Study On Stratum Deformation And Mechanical Effect In Construction Of Subway Station By PBA

With the rapid progress of the society and economy,the construction of urban rail transit enters a prosperous period and the underground excavation methods are also gradually emerged.As a hub of urban rail transit,metro stations are difficult to construct due to the large span and shallow depth.So the PBA(Pile Beam Arch)method is gradually proposed.This method combines the technical achievements of shallow-buried and undercutting excavation methods.It combines the small pilot tunnels,retaining columns,columns,buckles and other mature technology to form a collaborative support system to ensure the safety of construction,and this method has been successfully applied to a number of subway stations and other practical projects.However,the theory of the PBA method is behind practice largly,and the research on its mechanism of action is still not detailed enough,which makes it more difficult to be further popularized and applied in the construction of underground engineering.Based on the analysis of the research achievements both at home and abroad,this paper takes a subway project of Urumqi Metro Line 1 as a background.Based on the summary of the construction process and key steps of the PBA method through numerical simulation and theoretical analysis and combining with the measured data of surface subsidence in the field,the deformation,the stress of surrounding rock,the force of the surrounding rock and the optimization of the retaining pile during the construction of the PBA method are studied.The results show that the method can provide scientific theoretical guidance.The main contents and innovations of the research are as follows:(1)Relying on the project data of the subway station,the hydrogeological conditions,station structure types and construction steps of the station are introduced.At the same time,the key problems still existing in the application of the PBA method in underground engineering are discussed.(2)According to the characteristics of the project,the 3D model was established by using Midas-GTS-NX numerical simulation software to simulate the main process of the construction of the metro station by PBA method.The law of development of stratum deformation,surrounding rock stress and structure stress were obtained in different construction stages.The results show that the construction of small pilot tunnel and buckle arch is the key stage of the construction of the PBA method,and the settlement of ground strata is obvious.However,the structure and stratum displacement and stress of the whole station are both less than the allowable values,and the station is in a safe state.(3)In the process of excavation by the method,the overall stress system is composed of the arch structure,the retaining piles and the central steel pipe column.The piles are subjected to most of the load and are mainly subjected to axial forces.As the construction progresses,the peak axial force moves downwards as the excavated soil deepens.Due to the effect of side frictional resistance of the pile body,the axial force distribution of the whole pile gradually decreases from top to bottom.By calculation,the most unfavorable section safety factor is 10.20,which is greater than the safety factor required by the specification.The results show that the stress of the lining as a whole meets the specification requirements.(4)Because the retaining structure piles are dominant in the force of the frame system,the nine kinds of conditions are designed for comparative analysis of the two parameters of the pile diameter and pile spacing of the retaining pile.The results show that the different ratios of pile diameter and pile pitch have great influence on the final value and change law of axial force and bending moment,and have little influence on lateral displacement.Through the comprehensive analysis of the six indicators including the maximum axial force value,the maximum bending moment value,the maximum horizontal displacement value,the maximum surface settlement value,the maximum dome settlement value and the maximum surface elevation value in the final stage,the metro station perimeter was obtained.The best theoretical matching scheme for pile protection piles is 1meter pile diameter and 3meters pile spacing.