Optimization And Comparation Of Dewatering Method And Waterproof Curtain In Deep-buried Foundation Pit

This study based on Shanghai metro Line 10 Liyang station foundation pit project.When foundation pit is constructed in the soft soil areas of the Yangtze river delta,groundwater control is of top priority.South shield shaft of Liyang road metro station is sensitive to dewatering and excavation process.The extraordinary settlement caused by foundation pit excavation and dewatering is due to the partial loss of aquifer and the communication between confined aquifers and micro-confined aquifers near south end shield shaft.This shaft needs to be listed as a key observation object in the research process.Considering the special geological characteristics and local hydrological characteristics,the project need careful study.Finite element method and finite difference method are used in this project.The criteria of this study focuses on the environmental impact of deep foundation pit excavation.Several schemes provide an optimal solutions for engineering for reference.The main results of this study were as follows:(1)Analysis and Discussion on change regulation of confining head and settlement caused by foundation pit dewatering in complex geological condition under vertical waterproof curtain.Under the condition of simple waterproof curtain,stimulation of foundation pit excavation and dewatering were carried out.The overall dewatering results were racket-shaped,because the water level near the shield shaft at the southern end was relatively deep and settled.The results of numerical analysis show that with the increase of the opening time of dewatering well,the influence area of settlement is increasing.At the beginning,the hydraulic funnel area caused by the change of confined water is larger than the settlement funnel area,but it will be consistent with time increasing.Because of the water-retaining effect of diaphragm wall,there is basically no correlation between hydraulic gradient and settlement inside and outside wall.(2)Analysis and optimization of vertical barrier's insertion depth.Through numerical simulation,it is found that with the increase of diaphragm wall insertion depth,control for drawdown of water level and settlement has also been gradually increase.But before the insertion depth reaches the bottom of confined aquifer,increase the depth of diaphragm wall effect is not obvious.When the diaphragm wall insertion depth reaches the bottom of the confined aquifer,the diaphragm wall acts well in controlling environment impact.However,at the same time the insertion depth is deep,the requirements for constructing diaphragm wall and the engineering cost is relatively high.(3)Discussion on effectiveness of horizontal barrier scheme and optimization of waterproof curtain.Through numerical simulation,it is found that the horizontal barrier and vertical barrier with appropriate depth can effectively reduce settlement and drawdown of water level,which meets the requirements of foundation pit construction on environmental impact and has a relatively low cost.It can be used as the optimal scheme of this project.(4)Investigation on the regulation of leakage in different part of waterproof curtain.In the scheme of horizontal barrier,when leakage happens at pumping layer,the settlement and drawdown funnel will regenerate and do much more damage to improvement than the other part.The most dangerous part of leakage of the retain structure is the diaphragm wall section in the pumping layer,which need extra attention.