A Micromechanical Model for Numerical Study of Rock Dilation and Ductility

The newly implemented micromechanical model in the CA2 computer program was studied in this work. The purpose was to address some of the issues in the numerical studies involving the Bonded Particle Model (BPM) including unrealistically low qu/σ t ratios, overall dilation behavior, and the post-failure response of rocks. The plasticity model allows both tensile and shear softening of the filling material at the contact points of the particles. It is shown that for a more ductile material, there is less scatter of micro-cracking at the peak load. Furthermore, the ductility parameter appears to be a good tool in controlling the ratio of compressive to uniaxial tensile strength of rock. While the ductility of the filling at the contact points of the particles has a drastic effect on the macroscopic post-peak rock behavior in the direct tensile testing, its role in dictating the post-peak rock behavior in compression is negligible and needs further study. The combined effect of ductility and initial micro-cracking on rock strength characteristics was studied as well. The numerical results suggest that the ratio of Brazilian to direct tensile strength of the simulated material is affected by the initial micro-crack intensity; this ratio is around 1 for a material with no initial micro-cracks but it gradually increases as the initial micro-crack intensity is increased. In terms of the overall dilation behavior, it is shown that the macro-dilation angle can be controlled by means of the micro-dilation angle in a positive correlation provided that the average grain size is sufficiently small or when a joint is involved. As the grain size increases, the resulted macro-asperities suppress the functionality of the micro-dilation angle and consequently, the macro-dilation angle cannot be controlled. Further, it is shown that the genesis pressure can help to govern the overall dilation behavior. This parameter is also able to control the post-peak behavior of a bonded particle system. It is shown that high values of the genesis pressure yield to more brittle BPM system with greater dilation angles and steeper post-peak curves