Study On Stability Of Excavation Face Of Shield Tunnel In Sandy Cobble Strata Of Chengdu

The sand pebble stratum in Chengdu is characteristic with strong dispersion,poor stability and special stress mode,and the formation deformation process caused by shield construction is a typical discontinuous medium problem.Sand and pebble stratum construction is more prone to collapse accidents.At present,there are few studies on the stability of the excavation surface of shield tunnel in sand pebble stratum,and the study on its instability mechanism is mostly based on the failure mechanism of sand and soil stratum,therefore the research of it needs to be more in-depth.This paper takes one certain interval of the shield tunneling engineering of Chengdu Metro Line 17 as the engineering background,by simulating the construction process and analyzing the failure mechanism of the excavation surface to provides a theoretical guidance for similar projects,which has certain practical significance.This thesis is mainly carries on the following research work :1 In order to simulate the physical characteristics of sand and pebble formations,PFC was used in this paper to conduct a three-axis numerical simulation experiment.Through the stress-strain curve,analyze the influence of microscopic parameters of the model on the macroscopic mechanical parameters of sand and pebble formations and summarize the corresponding relationship between the macro and micro parameters to achieve the parameter calibration;2 According to the mechanical principle and working mode of shield machine in the practical project,the simulation model of shield machine was established to ensure the rationality of the driving motion of cutter plate;Compile the tunneling action language of the shield machine in PFC program to make the rotation of the cutter head and screw excavator the same as the actual one,and analyze the impact of the tool form on the formation disturbance;the formation model and monitoring function are set to realize the visualization of the simulation process;3 By changing the excavation rate to continuously increase the supporting force to be larger than the maximum supporting force or reducing the supporting force to be less than the minimum supporting force to make the excavation surface become instability,and then analyze the instability process to find out the criteria for judging the instability of the excavation surface under the two conditions,and explore the reasonable range of supporting force;4 By analyzing the damaged shape of the excavation surface of tunnels with different depths to explore the failure shape of the excavation surface and the deformation range of the stratum,analyze the development process of the formation cavity of the damaged of the excavation surface,and research the development process of the instability of the excavation surface to the surface subsidence;5 To Simulate and count the surface settlement values of tunnels with different depths of excavation under stable conditions,explore the impact of shielding process on the ground settlements under different depths to verify the feasibility of the criteria for the destruction of the excavation surface proposed in this paper.