Study On The Settlement Characteristics And Stability Of Pba Construction Method Of Subway Station In Loess Area

The construction of subway in Xi’an loess area in busy urban areas or near existing building structures has become an unavoidable engineering problem.How to quickly and effectively construct and reduce the impact on the city is an urgent problem to be solved in the process of urban underground construction in loess areas.It has certain academic value and engineering application value to carry out the research on the surface settlement law of PBA construction method of shallow buried underground excavated large-span station in loess area.Relying on the construction project of the main body PBA construction method of the shallow buried underground excavation large-span station of the Xi’an Loess Area Metro Line 8 as the background,a three-dimensional numerical model is established through the finite element software ABAQUS,the whole process of the proposed station construction is simulated,and the various construction stages of the station PBA construction method The law of induced surface settlement and the mechanical effect of the main structure of the station during the construction process are studied.The main work and conclusions are as follows:(1)The station adopts a pilot tunnel structure of "4 small pilot holes on the upper floor+2 large pilot holes on the lower floor".The construction of the new PBA method is appropriately simplified to simulate the entire construction process of the station.The results show that the final settlement of the station conforms to the shallow buried and darkened construction.The settlement control standard of excavation station is that the bottom uplift value of the pilot tunnel construction stage is relatively large,and the surface settlement is mainly concentrated in the early stage of the PBA construction method.The settlement caused by the pilot tunnel excavation accounts for the largest proportion,followed by the arching stage settlement.The surface settlement trough curve is roughly symmetrical about the center line of the station.The horizontal displacement of the surface takes the center line of the station as the axis,and reaches the maximum at about 14m from the center line of the station.The law of horizontal displacement of the surface generally shows a trend of the soil on both sides of the station moving and shrinking to the center axis.(2)Numerical simulation is used to study the excavation sequence of the new pilot tunnel structure of the station,and analyze the settlement and surface deformation characteristics of the station at various stages.Use the analytic hierarchy process to establish a systematic hierarchical structure and evaluation index matrix,compare the evaluation indexes of each construction plan,calculate the judgment matrix of each index,and determine the optimal pilot tunnel excavation sequence:first excavate the upper pilot tunnel,then Excavate the lower pilot hole,first excavate the pilot holes on both sides and then dig the middle pilot hole.(3)Research is carried out on the influence of the size of the pilot tunnel of shallow buried and concealed station on the surface settlement of the station and the force of the station structure.Numerical simulation is used to study the construction conditions of different pilot tunnel sizes for shallow-buried,hidden-excavation and large-span stations,and the analytic hierarchy process is used to evaluate the various conditions.Take the surface settlement value at the pilot tunnel stage,the surface settlement at the arching stage,the width of the settlement trough,the station structure stress,and the pilot tunnel construction space as the main evaluation indexes to establish an evaluation index matrix,calculate the judgment matrix for each index,and comprehensively consider the structural safety,For factors such as construction space and impact on the surrounding environment,it is recommended that the design size of the pilot tunnel should not exceed 5.0m,and 4.0m is the best.