Numerical study of the mechanical behaviour of unwelded block in matrix rocks under direct shearing

Block in matrix rocks (bimrocks) are known as a challenging material for engineering geologists due to their complex composition and associated difficulties in the determination of their mechanical properties. A novel numerical approach has been developed for the simulation of unwelded bimrocks based on the discrete element method. Three different types of contacts, including matrix–matrix, matrix-block and block-block, were assigned to the particles to mimic the mechanical behaviour of unwelded bimrocks. To investigate the validity of numerical models, the uniaxial compression and direct shear tests under different normal stresses were simulated and their results were compared with the physical experiments (glass bead blocks cemented by matrix plaster). The numerical uniaxial compressive strength (UCS) and direct shear tests at low volumetric block proportion (VBP) were found to be in good agreement with the experimental data, while they diverged with an increase in both VBP and the applied normal stress. It was found that an increase in VBP can lead to a decrease in both UCS and cohesive strength while the frictional strength increases. Also, it was concluded that an increase in the failed block-matrix contacts can lead to a decrease in the cohesion and UCS. At the same time, the occlusion effect of blocks can increase the internal friction angle. Detailed analysis on cracking mechanism showed that the increase in VBP results in a decrease in both crack initiation and damage stresses. Sensitivity analysis on micro-parameters showed that the tensile strength of block-matrix contacts is the most effective parameter on the strength.