Full-scale laboratory testing of a buried pipeline in sand subjected to cyclic axial displacements

This paper describes a full-scale laboratory study of the axial sliding behaviour of a trenched pipeline surrounded by sand backfill. Cyclic axial displacements are applied to a heavy pipe buried in a narrow trench (less than three pipe diameters wide), using various backfill cover depths and two different soils: dry Hostun sand and a damp, silty sand. A novel testing tank is employed, with compressible foam seals to allow the pipe to settle as it moves axially, and a pressure bag system to simulate backfill depths exceeding the height of the tank. The test pipe is instrumented to measure: (a) the axial soil resistance developed on an isolated central section of pipe (thus avoiding tank boundary effects) and (b) the normal and shear contact stresses at a number of points around the pipe circumference. The results indicate that both the axial resistance and the normal stress distribution around the pipe can undergo considerable changes when a pipeline experiences cyclic axial displacements. An extreme case identified here is the potential for a compacted damp sand backfill to ‘arch’ completely over the pipe as a result of differential settlement, leading to unexpectedly low axial resistance. To address some limitations of the design method currently used in industry, a new approach for estimating axial resistance is suggested and applied to the present test data.