Effect of Dewatering in a Confined Aquifer on Ground Settlement in Deep Excavations

Dewatering must be conducted for shallow confined aquifers located below deep excavations. However, its effects on ground settlement remain unclear, due to the complicated seepage and stress condition of the strata. In this study, a coupled numerical method was adopted to study the effects of dewatering in a confined aquifer on ground settlement during excavation. A hypothetical excavation model depicting the soil layers that are typical in Shanghai was developed in a finite difference software. Excavation and dewatering in a confined aquifer were simulated. The numerical results were analyzed in association with those of an identical excavation in which the confined aquifer was removed through modification of its hydraulic conductivity. Comparative results showed that dewatering in the confined aquifer during excavation aggravates ground surface settlements, especially for the ground surface twice the maximum excavation depth (2 He) behind the wall, where He was the excavation depth. The dewatering-induced settlement beyond 2 He showed a similar profile as groundwater drawdown in the confined aquifer owing to the insignificant deformations of the strata above the confined aquifer. However, the settlement profile within 2 He was inconsistent with the drawdown profile, due to the effects of unloading inside the excavation and soil-wall interaction. At the end of excavation, the confined aquifer was significantly compressed due to dewatering. However, it was found that the aquitard above the confined aquifer expanded dramatically with its maximum expansion located near 0.5 He. This may be attributed to the local soil arch and insignificant consolidation in the aquitard. Additionally, postexcavation pumping resulted in significant compression of the aquitard and further aggravate the ground surface settlement.