Optimizing methods of excavation dewatering for deformation control in strata with leaky aquifers: a case study in Tianjin, China

Excavation mostly adopts mixed wells that simultaneously extract phreatic and confined water when the aquifer is close to the excavation base. However, it usually leads to excessive deformation in strata with leaky aquifers. Currently, there is a lack of research and countermeasures for this problem. This study analysed striking influences caused by excavation dewatering with leaky aquifers. When excavation reached 8 m depth, the groundwater level outside dropped 4 m, exceeding the 1 m allowable fluctuation and causing about 14 mm of building settlement. A countermeasure, adopting deep-shallow-well while incorporating groundwater recharge, was proposed. Its mechanisms and advantages were also discussed through numerical simulation, and it was subsequently verified in the engineering project. The mixed-well scheme induced greater confined aquifer drawdown (approximately 270% above requirement) and advanced pressure relief (approximately 74 days earlier) than the deep-shallow-well scheme. External recharge can effectively protect adjacent buildings. Early adoption of the optimized scheme could reduce settlement from 26 to 3 mm due to the difference between soil compression and rebound modulus. Therefore, for excavation projects in leaky aquifers, the mitigation strategy combining deep-shallow-wells with recharge is recommended.