Research On The Disturbance Of EPB Shield Tunneling In Sand And Cobble Strata And The Calculation Method Of Excavation Surface Support Force

Earth pressure balance(EPB)shield is widely used in urban subway tunnel construction because of its advantages of less influence by the external environment and low operating cost.However,it is difficult to control the disturbance of stratum and the stability of excavation face during construction in sandy pebble stratum,and even cause surface collapse in severe cases.Based on the Phase 1 of Luoyang Rail Transit Line 2,this paper studies the analysis method of stratum disturbance and excavation face stability caused by EPB shield tunneling in sandy and pebble stratum by theoretical analysis,field test,indoor test and numerical simulation.The main contents and research results are as follows:(1)Based on the actual engineering and the characteristics of shield tunneling construction,the measurement method of sand and pebble stratum disturbance in EPB shield tunneling site is put forward,and the shield tunneling parameters,surface settlement of typical sections,stratum layered settlement,horizontal earth pressure and pore water pressure of stratum in the monitoring interval of shield tunnel are monitored;Through the statistical analysis of monitoring data,the relationship between shield tunneling parameters and stratum disturbance is analyzed,and the disturbance law of shield tunneling to sandy pebble stratum is preliminarily defined.(2)The finite element numerical simulation method is used to simulate the shield tunneling process in the monitoring section.According to the numerical simulation results,the general law of transverse and longitudinal surface settlement and stratum layered settlement caused by shield tunneling is emphatically analyzed;By comparing the field measured data with the numerical simulation results,the rationality of the numerical simulation results is verified.(3)On the basis of the non-limit state earth pressure theory and plastic limit equilibrium analysis method,considering the loading and unloading stress path of a point in front of the shield excavation face,the theoretical relationship between the displacement change rate of the excavation face and the excavation rate and the supporting pressure is established.The calculation method of excavation face support force is put forward,which can consider shield tunneling speed,screw machine speed and cutter head speed.(4)Two groups of large-scale indoor triaxial shear tests under different stress paths are carried out on sandy pebble samples taken from the project site,and eight parameters in the derived formula for calculating the supporting force of shield excavation face are determined.Based on this,the influence of the parameters of sand and pebble stratum,shield tunneling speed,screw machine speed and cutter head speed on the support force of excavation face is analyzed(5)Considering 32 kinds of shield tunneling conditions,which are composed of different buried depth ratio,different internal friction angle and active and passive displacement of excavation face,the effect of displacement change rate of excavation face on the evolution of plastic zone of soil in front of excavation face,the stress distribution in soil layer around excavation face and the settlement law of surface in front of excavation face are analyzed by finite element numerical simulation method.Compared with the theoretical calculation results,the rationality of the theoretical formula is verified.The research results of this paper are not only helpful to analyze the disturbance of EPB shield tunneling in the sandy pebble stratum,but also of great significance to the dynamic stability control of excavation face during shield tunneling.