The use of recycled steel slag and tire-derived rubber as a combined backfill for geogrid-reinforced retaining walls offers a potential resource-recovery route, yet its influence on seismic performance has not been systematically quantified. To address this issue, a two-dimensional nonlinear finite-element model was used to perform 120 time-history analyses for a conventional backfill and nine modified backfills under the El Centro, JiJi, and Kobe ground motions scaled to peak ground accelerations of 0.05–0.20 g. The results showed that increasing the rubber content in the steel slag–rubber composite reduced wall lateral deformation and surface settlement, whereas GZ2 produced the largest wall deformation and XJ20 generated the greatest reinforcement stress in the Kobe-wave case at 0.20 g. When wall deformation, surface settlement, and reinforcement stress were considered together, the composite containing 30% steel slag and 15% rubber particles (GK15) provided the most balanced seismic response, producing the lowest settlement at both monitoring locations while avoiding the pronounced reinforcement-stress increase observed for XJ20; therefore, within the investigated model and seismic-input range, GK15 is recommended as a recycled backfill option for geogrid-reinforced retaining walls.

Current-induced vortex shedding imposes repeated cross-flow loading on offshore wind monopiles, yet its effect on the lateral pile-soil response is not explicitly represented in conventional p-y formulations. This study used a validated three-dimensional ABAQUS model of a large diameter monopile in sand to evaluate static in-line drag, equivalent peak cross flow lift, and stabilized harmonic lift at representative elevations. Static and post-cyclic p-y curves were extracted after up to 10⁴ cycles and compared with the American Petroleum Institute (API) sand relation. The in-line load governed the short-term displacement and bending-moment response, whereas repeated cross flow loading progressively reduced the mobilized soil resistance. After 10⁴ cycles, the reduction reached approximately 6.0–12.5% at z = D/2 and 6.75–13.4% at z = D across the representative low and high load cases. The numerical curves also exhibited a lower initial soil reaction modulus than the API prediction within the investigated shallow zone. A modified p-y formulation was therefore developed by combining a nonlinear depth dependent reference modulus with a cycle dependent degradation coefficient. The formulation provides an engineering representation of VIV-related stiffness degradation for the investigated monopile sand system and loading range.