Dynamic Behavior of Geogrid-Encased Stone Columns-Improved Sand Under Three-Stage Seismic Loading

Geogrid-encased stone columns (GESCs) are an effective ground improvement technique for mitigating earthquake-induced liquefaction in saturated sands. This study investigates the dynamic response and liquefaction resistance of GESCs through 1:10 scale shaking table model tests under a three-phase loading protocol comprising static, seismic, and post-seismic stages. Four real earthquake records (EI, WC, TR, and RG) were applied to model foundations constructed in untreated sand and sand improved with ordinary stone columns (OSCs) and GESCs. Settlement, excess pore water pressure (EPWP), column-soil stress ratio, and acceleration response were continuously monitored. Results indicate that seismic loading contributes most significantly to settlement and EPWP generation. Both OSC and GESC foundations exhibit greater sensitivity to the EI and RG motions. The lateral earth pressure in OSCs undergoes more pronounced variation across the loading stages, whereas GESCs maintain more stable confinement. Despite seismic densification and geogrid confinement, the column-soil stress ratio decreases in the post-seismic stage due to structural degradation. Acceleration spectra for OSC and GESC display multiple peaks, in contrast to the single dominant peak in untreated sand. Under EI and RG inputs, GESCs exhibit higher peak acceleration amplification factors ( n _p ) than OSCs, while the opposite trend is observed under WC and TR motions.