| The pile beam arch method(PBA),as an emerging construction method,has gradually been widely used in subway station construction projects in major cities due to its unique construction advantages.Due to the complexity of the construction process and the flexible and variable structural forms of this construction method,the changes in surface settlement caused by the pile beam arch method construction have received widespread attention from people.At present,many scholars have conducted some research on the construction process of the pile beam arch method,the surface deformation caused by construction,the deformation and stress of the support system,etc.However,there is still a slight lack of research on the surface deformation law caused by the pile beam arch method construction of subway stations in sandy soil layers in loess areas,the optimization of arch construction schemes,and settlement control.This article is based on the PBA underground excavation construction project of a subway station in Xi’an.Through on-site monitoring data statistical analysis,theoretical analysis of surface deformation,numerical simulation,orthogonal experiments,and other means,the variation law and control measures of surface settlement in each stage of the subway station pile beam arch method construction process are analyzed and studied.The main research content and results are as follows:(1)Systematically introduced the construction characteristics and structural forms of the pile beam arch method;Analyzed the surface deformation mechanism and influencing factors of shallow buried underground excavation pile method construction in loess areas;Based on the random medium theory,a linear analysis was conducted on15 sets of surface settlement monitoring data collected on site.It was found that when the correction coefficient a of the pilot tunnel area is within the range of 0.002～0.005 and the shape correction coefficient b is within the range of 0.3～0.6,the simplified formula based on the random medium theory predicted the surface settlement curve can better conform to the actual surface settlement curve generated during the small and medium-sized pilot tunnel construction stage of the pile beam arch method in the region.(2)A three-dimensional finite element model was established using numerical analysis software to numerically simulate the construction process of subway station pile beam arch method.The surface deformation laws caused by each construction stage of the station were further analyzed.Based on this,the magnitude of surrounding rock deformation and surface subsidence caused by each construction stage was compared.The results showed that in the construction of subway station pile beam arch method in sandy strata in loess areas,The surface settlement generated during the arch construction stage accounts for the largest proportion of the total settlement,55.36%,followed by the small pilot tunnel construction stage,accounting for 30.76%.The proportion of surface settlement generated during the formation of the pile column beam system and the excavation of the remaining main body of the station is equivalent and the smallest,with a total proportion of 13.88%.(3)Based on the analysis of on-site monitoring data of the supporting project,the changes in surface settlement during the actual construction of the pile beam arch method,as well as the convergence of clearance and arch settlement during the construction stage of the small pilot tunnel,were summarized;Comparing the analysis results of on-site monitoring data with the numerical simulation analysis results,it was found that the analysis results of the two methods are basically consistent: the construction stage of the buckle arch in the construction of sandy strata pile beam arch in the loess region has the greatest impact on surface settlement,further indicating that the buckle arch construction stage is the key stage for controlling surface settlement in the construction of sandy strata pile beam arch in the loess region.(4)Based on orthogonal experimental theory and numerical simulation,the construction process under 9 different working conditions was simulated for different arch excavation steps,advanced grouting quality of small conduits,and initial support strength.The surface settlement results under 9 working conditions were compared and used as experimental indicators for range analysis,The calculation results indicate that the degree of influence of the above factors on surface settlement varies from large to small: initial support strength ">" advanced small conduit grouting quality ">" arch excavation step distance;On this basis,the vertical displacement and surface settlement of the strata after the completion of the buckle arch construction under different levels of the above factors were analyzed,and the buckle arch construction plan was optimized to control surface settlement.Recommended values for construction parameters were provided. |
