Strain softening model considering elastic-plastic coupling effect

There are some nonlinear characteristics presented for the post-peak deep rock mass,such as strain softening,elastoplastic coupling and plastic deformation failure.A better understanding of rock mass mechanical deformation characteristics around the excavation helps us to predict the displacements or the shape of failed zone,and subsequently assist the design of proper support systems.However,the use of one-dimensional model,such as Mohr-Coulomb Model or Strain-Softening Model could not simulate the rock deformation under high confining pressure in deep underground engineering.Therefore,through triaxial cyclic loading-unloading test,the mechanical parameters of rock at the different stages of plastic accumulation under different confining pressures are obtained,the rock characteristics deformation under the combined influence of plastic deformation and confining pressure are studied.By constructing the two-dimensional function of stress-plastic shear strain and confining stress to describe the above characteristics,a mathematical model of cohesion,friction angle,elastic modulus and dilatancy angle with plastic deformation considering the influence of confining pressure is established.The results show that the cohesive and friction angle can be described by the exponential function and the line function respectively with the plastic deformation and confining pressure.The two-dimensional function constructed by this method can achieve a fitting accuracy of ninety-five percent.The elastic modulus increases with confining pressure which can be described by negative exponential function,whereas decreases with plastic deformation which can be described by linear function.The dilation angle linearly decreases with confining pressure and decreases at first then increases with plastic deformation,which can be described by the difference between two exponential functions.Furthermore,the model is introduced to the simulation of the rock triaxial compression test by FLAC3D,the results confirm the accuracy and rationality of the proposed model.Compared with the strain softening model (S-S Model) and the Mole-Coulomb model (M-C Model),the proposed model has a remarkable impact on the stress distribution of surrounding rocks as the plastic zone expands in the deep circular cavern excavation,which is mainly reflected in that the tangential stress peak is further transferred to the deep surrounding rocks,and the radial stress decreases accordingly.For the displacement field distribution,with the expansion of plastic zone,the displacement obtained by the proposed model gradually increases,which is consistent with the characteristic that the displacement of rock mass increases with the increase of damage range after fracture.