Progressive Damage Characteristic and Microscopic Weakening Mechanism of Coal Under Long-Term Soaking

Water–rock interaction is an essential factor in the weakening of coal rock masses. Long-term soaking (LTS) as a type of water–rock interaction is widespread in mining engineering and shaft engineering. In this paper, progressive damage characterization of LST coal samples is obtained by a uniaxial compression experiment. Combining physical properties tests in coal samples and hydrochemistry tests in soaking solution quantitatively characterize changes in mineral compositions and microstructures of samples. The weakening mechanism of LTS coal is discussed at a micro-scale. The results show that mechanical parameters and acoustic emission (AE) signals of LTS coal samples continuously decline during loading. The crack initiation stress σ CI is regarded as the threshold value for long-term stability of coal rock masses, and the crack initiation stress ratio σ CI / σ P in a sample soaked for 360 days decreases by 10.8% compared with a saturated sample. With increasing of immersion time, the total number of cracks decreases while the percentage of shear cracks increases. The deformation failure of a sample transforms from tensile failure predominantly to tensile–shear composite failure. The variation of physical properties in coal samples as well as Ca 2+ , Na + and K + concentrations in soaking solution indicates that the dissolution of albite and calcite as well as the swelling of illite are observed throughout immersion process, which leads to an increased pore-crack number and enhanced connectivity in coal samples. As physical properties continue to evolve, the cementation of pre-existing weaknesses between coal matrix is more easily weakened, promoting the slip between matrixes. Under the effect of axial load, the failure surface of a LTS sample will develop along internal pre-existing weaknesses which is most favorable to induce failure. The study results can provide help for reliability assessment and maintenance of underground bearing structure under LTS.