Research On Waterproof Effect And Excavation Deformation Of Retaining Structure Of Secant Piles In PBA Method Constructed Subway Station

As China's urbanization process accelerates,the construction of urban rail transit has become an important measure to improve a country's overall national strength,economic strength,and modernization level.In the past subway excavation construction,in order to ensure the waterless construction of the excavation surface,Precipitation during construction is a preferred method.however,as the city's groundwater resources are insufficient,carrying out non-precipitation construction has become a new trend in subway underground excavation construction.Secant pile is a kind of economical and fast pile wall structure which can play a protective role,and can effectively achieve the water stop effect required by the project.This article relies on the Xitucheng Station of Beijing Metro Line 27 Phase 2?Changping Line South Extension?Project,and mainly studies the following items:?1?Experimental study on the performance of plain concrete pile materials.The mix ratio design of plastic concrete,a total of 10 groups,after the end of curing,plastic concrete cube compressive strength test and relative impermeability test.Through the control variable method,analyze the influence of water-cement ratio,bentonite content and sand rate on the strength and impermeability of plastic concrete,and summarize the influence law,and finally select the best mix ratio suitable for plastic concrete The best mix ratio is group A-4,and the specific mix ratio is cement:water:sand:stone:fly ash:bentonite=1:1.81:8.53:6.84:0.76:0.76,Group A-4 will be used as plain concrete pile material for the pilot project of Xitucheng Station.?2?Analysis of strata stability.In this paper,we take Xitucheng Station as the research object,establish a finite element model of the PBA four-guided tunnel underground excavation with secant pile as the side pile,and compares it with the underground excavation station with the diaphragm wall as the side pile.According to the actual working conditions,the finite element calculation of the subway station is carried out through the numerical simulation software FLAC^3D.In the process of excavation,the displacement law of the bite pile body,the integral relationship of the secant pile and the analysis of the ground settlement around the underground excavation station under various working conditions are revealed,and the prediction model of the excavation polygon in the limited space is established.Through the prediction model,it is found that the deformation curve of the enclosing pile is in the form of a quadratic curve.Compared with the diaphragm wall enclosing structure,the displacement deformation of the secant pile and the overall settlement of the subway station will increase.?3?Analysis of impermeability of secant piles.By summarizing the geometric defect law of the secant piles due to factors such as stratum and equipment during the construction process,and according to the geometric defect law,four sets of analysis conditions were set up to establish the finite element model of the underground excavated subway station.Finite element calculations are performed by Midas gtsnx software.The change characteristics of total head distribution,seepage flow,seepage velocity and seepage slope under the action of seepage are obtained,which reveals the weakening effect of different geometric defects on the seepage resistance of the occluded pile,which provides value suggestions.The existence of geometric defects will cause high head ratio,seepage velocity and seepage slope drop,which will increase the seepage energy of the water flow,and eventually increase the seepage flow of the retaining pile,which will adversely affect the water-stopping effect of the secant pile.