Numerical investigation of the mechanical properties of coal masses with T-junctions cleat networks under uniaxial compression

Evaluating the mechanical properties of coal masses is essential to the further understanding of coal bumps, pillar design, roadway support design and movement of overlying strata, etc. It is generally understood that the cleat network of a coal mass plays a major role in determining its mechanical behavior. Based on an extensive review of the characteristics of coal cleats, a novel method was proposed to generate a realistic model of T-junctions cleat network. Using the synthetic rock mass (SRM) approach, five sets of cubic cleat networks, with uniform sizes of 1 × 1 × 1 m3, were initially generated. Afterwards, smaller cubic cleat networks, with edge lengths of 5 cm, 10 cm, and 15 cm, were randomly cut from the large cubic cleat networks and then embedded into bonded-particle models (BPMs) to form SRM coal specimens of varying sizes. The SRM coal specimens were tested under unaxial compression conditions and the results of the tests were compared with the experimentally determined mechanical properties. The scale effect of uniaxial compression strength (UCS), relationships between failure modes and stress-strain behaviors, and the principal causes of fluctuations in the stress-strain curves were intensively investigated. The results demonstrated that the existence and intensity of scale effect mainly depends on sampling methods and cleat network geometry. SRM coal specimens whose failure modes were intact matrix failure and planar failure, and block failure and step-path failure exhibit typical strain softening behavior and fluctuating ductile behavior, respectively, in their stress-strain curves. Moreover, the fluctuations in the stress-strain curves are attributed to the changes in cleat aperture throughout the uniaxial compression tests.