MULTI-BOREHOLE 3D DC RESISTIVITY DETECTION METHOD FOR GROUTING MONITORING AND EVALUATION IN TUNNELS: A CASE STUDY IN NORTHWEST CHINA

Low-frequency electrical methods are increasingly being used for detecting water-bearing structures within tunnels, particularly through boreholes on the tunnel face, including horizontal probe boreholes and grouting boreholes. This case study focuses on grouting process monitoring and evaluation in an under-river tunnel located in Northwest China. The geological conditions of the tunnel pose a considerable risk for water inrush, with several notable incidents having previously occurred. This has resulted in tunnel construction being delayed by over two years. In this case study, a probing borehole reveals a large stream of water flow, and grouting is needed to prevent the potential threat of a water inrush disaster. To guide the grouting operation, multi-borehole three-dimensional (3D) direct current (DC) resistivity detections are conducted. Novel electrode arrays and arrangements are proposed for the irregularly distributed boreholes on the tunnel face, with their feasibility and effectiveness verified through numerical analysis. Furthermore, the attributes of the two tomography modes are discussed. Based on the proposed method, two detections are carried out before and after grouting to monitor and evaluate the grouting results. The initial detection results provide a basis for selecting grouting positions. However, the water stream in the boreholes further increases after this grouting. Subsequent detection results indicate a shift in the position of the water-bearing structures. The interpretation result indicates that the high-pressure grouting may have induced extra fractures in the surrounding rock, leading to a shift in the position of the water-bearing structures. The constructors then adopted an appropriate grouting strategy and successfully passed through this fractured section. This case study highlights the effective application of the multi-borehole 3D electrical detection method for monitoring and evaluating grouting.