Influence of the opening timing of recharge wells on settlement caused by dewatering in excavations

Abstract

In deep excavation engineering, the head of the confined aquifer below the excavation bottom must be lowered when the anti-uprush safety factor is not satisfied. However, the drawdown of the confined aquifer may cause ground settlement around the excavation and endanger adjacent buildings, tunnels or municipal pipes. Artificial recharge (AR) is a measure that has been applied in such situations to recover the water level and control the settlement. However, practical engineering cases have shown that AR could control only the development of settlement and could hardly recover the settlement caused by long-term dewatering. Therefore, three recharge schemes, namely, “recharge first”, “simultaneous pressure relief and recharge” and “pressure relief first”, are studied using numerical modeling. The results show that the opening timing of the recharge wells and pressure relief wells has a significant impact on the final settlement. In different recharge schemes, different stress paths of the soil in the confined aquifer and overlying aquitards cause different plastic compressive deformations. To reduce plastic deformation, recharge should begin at the same time as pressure relief when the surrounding environment of the deep excavation must be strictly protected. Additionally, the case study and numerical simulation both show that the most significant settlement or uplift occurs in the deep soil layer rather than at the ground surface, a phenomenon that occurs because the vertical displacement of the soil adjacent to the relief and recharge wells is larger than that in the soil far away, leading to a vertical soil arching in the strata above the confined aquifer.