Cyclic deformation regimes in liquefiable sands and sheet-pile systems

Abstract

Liquefiable sands under undrained cyclic loading exhibit two distinct deformation regimes: cyclic mobility and residual deformation accumulation, corresponding to fundamentally different mechanisms of shear-stiffness degradation and strain accumulation. In boundary value problems, these regimes may coexist and transition during loading, producing system-level seismic responses that are not captured by liquefaction triggering alone. To investigate the implications of such regime transitions, a sheet-pile-supported liquefiable sand system was analyzed under seismic loading with varying embedment ratios. An enhanced constitutive model, SANISAND-MSf, was validated against cyclic direct simple shear tests to reproduce both cyclic deformation regimes associated with cyclic mobility and residual deformation accumulation, and further validated using centrifuge tests on medium-dense Ottawa F-65 sand. The evolution of shear stiffness is further interpreted using a cumulative average shear modulus proxy, Gave, as a supporting indicator of deformation regime across element and system scales. A representative sheet-pile configuration was examined across a range of embedment ratios under seismic excitation. The simulations show that, within the range of embedment ratios considered, decreasing embedment ratio leads to an increase in sheet-pile head displacement that approaches a plateau rather than continuing to increase monotonically, even as toe-fill support is progressively reduced. This response is attributed to the transition from cyclic mobility to residual deformation accumulation, whose collective effect governs local stiffness and strain development and, in turn, controls system-level response. Additional analyses demonstrate that this displacement plateau can be mitigated by reducing the contrast in cyclic resistance between the two deformation regimes or by densifying the toe fill adjacent to the sheet pile, highlighting the importance of deformation-regime transitions when assessing embedment requirements for sheet-pile systems in liquefiable deposits.