Soil resistance under pipe vertical uplift in compacted clay: shallow tensile and deep shear failure mechanisms

The paper investigates the mobilization of soil resistance against pipe vertical uplift in compacted Regina clay using both physical and numerical modelling. Results of the physical model tests reveal that the measured uplift resistance is much lower than the one estimated by the relevant equation in engineering design guidelines for buried pipelines. This discrepancy is elucidated through finite element (FE) and extended finite element (XFE) analyses of results from model tests with various pipe diameters and embedment depths. XFE numerical results reveal the presence of tensile failure in the form of a fracture that reduces the uplift resistance. This fracturing cannot be captured explicitly in FE modelling whose predictions are consistent with design guidelines. It is found that for shallow embedment depths (H/D < 5) and high cohesive strengths (c/γH > 5), the soil above the pipe is set in flexure where a hybrid tensile-shear failure mode lowers the uplift resistance. By contrast, tensile failure does not develop at greater embedment depths with lower cohesive strengths, thus yielding an uplift resistance close to the design guidelines. A correlation between the reduction in uplift resistance in compacted clay and its normalized cohesion is developed to enrich the current design guidelines by including hybrid tensile-shear failure modes at shallow embedment depths.