Field experiment and numerical simulation on sequence dewatering of a sand inrush tunnel under high water pressure in semi-diagenetic rock: a case study

The semi-diagenetic sandstone stratum has the characteristics of weak cementation and low strength. Gushing induced sand inrush in semi-diagenetic surrounding rock under high water pressure is one of the most difficult worldwide disasters during tunnelling. In the construction of Wangjiazhai Tunnel in Lincang-Qingshuihe expressway, gushing and sand inrush disasters in semi-diagenetic surrounding rock occurred numerous times under high water pressure causing serious economic loss, environment influence and construction delay. Conventional grouting reinforcement and drainage measures in tunnel cannot effectively solved the disaster problem resulting in repeated gushing and sand inrush, rescue and re-excavation without confidence construction period. In order to solve the surge and sand inrush disaster problem of the tunnel, this study proposed a surface ultra-deep pumping well dewatering method. The pumping well exceeding 250 m deep was designed and sequentially pumping scheme was adopted adapted to the progress of the excavation face. Three deep wells were successfully installed, and pumping experiments was carried out which inversed hydraulic parameters and confirmed the feasibility of well structure. The pumping capacity of these wells exceeding 700 m³/d and a maximum drawdown of 70 m. Based on the field experiments, a sequential dynamic dewatering scheme was proposed. The sequential dynamic dewatering scheme was verified by numerical simulation. The results show that this scheme can reduce the groundwater level above the tunnel by more than 50 m and effectively reduce the groundwater level in front of the tunnel excavation face. The design ideas and results of this study provide a reliable solution for the control of tunnel surge and sand inrush disasters under similar challenges.