Effect Of Double Foundation Pits Excavation On Close-fitting Existing Line Subgrade And Optimization Of Supporting Parameters

With the continuous progress of urbanization in China,urban traffic lines are expanding from the ground to the underground,which on the one hand facilitates the daily travel of residents;on the other hand,due to the limited soil resources,more and more new projects are close to or even close to existing lines.Among them,many researches have been done on the influence of single foundation pit excavation on metro tunnel,but few researches on the project of double foundation pits excavation close to the ground line,as well as the related deformation law and optimization of the existing line and retaining structure.Once it appears,the risk of single foundation pit excavation is doubled.Therefore,it is necessary to study the excavation of double foundation pits close to the ground line.Based on a new subway station double foundation pits project,this thesis adopts the research method of theoretical analysis,practical monitoring and numerical simulation.The main research contents are as follows:(1)Based on the relevant data and literature,this thesis introduced the deformation of the retaining structure and the surface outside the pit caused by the excavation of the foundation pit,and summarized the various factors which affect the deformation of the foundation pit,then the follow-up research object and research method combined with the project were determined;(2)Based on the actual engineering case,the three-dimensional finite element analysis model of the foundation pit was established by FLAC 3D,and then the simulation calculation corresponding to the actual working condition was carried out.With the excavation of the foundation pit on both sides,the change law of the foundation settlement and the horizontal displacement of the retaining pile close to the existing line was analyzed,and the calculation results of the subgrade settlement were compared with the actual monitoring data of the project;(3)On the basis of the established effective numerical model,the single parameter change method was used to simulate and calculate the four supporting parameters of pile diameter,pile spacing,pile length and bolt inclination,and the influence of the change of support parameters on the horizontal displacement of the retaining pile and the settlement of the subgrade of the existing line were obtained.Combined with the safety and economy,the optimization of the foundation pit support scheme was determined.