Prediction of city tunnel water inflow and its influence on overlain lakes in karst valley

The prediction of water inflow before tunnel excavation is the key to reducing the influence of tunnel construction on the surrounding groundwater environment. Based on hydrogeological conditions revealed during tunnel construction, three empirical formulas are adopted to predict water inflow of the Nanshan tunnel and further clarify its influence on the overlain Tushan lakes in Chongqing, China. Monitoring data with respect to tunnel water inflow, variations of water level in Tushan lakes and the volume of infiltrated water are studied. The field monitoring data are compared with the predicted values to determine the most suitable prediction method. As a result, the ground dynamics method appears to be the most suitable for predicting tunnel water inflow in the Nanshan area. The radius of the influence range caused by tunnel construction ranges from 520 to 643 m. The induced precipitation funnel caused by the Nanshan tunnel overlaps with that of the Tongluoshan tunnel. Analysis of monitoring data indicates that the amount of tunnel water inflow is almost equal to the sum of water leakage in Tushan lakes and infiltrated water, which means that tunnel water inflow mainly comes from surface water. Groundwater has a strong hydraulic connection with Tushan lakes. It is inferred that if no measures were taken, Tushan lakes might dry out. To cut off hydraulic connections and further reduce tunnel water inflow, the clay seepage method and curtain grouting method were applied in the tunnel. The effectiveness of these two methods is validated through field monitoring data collected after tunnel breakthrough. The research results could provide guidance and references for similar projects.