Laboratory model tests on artificial groundwater recharge in deep excavation

Dewatering inside an excavation can induce significant surrounding ground settlement. The artificial groundwater recharge is commonly employed to limit the dewatering-induced ground settlement. Generally, it is necessary to carry out a series of preliminary tests to check the recharge effects and then determine the formal recharge operation scheme. If the scaling physical modelling of various recharge conditions can be reproduced in the laboratory environment, it will provide a more economical and efficient method to establish optimal recharge schemes. In this study, based on a practical excavation of metro station in Tianjin, the laboratory-scale dewatering and recharge tests are conducted to explore the feasibility of reproducing the effects of artificial recharge on limiting groundwater drawdown and ground settlement in the laboratory. With installation of observation wells, displacement sensors and stress sensors, the time-history variations of water level inside and outside the excavation, enclosure wall deflections, pore water pressure and earth pressure at both sides of the wall, and ground surface settlement are monitored during the dewatering and recharge tests. The results show that: (1) The groundwater drawdown induced by dewatering and groundwater level rise caused by recharge can be well reproduced in laboratory environment, demonstrating the feasibility of laboratory-scale model tests on dewatering and recharge. (2) When recharge is carried out in close proximity to the enclosure wall, the water pressure around the bottom of the recharge wells increases significantly, which can make the enclosure wall at the same buried depth move inward to the excavation, subsequently leading to ground settlement outside the excavation. In practical engineering, engineers should carefully determine the recharge rates of the wells located near the enclosure wall to prevent large wall deflections and subsequent settlement increment. For the projects that require strict control over surface deformation, it is not advisable to place recharge wells in the vicinity of the enclosure wall.