Mechanical and degradation properties of rock under triaxial loading-unloading stresses: Comparative insights into coal and sandstone

Abstract

Underground mining catastrophes mainly stem from the gradual weakening of coal and its adjacent rock formations, induced by complex stress variations occurring during loading and unloading processes. Although several cyclic loading and unloading tests have been conducted on rocks to evaluate this concern, there is a dearth of understanding of the distinct damage mechanisms that occur in coal and rocks under complicated triaxial cyclic loading-unloading (TCLU) stresses. Hence, a series of TCLU experiments considering variations in the upper stress limit and cycle numbers are conducted on coal and sandstone in this paper. Synchronised ultrasonic velocity measurements are performed, and the energy dissipation and wave velocity evolution methods are combined to characterise the rock damage process. The findings indicate that the dissipation energy–inducing damage increases with the upper stress limit in the single cycle. Compared to coal, sandstone shows a higher ability to store elastic energy. The rate and magnitude of energy release responsible for the structural deterioration in the sandstone appear notably larger than that in coal. Moreover, the differentiation of the sensitivity of acoustic waves to stress variations under TCLU is examined for coal and sandstone. Meanwhile, upon reaching peak stress, the deterioration index of coal increases slowly, whereas that of sandstone shows a rapid increase. A quantitative relationship established among ultrasonic wave velocity, stress and energy using polynomials provides the possibility of on-site stability analysis using ultrasonic measuring techniques.