Seismic Suitability Assessment of Steel Slag–Rubber Composite Backfill for Geogrid-Reinforced Retaining Walls

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.