Pipe uplift in saturated sand: rate and density effects

A key parameter for assessing the unburial behaviour of offshore pipelines, such as during upheaval buckling, is the soil uplift resistance. Despite much research on uplift response, two variables remain relatively unexplored: the effect of low relative density backfill and partially drained soil response. To address this shortcoming, an investigation into the plane-strain uplift resistance of pipe segments in saturated sand was carried out. The model tests, targeting drained and partially drained soil behaviour, demonstrate that soil state strongly influences the pipe–soil failure mechanism, and hence the mobilised uplift resistance. In very loose sand the normalised peak uplift resistance remained constant beyond a critical embedment depth, indicating a possible transition between different failure mechanisms. Faster rate tests result in positive excess pore pressures, which serve to further reduce the uplift resistance. Tests at varying uplift rates and densities identified a transition relative density between net contraction and dilation for a given embedment. These findings suggest that a minimum backfill density should be ensured prior to pipeline operation, to avoid the negative effects associated with contractive behaviour of loose soil.