Effect of Infiltration on Single-Pile and Monopile-Raft Foundation Embedded in Unsaturated Sand

The present study discusses the effect of hydraulic loading in terms of water infiltration caused by rainfall on load-mobilization mechanism of single-pile (SP) and monopile-raft foundation (MPRF) embedded in unsaturated sand by means of the finite element-based computer program Plaxis3D. Soil was modeled using the conventional Mohr-Coulomb model incorporating Bishop's effective stress, and the hydromechanical behavior was incorporated using the van Genuchten model. A fully coupled flow deformation analysis based on Biot's theory of consolidation was employed to model the behavior of soil during water infiltration. Water infiltration at different rates ranging from 5 to 864 mm/day was simulated using the time-dependent flow boundary condition at the ground surface. After successful validation of the developed numerical model using the experimental study available in literature, the evolution of suction, effective saturation, and suction stress during the different rates of infiltration was investigated and the responses of SP and MPRF in terms of settlement and axial forces were obtained. The results indicated the contribution of suction stress in increasing the stiffness of soil to an infiltration rate of 240 mm/day, thereby increasing the capacities of both SP and MPRF. The mobilization of shaft resistance was greatly dependent on the suction stress, and a reduction in the suction stress upon infiltration led to a lesser mobilization of shaft resistances (reduced around 6%). This caused a higher pile deformation, leading to further mobilization of pile base resistance. The rate of infiltration and the raft dimension has significant effect on mobilization of raft and pile resistance within MPRF due to raft-soil and pile-raft interactions. A higher rate of infiltration led to a greater mobilization of pile resistance due to positive pile-raft interaction brought by reduction in the contribution of raft base resistance that was associated with reduction in Bishop's stress. The outcome of the present study would help in better understanding the effect of hydraulic loading on the behavior of SP and MPRF and can be extended to pile group and combined pile-raft foundation.