Identification and Staged Remediation of Seepage Anomalies in a Deep Excavation in Water-Rich Karst: A Case Study Using Sonar-Based Seepage Detection

Water inrush in deep excavations in water-rich karst is difficult to control because concealed cavities and fractures can preserve local hydraulic connections after regional grouting. This study analyzes a deep metro-station excavation and develops a closed-loop framework comprising source–pathway diagnosis, regional treatment, post-treatment anomaly screening, targeted reinforcement, and monitoring feedback. Nine major inflow incidents occurred near the third strut level; the principal inflow was initially approximately 110 m3/h and stabilized at 40–60 m3/h. Borehole logs, hydrochemistry, and coupled groundwater responses linked the inflow to confined karst water, connected cavities and fractures, incompletely sealed boreholes, and diaphragm-wall defects. Regional treatment combined cavity-filling grouting, a jet-grouted basal blanket, deep external curtain grouting, and local joint sealing. A post-treatment sonar survey in 14 boreholes identified 17 measurements above 1.0 × 10−3 cm/s in Boreholes 12, 11, 14, and 7, mainly at depths of 19–31 m. These site-specific relative anomalies guided repeated sleeve-valve-pipe grouting and joint reinforcement. Subsequent construction was completed without another concentrated inrush of comparable magnitude. The adjacent wall point ZQT5, however, increased from 27 to 83 mm, demonstrating that hydraulic remediation and retaining-structure response must be controlled together. The framework improves the spatial focus of post-grouting verification, while the sonar results remain screening indicators rather than independently validated Darcy velocities or universal risk thresholds.