Deformation Monitoring And Numerical Simulation Of Pile-anchor Support For Deep Foundation Pit Of High-speed Railway Station

The engineering geological environment and the hydrogeological conditions faced by the construction of the deep foundation pit of the station is complex and changeable,and most of them are close to dense buildings and existing underground structures.Taking the foundation pit project of a certain station in Beijing as the research background.Based on theoretical analysis method,the common support structure forms,the advantages and disadvantages of pile-anchor support,and the current development situation in deep foundation pit engineering are discussed.Based on on-site monitoring method,the time history curve of various monitoring results during the entire process of foundation pit construction are drawn,and a detailed analysis are conducted on the changes in specific monitoring items.Based on numerical simulation method,the deformation and stress characteristics of the entire construction process of the pile-anchor support system for foundation pits in typical strata in Beijing are studied,and then the influencing factors of the pile-anchor support system are comprehensively explored.The main conclusions are as followed:(1)The elastic foundation beam method is more conservative compared to the elastic-plastic foundation beam method in the calculation and verification of pile-anchor support structures.The three-dimensional finite element method can reflect the spatial stress characteristics of the structure and fully consider the joint action of soil and pile-anchor support system.(2)With the increase of excavation depth of the foundation pit,the overall time history curve of the same measuring point shows a trend of first increasing and then stabilizing.The deformation of the retaining pile at various points along the depth shows a pattern of "large deformation at the top,protruding at the middle,and small deformation at the bottom".(3)The middle area of the long side of the foundation pit and the interior of the foundation pit near the center point are the locations where the deformation and stress of the entire foundation pit are greatest.In practical engineering,the stiffness of structural components within a certain range of internal and external corners of the foundation pit can be appropriately reduced,and the stiffness of structural components in the middle of the foundation pit can be strengthened.(4)When the stiffness of the retaining pile increases,the longitudinal spacing between anchor cables increases,the number of anchor cable arrangements decreases,the angle of anchor cable arrangement increases,and the length of anchor cable arrangement decreases,the horizontal deformation and surface settlement of the retaining pile increases.The internal force of the anchor cable is less affected by the stiffness of the retaining pile and the number of anchor cable arrangements,but is closely related to the longitudinal spacing.(5)The sensitivity ranking of various influencing factors of pile-anchor support are as followed: length of anchor cable arrangement,number of anchor cable arrangement channels,longitudinal spacing of anchor cables,stiffness of retaining piles,angle of anchor cable arrangement.In the design of pile-anchor support system in foundation pit engineering,the focus should be on anchor cable design.The research results can provide reference for the design and construction of deep foundation pits for stations in the Beijing area.