Study On Deformation Characteristics By Pre-Dewatering Before Excavation Of Foundation Pit With Suspended Impervious Curtain In Phreatic Aquifer

In recent years,with the rapid development of infrastructure construction in China,the number of deep foundation pit projects is increasing.The influence of dewatering on the supporting structure of foundation pit and the surrounding soil deformation becomes more and more prominent.The related research mainly focuses on deformation caused by dewatering after excavation,but pays little attention to the deformation caused by predewatering before excavation,and the related research is not comprehensive enough.In the design of foundation pit in the stratum with large permeable layer thickness in phreatic area,considering the difficulty and cost,suspended impervious curtain and dewatering in pit are often adopted.The change of seepage field inside and outside the foundation pit caused by pre-dewatering will have a significant impact on the deformation of the supporting structure and surrounding soil.Therefore,it is quite necessary to study the deformation characteristics and influence rules of foundation pit caused by suspended impervious curtain pre-dewatering in phreatic area.With the help of finite element software,this paper which is based on the deep and large foundation pit project of Guangzhou Second Expressway,simulates the whole process of dewatering and excavation of foundation pit and compares the simulation results with the measured data to verify the rationality of the model parameters.Then,based on this model,the supporting structure and soil deformation characteristics and influencing factors caused by pre-dewatering of suspended impervious curtain foundation pit in phreatic area are further studied.Finally,the control method of deformation caused by pre-dewatering is discussed in combination with practical engineering.The main research results are as follows:(1)A three-dimensional finite element model for foundation pit is established based on the field investigation results.The simulation results agree well with the measured data.It suggests the numerical model can reflect the deformation law of foundation pit and can be further studied.(2)The change law of pore water pressure,the effective depth of pre-dewatering inside and outside foundation pit,and the distribution curve of groundwater level outside the foundation pit after dewatering are studied by numerical model.The distribution curve of groundwater level outside the pit is fitted by the formula of groundwater level distribution when the reservoir water level drops.Finally,based on the fitting formulas,the calculation method is used to calculate the ground surface settlement of foundation pit caused by pre-dewatering.The results suggest that the calculated results agree well with the numerical simulation results.(3)The numerical simulation results show that the pre-dewatering of suspended impervious curtain foundation pit will cause significant "cantilever" horizontal displacement of diaphragm wall.The displacement of top part of diaphragm wall is the largest,and the "kick-type" deformation occurs at the bottom.After dewatering,the horizontal displacement of the soil inside and outside the foundation pit is directed to the axis of the foundation pit.The horizontal displacement of the diaphragm wall is caused by the coordination of deformation and the lateral pressure of the soil outside the foundation pit.After dewatering,the settlement of the soil inside foundation pit is significantly larger than that outside the pit,and the diaphragm wall has a certain degree of lateral displacement under the action of the pressure difference between the soil inside and outside the pit.(4)The influence factors of ground surface settlement and horizontal displacement of diaphragm wall are analyzed by numerical simulation method from four aspects: predewatering depth,diaphragm wall depth,soil permeability and foundation pit length.The results show that the ground surface settlement and the horizontal displacement of diaphragm wall increase with the increase of dewatering depth,but the ground surface settlement increases linearly,while the increase amplitude of horizontal displacement of diaphragm wall decreases.The larger the depth of diaphragm wall,the smaller the ground surface settlement,but the decreasing amplitude decreases gradually.The influence of diaphragm wall depth on the horizontal displacement is small;with the increase of permeability of weak permeable layer,the ground settlement outside the foundation pit and horizontal displacement of diaphragm wall increase gradually,but the increase amplitude decreases gradually.Foundation pit length has a greater impact on ground surface settlement and horizontal displacement of diaphragm wall,and corner effect will significantly reduce the range of maximum and maximum surface settlement and horizontal displacement of diaphragm wall.(5)The control methods of surface settlement and horizontal displacement of diaphragm wall caused by pre-dewatering are briefly discussed.The results show that when the length of foundation pit is less than 80 m,the ground surface settlement and the horizontal displacement of the diaphragm wall will decrease significantly;the ground surface settlement will gradually stabilize after the depth of the diaphragm wall is greater than 26 m;the horizontal displacement of the diaphragm wall caused by dewatering will be reduced significantly by "first supporting and then falling".The depths of predewatering will also have a significant effect on controlling the surface settlement and the horizontal displacement of the diaphragm wall.