Analysis of soil-structure interaction for pile-raft and slab foundations under complex soil stratification with account for creep, liquefaction and cyclic loading

Abstract

The persistent gap between predicted and observed settlements of pile-raft and slab foundations on sites with complex soil stratification remains an open issue in design practice despite the advanced apparatus of numerical modelling: the standard linear-elastic formulation does not reproduce the combined influence of the rheology of soft interlayers, the cyclic degradation of contact resistance and the possible liquefaction of water-saturated sand. The study compares two structural solutions for the substructure of a seventeen-storey building resting on a five-layer profile by means of an author's computational model that integrates load redistribution between the slab and the piles, a time-dependent settlement forecast based on a nonlinear creep scheme and an assessment of the liquefaction potential of the sand horizon through the cyclic stress ratio. Engaging the slab is found to reduce the settlement of an individual pile by approximately 29% and to transfer about 29% of the total load to the raft, while the creep contribution to the final settlement after twenty-five years reaches 38%, and cyclic loading with about ten thousand cycles lowers the unit shaft resistance to 0.82 of the initial value. The cyclic stress ratio calculation shows that the factor of safety against liquefaction drops below unity only in the upper part of the saturated layer, which alters not the tip bearing capacity but the load transfer pattern in the middle third of the pile. The joint account of the three factors shifts the design settlement from a quasi-instantaneous to a long-term value and moves the critical interaction zone from the slab base to the contact of the shaft with the rheologically active interlayers.