Research On Deformation Law Of Double-Line Shield Tunnel Undercrossing Existing Railway Subgrade Based On Response Surface Method

Railway network and subway lines are crisscrossed in the city.Tunnel running under existing railways has become a typical project.In such projects,in order to ensure high smoothness and stability of the line,the settlement of the railway is strictly controlled according to the control standard of subgrade deformation.Based on the background of Beijing Metro Line 16 section 18 double-line shield tunnel crossing the Beijing-Kowloon Railway,the influence laws of different factors on subgrade settlement are explored by theoretical analysis and numerical simulation.At the same time,the response surface method is used to fit the function of subgrade normal service limit state under different conditions,and the reliability index corresponding to each condition is solved by first-order and second-order moment method,and the sensitivity of each influencing factor is analyzed by grey correlation degree.Finally,the risk assessment model of double-line shield tunnel crossing the Beijing-Kowloon Railway is established by combining the AHP and fuzzy comprehensive evaluation method.According to the engineering design parameters,the risk assessment of double-line shield tunnel crossing the Beijing-Kowloon Railway is carried out and the corresponding layer control measures are put forward.The main conclusions are as follows:(1)Taking shield tunneling underneath railways in soft-hard interacting strata as research background and through literature research,it is concluded that formation deformation caused by double-line shield tunnel construction mainly depends on the nature of soft rock and the proportion of soft rock in rock strata.As far as shield tunneling itself is concerned,the main factors influencing formation deformation are shield tunneling parameters,tunnel buried depth,tunnel spacing and excavation surface spacing.By using Midas GTS NX,a three-dimensional numerical model for shield tunneling through existing railways is established.The deformation laws of surface and subgrade caused by Shield Tunneling Excavation under different conditions are analyzed.It is proposed that reasonable burial depth and spacing should be selected in the design stage and driving parameters should be controlled in the construction stage in order to control the deformation of surface and subgrade.(2)Considering the internal friction angle of silt layer φ Based on the randomness of elastic modulus E,the numerical models of different tunnel spacing,grouting pressure and soil chamber pressure are established respectively.The response surface method is used to fit the limit state function of Subgrade under different conditions,and the reliability index and failure probability under different conditions are solved by first-order and second-order moment method.Statistical regularity discovery has the same regularity trend as deterministic analysis of influencing factors.Sensitivity analysis of influencing factors is carried out by using grey correlation degree,and the order is tunnel spacing S>internal friction angle φ> Soil chamber pressure F > grouting pressure P > elastic modulus E.(3)Risk identification of double-line shield tunneling underneath railways is carried out by investigating a large number of documents.Risk conditions of subgrade deformation over limit are divided into three aspects: geological conditions,tunnel design and excavation design parameters,and the lower level is subdivided into 11 indexes.Analytic hierarchy process(AHP)is used to determine the impact weight of each factor on the over-limit deformation of subgrade.A risk assessment model is established by combining the fuzzy comprehensive evaluation method.Risk assessment is carried out on section 18 of Beijing Subway Line 16 to determine that the project risk level is medium risk.(4)Based on the risk assessment results of this paper,the foundation settlement control measures are proposed from three aspects: excavation parameters,synchronous grouting,secondary grouting and subgrade reinforcement.Through the comparison of field monitoring data and numerical simulation results,it is concluded that the subgrade settlement control measures are effective.