Integrated geophysical detection of water inrush from foundation pit near the river: a case study of Nanjing subway station

A sudden water inrush will inundate the foundation pit and bring great difficulty to construction. Subway construction near a river is more complicated since the river water is usually closely related to the groundwater, providing a continuous water supply for the water gushing inside the foundation pit. This study took the Nanjing Shangyuanmen station water inrush accident as a case study. We proposed a comprehensive detection system that combined different geophysical methods for delineating the water inrush hazards around the foundation pit. Considering that the foundation pit was only 400 m away from the Yangtze River in the north, the combination of transient electromagnetic (TEM) and surface electrical resistivity tomography (ERT) was used to realize the rough exploration of the water supply outside the foundation pit. Since the hydrogeological conditions were complex in the foundation pit, the cross-hole ERT and surface ERT were adopted to determine the distribution range of the water-rich areas and the connection to the water-inrush channel. To provide a plane position for the grouting treatment, we integrated the detection results of the different methods with drilling information and divided the water-rich area into three parts, through which we determined the main water gathering area. The integrated detection system provides guidance for the later treatment of the water inrush hazards of a subway foundation pit near a river.