| With the rapid development of China’s economy and technology and the continuous increase of urban population density,the available space above the ground has been shrinking,a large number of underground space projects have been constructed,thus there are more and more deep foundation pit projects.Due to the particularity of the location of foundation pit engineering in the city,there are many neighboring buildings nearby.The selection of appropriate retaining structure can effectively restrain the deformation of the foundation pit,reduce the ground settlement and the uplift of the pit bottom,thus improving the overall stability of the foundation pit and reducing the excessive influence on the surrounding existing buildings.The collapse of foundation pit has occurred frequently at home and abroad.If progressive collapse occurs to the retaining structure,it may lead to excessive deformation of the retaining structure and then lead to the occurrence of foundation pit safety accidents.Therefore,it is necessary to study the overall stability of deep foundation pit with partial failure of retaining structure.Based on the foundation pit engineering of the second expressway of Guangzhou Baiyun International Airport,this paper uses the combination of on-site investigation,theoretical analysis,on-site monitoring and numerical simulation to study the spatial effect of the foundation pit excavation process,the mechanical effect of the retaining structure and the influence of foundation pit retaining pile and horizontal support partial failure on the overall stability of foundation pit are systematically and deeply studied.Besides,the influence of design parameters of retaining piles structure with interior bracing on the redundancy of foundation pit is studied by relying on engineering.The main contents and research results are as follows:(1)Based on the Midas-GTS NX finite element numerical analysis software,a three-dimensional finite element foundation pit model for simulating the actual excavation process is established,which is consistent with the actual excavation process.The results of field monitoring and numerical calculation are comparative analyzed in this paper and the distribution law of the two is similar,which verifies the rationality of the numerical model and its parameter selection and can be used for subsequent expansion analysis.(2)The whole process of foundation pit excavation is studied by numerical simulation method,the variation of surface settlement outside the pit and the deformation of the retaining structure under different excavation depths and the mechanical effect of the foundation pit at different locations are analyzed.The results show that the maximum horizontal displacement of the retaining pile appears near the bottom of the pit and the vertical position of the horizontal support has a great influence on the distribution of the internal force of the pile.There is a significant spatial effect in the foundation pit and the deeper the foundation pit is,the stronger the spatial effect will be.The deformation in the middle of the pit is larger than the deformation of the corner of the foundation pit.When the distance from the pit angle exceeds 3 times the excavation depth of the foundation pit,the deformation can be equivalent to the plane strain state.The settlement curve of the ground surface outside the pit is “groove type”,and the maximum settlement appears approximately 0.7 times the final excavation depth of the foundation pit.(3)The influence of partial failure of the retaining pile and horizontal support on the overall stability of the foundation pit is studied by using the alternate path method,and the stress deformation and foundation pit displacement of the support system after partial failure are obtained.The results show that the moment transfer coefficient of adjacent damaged piles and the relative size of the single pile safety factor determine whether progressive collapse continues to occur.The outline of the collapsed area formed by the failure is in the shape of an arc,when the damage scope is increased to a certain extent,the range of the collapse area in the width direction is no longer increased,while the range in the length direction continues to increase.With the increase of the damage range,the unloading effect caused by the soil collapse gradually increases,when the soil arching effect generated by the soil is insufficient to cause damage to the adjacent support pile,the continuous damage will be terminated.When partial failure occurs to the horizontal support,the remaining un-failed rods are not evenly distributed to the load that is originally taken up by the removed rod,the released load is basically only applied to the nearest rods on both sides of the removed rod,this may result in a large axial force of the adjacent support,which would trigger progressive collapse to the support.(4)The numerical simulation method is used to compare and analyze the stress deformation characteristics of the retaining structure under different supporting parameters,and the influence rules of different design factors on the retaining structure and foundation pit displacement are obtained,then the influence of supporting parameters on the redundancy of the foundation pit is studied under the partial failure of 5 piles.The results show that the moderate increase of retaining pile and support stiffness is beneficial to reduce the deformation of the foundation pit,the embedded depth of the retaining pile and the vertical position of the support have a significant influence on the foundation pit.When the embedded depth is insufficient,the pile body will have a larger overall lateral displacement,beyond that,properly encrypting the lateral spacing of the horizontal support can provide more force transmission paths for the support after partial failure of the foundation pit,and the foundation pit has higher ability to resist continuous collapse.The position of the third horizontal support is critical to the safety of the foundation pit,which is more influential than the other two supports. |
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