Ground Settlement Induced By Dewatering And Excavation And Its Control Measures

The scales of deep excavations extend persistently in the process of rapid development of urban underground space utilization. Due to the high underground water level in the coastal area, dewatering has become a necessary step in underground construction. The value and influence area of the ground settlement induced by dewatering is much larger than that induced by unloading in deep excavations. The large range of ground settlement not only endangers the surrounding underground structures, but also contributes to increase of the regional land subsidence. Therefore, it is urgent to research the law of ground settlement induced by dewatering and excavation so that corresponding control measures can be proposed.In this paper, the ground settlement induced by the coupling effect of dewatering and excavation is studied by field test and numerical analysis. The evolvement rules of the soil movements caused by deep excavations are discussed and the regulation effects of different measures in controlling the ground settlement are analyzed. The main works and achievements are as follows:(1) The applicability of different soil constitutive models in numerical analysis of dewatering and excavation. According to the characteristics of deep excavations, two-dimensional finite element model was established considering the soil-water coupling effect of the saturated soil. The methods of “element remove” and “water flow controlling” are adopted to simulate the excavation and dewatering process respectively. Based on above-mentioned model, the Elastic model, Mohr-Coulomb(MC) model and Modified Cam Clay(MCC) model are adopted to analyze the influence of soil constitutive models to soil seepage field, displacement of both retaining structure and surrounding ground. It indicates that MCC model is more suitable constitutive model for deep excavations comparing with the Elastic model and MC model.(2) Response of the ground settlement induced by deep excavation and dewatering. The transient consolidation method by controlling water flow is applied to analyze the excavation and dewatering progress and investigate the variation of ground settlement. Compared to the data obtained from field test, the calculated results are consistent with the field measurements. This indicates that MCC is rational to simulate the coupling effect of dewatering and excavation. Then the influences of the depth of dewatering well and the location of recharge well on the ground settlement are investigated by parametric studies.(3) Corresponding measures and their effectiveness to control of the ground settlement induced by excavation and dewatering. Numerical analysis is conducted to a typical deep excavation case in Shanghai, which revealed that dewatering in the confine aquifer was the main reason for the ground settlements with a larger range. The influence of different measures to control the ground settlement and their efficiency are studied with parametric studies on this case. Results show that the ground settlement decrease with increase of both the waterproof depth and the recharge water flow. The horizontal displacement of the retaining structure would increase because the increasing of water pressure difference between outside and inside of excavation.