Study On The Stability Of Deep Foundation Pit Adjacent To Existing Railway Line

In order to alleviate the problem of urban traffic congestion and effectively use underground space,deep foundation pit engineering often occurs in the construction of subway stations.When the deep foundation pit is adjacent to the existing railway line,the existence of the train load will inevitably affect the stability of the deep foundation pit;at the same time,the excavation of the deep foundation pit will also cause the railway subgrade and the rail track to be deformed.Based on the deep foundation pit of Shangshagou Metro Station on Line 3 of Nanchang Subway,this paper combines theoretical research,monitoring data analysis and finite element simulation to analyze the stability of the deep foundation pit adjacent to the existing railway line,and provide reference for similar projects.Main research contents and results are as follows:(1)The deformation mechanism and mode of deep foundation pit are summarized from three aspects: deformation of enclosure structure,surface settlement and foundation pit bulge.The instability of the deep foundation pit is summarized as the overall instability of the deep foundation pit,the broken wall of the retaining wall,the kicking of the retaining structure,the foundation pit bulge,the inrush of the pit bottom and the destruction of the soil.At the same time,the effects of geological conditions,foundation pit size,foundation pit design and foundation pit construction on the stability of deep foundation pit are expounded.(2)According to the support scheme and monitoring plan for deep foundation pit excavation,with deep foundation pit excavation,the development rules of horizontal displacement of underground continuous wall,axial force of foundation pit support and surface settlement are summarized,the development difference between the construction parameters away from the railway side and the adjacent railway side,the end and the standard section of the foundation pit are pointed out,and the corresponding construction suggestions are put forward.Meanwhile,The development of railway subgrade is summarized into the synchronous settlement stage,the uneven settlement stage and the stable settlement stage,and the development trend of each stage is analyzed one by one.Furthermore,the stability of the diaphragm wall is verified in the absence of the isolated reinforcement structure.(3)In order to explore the mutual deformation effects of deep foundation pit and adjacent existing railway lines during construction,verify the effectiveness of Isolation reinforcement structure,the whole process of deep foundation pit excavation is simulated by using finite element software ABAQUS.It is found that excavation of deep foundation pits will result in track gauge deviation and orbital settlement difference,when the train speed limit is 80~120km/h,the existence of train load increases the horizontal displacement and bending moment of the underground continuous wall by 71% and 36% respectively,and shift the position of two indicators to the peak.At the same time,the isolation and reinforcement structure can effectively reduce the mutual deformation between the deep foundation pit and the adjacent railway line,but it is easy to cause a sudden change in the displacement,which makes the space effect of the foundation pit more complicated,and it cannot be ignored in similar projects.(4)In view of the conservative design of the isolation and reinforcement structure,the characteristic parameters are set as the horizontal displacement of the underground continuous wall.From the aspects of pile spacing,pile length,pile diameter and elastic modulus of bored piles,the influence of various factors on characteristic parameters is analyzed.The important level of important factors is extracted and orthogonal test is carried out to judge the influence degree of each factor: pile diameter> pile length> pile spacing> elastic modulus.At the same time,an optimization plan is proposed to save engineering cost and shorten the construction period.