Optimization Design And Deformation Characteristics Research Of Pile-Anchor Supporting Structure For Deep Foundation Pit In Changde

With the continuous improvement of urbanization level in our country,the usable land in cities has been greatly reduced.As a result,the use of cinstruction of underground spaces such as urban subway stations,urban utility tunnel and underground civil air defense facilities in cities has increased significantly.Taking a foundation pit project in Changde as the research object,this paper uses finite element analysis method to analyze the influence of anchor cable anchorage length,anchor cable inclination angle,pile diameter,pile length,net pile spacing,anchor cable prestress and other related factors on the deformation and mechanical characteristics of the supporting structure.Through orthogonal test,the above factors are comprehensively evaluated and analyzed to determine the optimal combination of detailed design parameters of pile-anchor supporting structure.Finally,the rationality and reliability of the optimized design scheme are verified by finite element analysis software PLAXIS and Lizheng deep foundation pit software.The main research contents and results of this paper are as follows:(1)The deformation and stress characteristics of pile-anchor supporting structure in deep foundation pit under three different working conditions are analyzed.Case 1:when the foundation pit is excavated to-2m,the maximum horizontal displacement of the row of piles is located at the top of the pile,The maximum bending moment of the row pile is only 46.6kN-m;Case 2:apply the first anchor cable at-1.5m and excavate the foundation pit to-6.5m,the horizontal displacement of the pile top decreases and the maximum displacement point of the pile row moves to the lower part of the pile row,The maximum bending moment of the row pile is 307.0 kN-m;Case 3:apply the second anchor cable at-6m and excavate the foundation pit to-10m,and the maximum horizontal displacement point of the foundation pit continues to move downward to-7.25m,The maximum bending moment of the row pile increased to 327.2 kN-m,With the excavation of the foundation pit,the maximum horizontal displacement point of the row pile is no longer the pile top.The displacement curve shape of row piles also become a "bow" shape,but gradually develops into the depth of the foundation pit.The surface settlement of the foundation pit is in the shape of a"concave" groove,the maximum position is 5m away from the foundation pit wall,the maximum uplift of the foundation pit bottom soil is 15.62mm,the maximum uplift is 3.03m away from the foundation pit wall,and the uplift in the middle of the foundation pit bottom is 14.4 mm,which is a plastic uplift.(2)PLAXIS software is used to conduct single factor analysis on the detailed parameter of pile-anchor supporting structure.The influence degree of relevant factors such as prestress of anchor cable,anchor cable inclination angle,anchor length of anchor cable,net spacing of piles,pile diameter and pile length on deformation and mechanical characteristics of supporting structure is studied.(3)Propose the optimal design scheme of the foundation pit supporting structure.The optimization design scheme adopts factors such as anchor cable anchoring section length,pile diameter,pile length,net spacing of piles,anchor cable prestress and the like as design variables,and takes the maximum horizontal lateral displacement of row of piles and the cost per linear meter of foundation pit as evaluation indexes.In order to solve the problem of multi-index contradiction,the fuzzy comprehensive evaluation membership degree is introduced to replace the original contradictory evaluation index.On this basis,through statistical analysis of fuzzy comprehensive evaluation membership degree,the optimization scheme of supporting structure detail parameters is obtained.Finally,the article verifies the rationality and reliability of the optimization scheme,completes the deepening design of the support scheme,and achieves good application effect.