Abstract
Soil liquefaction and associated lateral spreading pose severe geotechnical hazards to critical coastal and offshore infrastructure, particularly large-diameter monopile foundations. This study presents a dynamic numerical investigation into the response of monopiles embedded in layered liquefiable soil profiles subjected to combined cyclic lateral loading and earthquake-induced lateral spreading. A three-dimensional finite-element model incorporating an elastoplastic constitutive formulation (PM4Sand) was calibrated and validated against benchmark dynamic centrifuge test data. The research systematic analyzes the complex kinematic and inertial interactions between monopiles, overlying non-liquefiable crusts, intermediate loose sand strata, and underlying dense bearing layers. The results indicate that excess pore water pressure accumulation within the loose sand layer reduces dynamic lateral soil reaction, inducing a substantial shift of the maximum bending moment toward the lower non-liquefiable base layer. Furthermore, the presence of a stiff, non-liquefiable upper crust amplifies kinematic driving forces, increasing maximum pile bending moments by up to 38% compared to uniform liquefiable profiles. Coupled cyclic inertial loading at the pile head during the lateral spreading phase further exacerbates residual lateral deflection and permanent tilting. Based on parametric evaluations, modified strain-dependent degradation parameters for conventional $p$-$y$ spring formulations are proposed to improve the accuracy of simplified engineering design methods for monopiles in heterogeneous liquefiable sites.
