The Numerical Study Of The Strain Gradient Model Of Zonal Disintegration Of Rock Mass Near Deep Level Tunnels

Zonal disintegration as one of scientific phenomena in deep rock mass engineering characteristic response, some problems of which are more and more clear after thirty to forty years research and development. Although some scholars have made great progress and breakthrough in this field, the conditions, mechanism and deformation of this phenomenon are still made no unified conclusion.In this paper, the rock stain softening constitutive relation is used to give a detailed analysis for the changes of stress field of the surrounding rock mass in the deep tunnel based on the framework of strain gradient theory. The content is divided into3aspects as follows:(1) Based on the strain gradient theory, the thick-walled cylinder under the hydrostatic pressure is analyzed in the elastic state from two aspects. On the one hand, simplified equations and the corresponding stress and strain solutions of thick-walled cylinder are obtained through simplifying assumptions of the higher-order stress. On the other hand, the shooting method and the quasi Newton iteration method are used to solve the fourth-order ordinary differential equation with two-point boundary conditions, and the accurate numerical solutions of stress field and displacement field of the rock mass in thick-walled cylinder are acquired. The analytical solutions of conventional elastic theory, analytical solutions of the simplified gradient model and the accurate numerical solutions of the original gradient model are compared to test the feasibility and accuracy of the simplified gradient model and acquire the differences of the original gradient theory and conventional elastic theory in the thick-walled cylinder.(2) Considering the ductility of surrounding rock mass under the high, the strain softening model simplified of the complete stress-strain curve and the Mohr-Coulomb failure criterion are used to derive the elasto-plastic constitutive equations combined with the strain gradient theory. And the strain gradient elaso-plastic model of the surrounding rock mass in deep circular tunnel is established and the corresponding equilibrium equations and boundary conditions are received.(3) The numerical method is implemented in Matlab software to solve the strain gradient elasto-plastic model for the stress field of the surrounding rock mass in deep circular tunnel. The results are proved that the softening and the strain gradients introduced into the constitutive equations are the necessary conditions for the stress field volatility of surrounding rock mass in deep tunnel. Meanwhile, the influence of model parameters in the constitutive relationship, such as Poisson’s ratio, softening modulus, internal length scale, the gradient-dependent elastic parameter, lateral deformation coefficient and relative loading level, on stress and deformation in surrounding the tunnel rock mass is investigated based on numerical solutions of stress field.