Study On Deformation Behavior Of Subway Deep Foundation Pit Excavation In Sand And Silt Strata

With the rapid development of urban rail transit construction,the problem of deep foundation pit engineering in subway has attracted much attention.The deformation of the foundation pit excavation is closely related to its geological and hydrology conditions,sand and silt based strata has the characteristics of high sensitivity,high compressibility and low strength,near the river,the water level is high and is affected by the seepage of groundwater,the risk of foundation pit engineering under this geological and hydrology condition is higher.Therefore,it is very necessary to study the deformation characteristics of the foundation pit in the process of excavation in sand and silt strata and adjacent to the river.In this paper,Guangfo line construction 18 A Section Lijiao subway station foundation pit engineering as the research object,through field measurement and numerical simulation research methods combined,deep research on the deformation law of sand and silt geology and adjacent river foundation pit during excavation,the research results can provide reference for the design and construction of subsequent foundation pit(reentry line,Pavilion)and other similar foundation pits.The main contents and conclusions are as follows:(1)Analysis of field measured data of foundation pit,a preliminary summary of the deformation of the foundation pit enclosure structure and the law of the surface settlement in the pit.(2)Taking Guangfo line construction 18 A Section Lijiao subway station foundation pit engineering as the object of study,using the three-dimensional finite element analysis software MIDAS/GTS to simulate and analysis excavation process of foundation pit,the comparison between the numerical simulation results and the measured data verifies the correctness of the model and parameter values;and comprehensive measurement data and simulation results are used to summarize the deformation law of Subway Foundation Pit under the condition of thissite.It is concluded that the maximum horizontal displacement of the wall appears in the middle and lower parts(0.6h to 0.64 h,h is the depth of the diaphragm wall);The main influence area of the surface settlement of pit is 0 to 18 m from the edge of the foundation pit(0 to 1.1H,and H is the depth of the excavation),the secondary influence area is outside 18m(>1.1H)from the edge of the foundation pit,the presence of deep silt layer causes the secondary settlement to remain obvious;bottom of the pit because of excavation unloading first elastic uplift after the plastic upheav,the plastic uplift "bimodal" is located 2 to 3m away from the earth wall.(3)According to the measured data and simulation results analysis of the reasons for the excessive deformation of the retaining structure of the station foundation pit,and put forward reasonable suggestions;and discuss the effects of support stiffness on deformation of foundation pit.The study found that overloading on the south side of the foundation pit is the main reason for the excessive deformation of the wall,and the foundation pit is near the tributary of the ZhuJiang River its water level is relatively higher,wall deformation is greatly influenced by seepage of groundwater;the actual embedded depth of the wall of the foundation pit is 5.5m,existence disadvantageous situation that horizontal displacement of wall bottom exceeds its maximum horizontal displacement 1/3,it shows that the inlay depth of the Underground continuous wall is not enough,for safety and economic consideration,the proposed ground wall is embedded in deep 6.5m;the increase of supporting stiffness can reduce the deformation of the ground wall and the settlement of the pit surface,but there is a marginal effect and it has little influence on the deformation of the wall and the settlement of the pit.(4)According to sand and silt strata geological features,by changing the values of the main soil layer parameters affecting the deformation of the foundation pit simulation analysis,get the sensitivity of wall deformation and surface settlement to each soil parameter.Research shows that the order of sensitivity of wall deformation and surface settlement in the pit to silt layer parameters ranking is Poisson’s ratio,internal friction angle,soil modulus and cohesion;however,the order of sensitivity to sand layer parameter is: internal friction angle,Poisson’s ratio,soil modulus and permeability coefficient.