Uplift resistance of shallow-buried pipes in unsaturated sand under different saturation levels

Ground deformation caused by geological or human activities, such as tunnel excavation, landslides, and faulting, can seriously threaten the integrity of shallow-buried pipelines. However, most studies of buried pipelines rely on saturated-soil mechanics and neglect the influence of matric suction on pipe-soil interaction in unsaturated sand. This study conducted six laboratory model tests on shallow-buried pipes in sand under varying saturation conditions. Tensiometers were installed concurrently at the pipe crown and within a 45° shear band. Particle image velocimetry (PIV) was used to measure the displacement field, enabling quantification of the coupled evolution of suction, deformation, and failure during pipe uplift. Based on these observations, we propose a simplified mechanical model of pipe-soil interaction. The model assumes an inverted-trapezoidal wedge failure mode and applies limit-equilibrium theory. Equivalent suction stress is incorporated into shear strength to derive a closed-form expression for the ultimate uplift resistance. Results show that matric suction increases peak pipe-soil uplift resistance in unsaturated sand to approximately 3-4 times the value observed in dry or saturated conditions. The displacement required to reach this peak increases by about 30%. Around the pipe, soil fails as an inverted trapezoidal wedge. Under unsaturated conditions, the wedge width is approximately 6D (D is the outer diameter of pipeline) and the shear-band inclination about 40°, significantly larger than the 4D width and 25° inclination observed in dry or saturated sand. The simplified analytical model predicts both the current experimental data and published sand data with errors generally within ±15%. We establish an integrated framework linking the evolution of suction and displacement fields, identification of deformation and failure mechanisms, and simplified mechanics-based analysis. The simplified mechanical model can be used for engineering estimates and design verification of ultimate uplift resistance for shallow-buried pipelines in sand at various saturation levels.