Study Of Deformation And Stability In Process Of Shallow Metro Tunnel Excavation Under Sand Layer

Based on interval tunneling from Hexi station to Hedong station of Qingdao Metro Project I (Line 3), this paper combined theoretical analysis, numerical calculation and monitoring to analysis the deformation regularity and stability of surrounding rock of the sand soil of Shallow Metro tunnel excavation. Then analysis the construction risk and collapse mechanism.Firstly, this paper summarized the present situation and research progress of the domestic and foreign research of shallow metro tunneling induced ground deformation from four aspects:experience method, analytical method, numerical analysis and model test method. Through the statistics of the metro construction safety accident analysis, we know that collapse is the main type of metro construction safety accidents. So it is necessary, important and urgent to have a study of induced deformation regularity and formation stability of tunnel excavation.Secondly, the paper introduce the development, line planning of Qingdao Metro briefly and have a detailed introduction of engineering geology, hydrogeology of interval tunneling from Hexi station to Hedong station (Line 3). Based on analytic hierarchy process to analysis the construction risk, we found that the complex geological conditions and the surrounding environment is the most important risk factors. Based on Delphi ideal point method, choose longitudinal wave velocity, depth, span, groundwater, surrounding environment, construction unit management level as influence factors to establish the collapse prediction model to analysis the collapse risk. Put forward a set of shallow excavation tunnel collapse risk prediction system and then predict the collapse risk of interval tunneling from Hexi station to Hedong station and due to a typical case to validate the prediction system.Thirdly, describe the applicability of PECK formula and stochastic medium theory. In order to investigate the applicability of PECK formula in the sand layer and shallow excavation of metro tunnel, this paper fitting the data by using the PECK formula Based on the monitoring data and a typical section. In view of the current lack of polar coordinates solution of stochastic medium theory, this paper put forward the Cartesian coordinate solution and its detailed steps.Fourthly, by using the FLAC3D on sand soil layer in shallow strata induced by tunnel construction deformation analysis, we know that the lateral surface settlement influence range is about 2～2.5 times of hole diameter, the influence of longitudinal surface subsidence range is about 2 times the diameter of the tunnel and the maximum surface subsidence appears in the midline of the tunnel. The vault settlement trend is likely to the surface settlement trend but vault settlement is relatively large, In view of the different core soil length condition, analysis its influence on the stability of the tunnel excavation formation.Fifthly, this paper analysis the transverse influenced range of tunnel excavation by compared the monitoring data and numerical simulation results. This paper pointed out that the crown settlement monitoring has inevitable drawbacks by compared with the surface settlement and vault settlement. This paper put forward theoretical prediction of vault settlement as the monitoring data of the vault settlement is only about 40% of the actual settlement caused by the serious lag of monitoring. Then predict the fitting of the vault settlement and make the settlement of vault displacement curves based on particle swarm algorithm.Finally, classify the shallow metro tunnel collapse. Divided according to the position of collapse, which including the formation collapse instability of tunnel faces and surface collapse, it also includes beyond the collapse of soil excavation face and surface subsidence in small range. Divided according to the collapse time, which including the tunnel face and surface subsidence occurred at the same time or lag in tunnel excavation. Analysis the collapse reason deeply by three aspects:soil physical and mechanical properties, construction measures and buried depth of tunnel. Then discuss the mechanical mechanism of tunnel collapse based on the improved wedge theory. Then simulate the tunnel face collapse process by PFC2D and analysis the vertical displacement and horizontal displacement of stratum according to the different cross and vertical section.