The failing process of coal sample under loading conditions is investigated by using both laboratory experiment and 3D finite-discrete element method in the present paper. The cohesive zone model was used to characterize nucleation, growth and propagation of cracks, while the potential contact detection and interaction of fractured solids were examined by means of the penalty method in ABAQUS software, where the parallel computation was employed to accelerate the calculations. Uniaxial and Brazilian tests were performed in the laboratory to obtain the mechanical properties of the coal such as Young’s modulus, fracture energy, cohesive strength, friction angle, uniaxial compression and shear strength. Further, these properties were carefully calibrated prior to being taken as input arguments in the continuous-discontinuous modelling. All the simulating results were basically in agreement with that obtained from the tensile tests in laboratory. This study shows that such computational mechanics of discontinua can be employed to gain powerful insight into the failure mechanism of coal, which could also be a useful tool to clarify the collapse mechanism of coal block caving in mining engineering design and rock test scheme optimization.
As a special kind of rock, coal is generally at a complex stress state under mining conditions. Thus, understanding for the mechanical behavior of coal plays a very important role in designing rock structures such as coal mining, underground excavation. In the literature, numerical methods such as continuum and discontinuum are often used to describe the failure mechanism of rock (Bobet et al., 2009; Li et al., 2015; Lisjak and Grasselli, 2014). For example, plastic deformation and damage softening are perhaps the most studied problems in the continuum method while the internal length of geomaterial is usually not considered in its formulation, which is the most serious drawback because of its predictions significantly depended upon mesh size. To bypass the shortcomings mentioned above, an enriched or higher-order continuum formulation for the softening was developed (de Borst and Pamin, 1996; de Borst, 2002) and nonlocal continuum was also introduced (Bazant and Planas, 1998). However, interaction between fragments during the evolution of multi-cracks cannot still be taken into account in the enhanced continuum methods. On the other hand, discontinuous modeling techniques that are known as discrete element methods (DEM) treat the material directly as an assembly of separate blocks or particles, which was originally proposed by Cundell (1971) from the viewpoint of analogous molecular dynamics simulation to better account for and understand the interaction between the blocks. In discontinuous methods, the length scale can be automatically incorporated into the modelling, which naturally accommodates the real size of elements or particles to capture failure zone of the shear process.