Field Monitoring of Site Responses in a Ground with Transitional Silty Clay Layer during the 2022 M_S6.8 Luding Earthquake, China

Field monitoring of ground vibrations and excess pore water pressure (EPWP) during earthquakes is critical for understanding soil liquefaction and cyclic softening, yet such in-situ data remain scarce. Such data scarcity hinders the calibration of site response models for composite foundation of dams located in valleys. This study presents a field case at the Yingliangbao Hydropower Station dam site, where a vibration monitoring system originally installed for quality control during vibro-replacement stone column construction successfully captured seismic responses during the 2022 M_S6.8 Luding earthquake. The recorded data revealed that: first, significant vertical amplification within the soft soil profile, with the PGV and PGA at 4.0 m depth more than twice those at 11.0 m depth; second, under the in-situ seismic loading (PGA ≈ 0.16g), the predicted peak EPWP ratios were below 0.25, and the field-measured ratios were below 0.10, indicating no liquefaction or significant cyclic softening in the transitional silty clay layer. Notably, EPWP dissipated more rapidly in the improved zone than in the unimproved zone, thus providing direct evidence of the drainage effect of stone columns. These observations provide site-specific empirical evidence that stone column-improved foundations can substantially modify site response and EPWP evolution, and offer valuable field datasets for understanding the liquefaction and cyclic softening mechanisms.