Mechanical properties of rock-coal bi-material samples with different lithologies under uniaxial loading

In this paper, we performed uniaxial compression tests on fine sandstone-coal, coarse sandstone-coal, and oil shale-coal bi-material samples to investigate the lithology effects on their strength, macro-failure initiation (MFI), energy evolution, and failure characteristics, respectively. Uniaxial stress-strain characteristics of the samples primarily depended on the coal and were affected by coal-rock interactions. These interactions involved the following aspects: (1) The differences in the mechanical properties of rock and coal affected their stress states near the interface. They were in triaxial compression state with strength increasing or in triaxial compression-tension state with strength decreasing. (2) During the pre-peak stage, the rock deformation affected the values of input energy, elastic energy, and dissipated energy. Damage and energy accumulation in coal were also limited by rock deformation, resulting in higher strengths for the bi-material samples compared to pure coal samples. (3) During the post-peak stage, the rebound deformation of rock was accompanied by the elastic energy release, which promoted the failure of coal. MFI of the samples occurred within the coal. MFI models mainly included the tensile fracture of coal near the interface, the initiation and propagation of macro-cracks, and the development and evolution of micro-defects, accompanied by varying degrees of surface spalling or ejection failure. The coal showed the splitting ejection failure. The oil shale and coarse sandstone exhibited tensile failure or tensile-shear composite failure due to crack propagation in coal. Failure was not evident in the fine sandstone. The coal in the oil shale coal bi-material samples became more broken due to the most serious rebound deformation of oil shale. (C) 2020 The Author(s). Published by Elsevier B.V.

Automated summary

The study found that lithology plays a significant role in the strength and failure characteristics of bi-material samples, while coal-rock interactions also have an impact on the behavior of the samples.