Field pumping experiments and numerical simulations of shield tunnel dewatering under the Yangtze River

The water conveyance tunnel for the Changshu power plant is located under the riverbed of the Yangtze River. The tunnel passes through muddy silty clay (①) and silty clay (②). Because quicksand and water bursting occurred in the tunnel due to confined water and biogas in layers ② and ③, it can be concluded that the tunnel is intensely affected by the confined aquifer (layer ③). To obtain more accurate hydrogeological parameters of the aquifers, in situ pumping tests were carried out. The tests were divided into five stages and, during the test process, the flux and drawdown were monitored. Based on the measured data, the permeability coefficients of the confined aquifers were calculated by the formula and numerical methods. The calculated results indicate that the parameters derived from the two methods are very similar. The hydraulic connections of the confined aquifers were also inversed from the pumping tests. The dewatering schedule for tunnel recovery was simulated using ModFlow. Based on the numerical simulation, 21 wells, whose screens were installed in layer ③, were designed for the dewatering operation. During pumping, the water levels declined to the drawdown requirement. The results indicate that the dewatering approach is feasible for tunnel recovery and can provide solutions for similar projects.