Strength And Failure Mechanism In Macroscopic And Mesoscopic Of Brittle Rocks Under Tensile-Compressive Stress State Using Particle Flow Simulation

Many engineering practices have found that excavation of rock masses in deep underground high-stress areas is likely to cause tensile-compressive damage to brittle rock masses in surrounding rock disturbance areas.For situations where such tensile-compressive stresses dominate the stability of rock masses,using conventional compression and tensile strength criteria cannot be accurately applied,and research on rock mechanical properties under hybrid tensile-compressive stress conditions is needed.Because using the traditional physical test method unable fully probe into the internal failure process of brittle rock and the effect of grain distribution,so it can not effectively explain the failure mechanism.Therefore,in this paper,PFC2 D software is used to effectively simulate the tensile-compressive strength(tensile strength under lateral pressure)and fracture characteristics of brittle rocks through the improvemed parallel bond strength criterion and the grain-based model respectively.The main research contents and results are as follows:(1)In order to solve the problem that the conventional parallel bond model in PFC can not effectively simulate the high compressive strength to tensile strength ratio and a nonlinear strength envelope of brittle rocks,established a parallel bond strength criterion considering two tensile strength parameters and with convenient application.The numerical simulation results are in good agreement with the results of the tensile-compressive physical test in terms of strength and fracture characteristics of marble(As lateral pressure increases,inclination of the fracture surface gradually increases,the rupture area is more scattered,and the cracks appear as en echelon intermittent distribution trend,fracture mechanism change from tensile fracture to tensile-shear fracture).Verified the reliability of the improved parallel bond strength criterion.(2)Based on the parallel bond contact model(grains)and smooth joint contact model(grain boundaries),a marble grain-based model consistent with the actual grain size distribution was constructed.Through the verification and comparative analysis of numerical biaxial tension-compression test and the physical tests results,the marble grain-based model can simulate the tensile-compressive strength and fracture characteristics of marble effectively.On this basis,the micro-crack generation mechanism and mechanical property identification under the stress state of biaxial tension-compression,uniaxial tension and uniaxial compression were further analyzed from the meso-displacement field and the damage evolution process of marble is roughly divided into four stages by the development process of microcracks number and stress-strain curves: elastic deformation,stable fracture development,unstable fracture development and overall fracture.(3)Based on the grain-based model,the effects of grain size,homogeneity,and mineral composition on the tensile-compressive strength,macro-and meso-fracture mechanisms of rocks were further studied.The results show: as the grain size increases,the tensile strength under lateral pressure enhanced,the micro-crack distribution more dispersed,and it is difficult to form macroscopic shear fracture;When the model is more homogeneous,the fracture mode of the model is tensile fracture,as the model heterogeneity coefficient increases,the fracture mode gradually changes to shear fracture.The tensile strength under lateral pressure and uniaxial tensile / compressive strength decrease as the heterogeneity coefficient increases.The effect becomes more pronounced when the lateral pressure is larger;When the lateral pressure is large,increasing the feldspar content in the rock will reduce the brittleness of the rock,thereby the tensile strength increased.When the lateral pressure is low,the effect of mineral composition on the tensile strength of the rock is disappeared...