Diaphragm wall deformation and ground settlement caused by dewatering before excavation in strata with leaky aquifers

Abstract

In deep excavations, pre-excavation dewatering (PED) usually causes notable diaphragm wall (D-wall) deformation and ground settlement. However, this problem has not attracted sufficient attention. In this paper, the mechanisms and control methods of settlement caused by PED in a deep excavation in Tianjin, China are analysed. A hydraulic connection between the soil inside and outside the excavation area due to aquifer leakage was observed during PED. Therefore, although the first level of struts had been installed before PED, the deformation of the D-wall and ground settlement reached 6·9 mm and 12·5 mm, approximately 29·0% and 73·5% of the maximum allowable values, respectively. The monitoring results demonstrated that leaky aquifers could reduce the deformation of the D-wall but increased the magnitude and extent of ground settlement. Based on the numerical simulations of D-wall deformation and groundwater change, in this case, the ground settlement within the area extending two times the depth of unloading (two times the depth of dewatering, 2Hd) from the excavation was found to be caused by both the deformation of the D-wall and the drawdown of the groundwater, whereas the noticeable settlement in the area beyond 2Hd was caused by only the drawdown of the groundwater. When hydraulic connections exist between the soil inside and outside the excavation area, artificial recharge should be used to control the noticeable settlement induced by groundwater drawdown.