Study On Meso-parameter Calibration Of Sand Pebble Soil And Stability Of Tunnel Excavation Surface

This thesis focuses on the stability of tunnel face in sandy cobble stratum.Both theoretical analysis and numerical simulation are adopted to study the relationship between the micro and macro parameters of sandy cobble soil.Then,the PFC^3Dnumerical models of tunnel are established to analyze the failure zone of tunnel face instability under sandy cobble stratum.Based on the numerical model,the new failure mechanism is proposed to analyze the tunnel face stability.The main research contents are as follows:（1）A new approach has been devised for calibrating microparameters of sandy pebble soil in triaxial shear simulation to solve the problem of time-consuming time during calibration.Firstly,a numerical experiment scheme is designed based on orthogonal principle.Then,variance analysis is employed to analyze the sensitivities of the macroscopic response（i.e.Young’s modulus,peak strength,friction angle,cohesion）of numerical model with respect to each microparameters（i.e.effective modulus,normal-to-shear stiffness ratio,friction coefficient,tensile strength and shear strength）of linear contact bond.The microparameters with significant influence on the macroscopic response are singled out and their linear relations to macroscopic responses are estimated by multiple linear regression.Finally,using a multi-objective optimization method,the optimum set is obtained that gives the best agreement either in quantitative or in qualitative ways between the results both from the numerical simulations and laboratory,which verifies the feasibility and applicability of the approach.（2）In order to reflect the influence of the discreteness of sand cobble stratum and its sensitivity to deformation on the stability of tunnel face,the PFC^3D numerical models of tunnel are established.Firstly,the rationality of the numerical model is verified by comparing with model test,numerical models and theoretical model.Then,based on the PFC^3D models,the influence of cover depth ratio and friction angle on the limit support pressure is analyzed.Finally,the failure zone of tunnel face in sandy cobble stratum is studied,and the height of collapse arch in sandy cobble stratum is modified according to the numerical simulation results.（3）Based on the failure zone of numerical simulation in Chapter 3,the new failure mechanism of tunnel face suitable for sandy cobble soil layer is proposed,and the analytical solution of limit support pressure of tunnel face is derived by limit analysis method.Then,by comparing with numerical simulation,typical analytical model and model test results,the influence of various factors on the ultimate support force is analyzed,and the rationality and applicability of the model in this thesis is verified.Finally,the model is applied to the actual project,and good results are achieved.