Research And Application On Macro-Meso Mechanical Properties Of Sandy Cobble Surrounding Rock In Chengdu

With the development of a large number of urban tunnels,the security problems caused by crossing the cobble stratum have attracted more and more attention.The cobble stratum is composed of gravel and loose coarse particles with different particle size of sand and soil.In order to further research the engineering properties of sandy pebble soil,the micro-parameters are one of the main research contents of the characteristics.In this paper,the engineering characteristics of sandy pebble soil are studied through the comprehensive research method of particle sizing test,indoor large-scale three axis test and PFC three axis numerical simulation test.And the influence of the main parameters on the characteristics of sandy pebble soil is analyzed quantitatively.Some systematic laws are obtained,which lays the foundation for the study of stability analysis of surrounding rock of tunnel.The main results are as follows:(1)The large-scale triaxial test with different water content and densities are studied and the law of stress-strain relation and the law of shear strength are analyzed;A constitutive model based on the contact point of granular flow(without considering particle breakage.By using embedded fish language program.PFC3 D numerical simulation model was obtained and the numerical simulation of large-scale triaxial test was completed)(2)The stress-strain curves obtained from numerical simulation and laboratory are quite consistent.And the relative error of shear strength parameters from numerical simulation and laboratory is very small.This numerical simulation has a high degree of agreement;(3)Numerical simulation test results can be very good fit the results of laboratory tests.And study on the influence factors of the deformation characteristics and shear strength of sandy pebble soil from the microscopic scal.The effect of particle contact modulus,friction coefficient and stiffness ratio on the macroscopic mechanical properties of sandy pebble soil was analyzed in detail.Analysis results show that: With increasing particle contact modulus,sandy pebble soil macro initial deformation modulus showed a increasing trend,peak also has higher strength,axial strain at failure decrease,shear contraction deformation is getting smaller and smaller,the corresponding axial strain is smaller when the numerical model is expanded by the shear contraction steering;With increasing particle friction coefficient,the peak strength of sandy pebble soil also increased,peak strength and friction coefficient of the roughly proportional relationship,sandy pebble soil initial deformation modulus with the friction coefficient increases with the increase of the trend of increased slightly,but particle friction coefficient of deformation has little effect;The larger the stiffness ratio,the smaller the peak stress,the smaller the initial modulus,the smaller the volume strain to the minimum hourly strain,but the effect is not obvious on the characteristics of the sand and gravel soil.To a certain extent,these conclusions can be applied to the quantitative analysis of the influence factors on the engineering characteristics of sandy pebble soil.And can provide some references for the study of the characteristics of sandy pebble soil.(4)After excavation of the sandy pebble layer,soil before tunnel face is destroy firstly,deformation of soil which closer to tunnel is greater,and it is caused surface subsidence.The stress relaxiation area is spread to upper right,enventually it leads to the excavation face unstability.