Experimental investigation on mechanical properties of coal samples under different freeze–thaw cycles

Due to the extensive excavation of open-pit coal mines in northwest of China, the rock slopes formed by special environments are subjected to freeze-thaw (F-T) action, which has a certain impact on their stabilities. In order to evaluate the mechanical properties and micro damage characteristics of coal under different freeze-thaw cycles, uniaxial compression experiments combining acoustic emission tests were conducted. The results suggest that as the number of freeze-thaw cycles increased, elastic modulus of coal samples decreased, the samples showed ductile damage characteristics and initial compaction stage gradually increased. Compared with unfrozen-thawed coal sample, the compressive strength of the coal samples decreased by 23.27% after 10 F-T cycles, 31.06% after 15 F-T cycles, and 36.01% after 20 F-T cycles. The internal fissures in the coal samples transitioned from tensile fissures to shearing fissures, and the samples gradually showed tensile-shear combined failure. The final cumulative energy of the coal sample became lower, the cumulative energy duration increased and the time point of the energy surge was delayed with the increase of cyclic freeze-thaw times. A damage model based on the evolution law of the cumulative energy was established to bridge the gap between macro-micro damage mechanics.