Brittleness Evaluation of Saturated Coal Based on Energy Method from Stress–Strain Curves of Uniaxial Compression

Rock brittleness is an important index evaluating geotechnical stability and failure mode, and it is also influenced by pore fluid pressure, especially in deep underground mining. To evaluate the brittleness from stress–strain curves of uniaxial compression, uniaxial compressive tests were carried out on water-pressurized coal samples, and a new energetic brittleness index was proposed. The saturated coal samples were classified into brittle, transitional, and ductile groups according to the stress–strain behavior of uniaxial compression. It is found that the pore water enhanced brittle coal strength, but reduced strength of transitional and ductile coal. Compared to ductile and transitional coal samples, the brittle coal is of a lower elastic modulus, but showing a higher capability of elastic energy storage prior to peak stress. For brittle coal, significant acoustic emission (AE) events occurred at the peak stress, but for ductile and transitional coal substantial acoustic emission occurred in the post-peak-stress stage suggesting significant elastic energy storage and insignificant energy dissipation feature prior to peak stress. Assuming that the total energy consists of the accumulated elastic energy prior to the crack coalescence point, and the dissipated energy after the crack coalescence point, a new brittleness index was proposed based on strain energy integration from stress–strain curves of uniaxial compression. The new brittleness index shows great superiority in avoiding the subjective mechanical parameter determination and can be conveniently calculated from uniaxial compression tests.