| Summary: | The high stress environment brings many challenges in underground coal mining. In order to address the strength behavior of coal under various confining stresses and hence shed light on coal pillar design optimization, compressive tests were conducted under the lateral confinement of 0⁻8.0 MPa, and the strength enhancement mechanism was studied from the grain scale using PFC modeling. The results show that the coal strength and cumulative axial strain at failure increased with the confinement, while the Young’s modulus of coal is independent of confinement. However, this confinement-dependent strength property can be significantly weakened by existing cracks. Compared to the significant increase in peak compressive stress, the crack initiation stress slightly increased with the confinement. The strength component mobilized with the confinement enhancement. In the early stage of loading, the high confinement restrained the development of microcracks, while in the later stage, it enhanced the frictional resistance strength component. The two mechanism shifted the compressive strength of coal together and the latter one contributed to the strength component mobilization. The coal showed three failure modes sequentially with the increase of confinement, namely axial splitting, mixed failure and shear failure mode. With regard to failure envelope, the Mohr-Coulomb, Hoek-Brown and S-shaped failure criteria can generally represent the confinement-dependent coal strength with <i>R</i>-square larger than 0.9. The confinement of rapid strength promotion section of S-shape failure envelope falls in a range of 1.5⁻3.0 MPa. This leads to the difficulty of S-shaped failure envelope justification due to the soft nature and heterogeneity of coal.
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