simulate the relative ground movements in the longitudinal direction. The test facility was instrumented to measure pulling force, pipe wall strains, and soil stresses. The measured pullout force was significantly higher than predicted using the equations recommended in current design guidelines, which is attributed to the increase of normal stress on the pipe wall by shear-induced dilation of interface soil. The cavity expansion theory was successfully applied to calculate the normal stress increase. The distribution of measured strains was nonlinear along the pipe length. Assuming a parabolic distribution of the strains, simplified equations were developed to calculate pullout resistances and pipe wall strains from the relative ground displacement. The developed method reasonably predicted the pipe strains measured during the tests.