Research On Mechanical Behavior And Safety Control Technology Of Open Excavation And Floor-increasing Construction Of Underground Subway Station

In recent years,the scale of urban subway construction in China has gradually expanded,and more and more cities have formed a subway line operation network.With the construction of subway lines into the network,a large number of existing subway stations have been expanded into new transfer stations.In the construction process of transfer stations,the surrounding environment is more complex,and construction types such as ultra-small distance proximity and underpass often appear,which increases the construction difficulty.Open-cut construction is a common construction method in the process of station expansion,but there are problems in the construction process that the new structure and the existing structure have great influence,and the construction mechanical behavior is very complicated.In the construction process of Beijing Metro Pingguoyuan Station transfer station,the new type of open-cut co-construction is adopted:Underground two and three layers of excavation construction,after the completion of construction,in the upper open excavation construction,to build an underground layer,which saves the underground space resources.However,the cross influence between the underground station structure and the new foundation pit is more intense and the construction is more difficult.In this regard,relying on the Pingguoyuan Station project of Beijing Metro Line 6,this research adopts similar model test,numerical simulation,theoretical analysis and field measurement to analyze the stress and deformation law of underground dug station,the stability of the envelope structure,the formation failure mode of underground dug station,and the stability control measures of station and envelope structure.The research results are as follows:(1)Based on the Apple Orchard Station,a similar model test of open-cut extension was designed,and the numerical simulation analysis of open-dark excavation combined construction was carried out.It was concluded that the excavation of foundation pit caused the increase of the relative strain of the roof and side wall of the station,the overall uplift of the station in the area of foundation pit,and the longitudinal deformation of the station was symmetrically distributed with the axis of foundation pit as the center.The foundation pit area and 30 m area on both sides of the foundation pit are the longitudinal uplift range of the station roof.At the center of the foundation pit,the station cross section side wall shows an internal extrusion trend,the stress state of the station roof and the upper side wall changes,while the stress state of the bottom plate remains unchanged but the stress is relieved.The existence of the dug station is conducive to the stability of the embedded pile,and the bottom of the stilt pile located on the roof appears the sliding deformation of the kicking foot.(2)Through numerical simulation,the thesis reveals the stratum failure modes of underground excavation stations under different foundation pit widths and different embedment depths of retaining piles,summarizes three failure modes of through slip surface,intermediate soil and limited soil,and reveals the load transfer mechanism of substructure cross section.Based on thin layer element method and sliding wedge theory,the calculation method of side wall load is derived.Based on this method,the load structure calculation model based on elastic foundation beam is established,and the longitudinal reinforcement measures on both sides of the station are proposed.(3)The sliding mechanism of the suspender pile was revealed,and the influence of soil parameters and pile bottom friction on the sliding of the suspender pile bottom was clarified.It was concluded that the deformation characteristics of the suspender pile bottom changed from the sliding kick shape to the fish-belly shape after the connection,and the optimal width of the grouting reinforced area was 3m.The stability control measures of three envelope structures are given: connecting pile bottom to the first branch of the roof of the dug station,strengthening by grouting after pile and reserving back co MPacted soil.The effectiveness of the control measures is verified by the analysis of field measurement.