Abstract
The loading modes and roof lithology have a significant influence on the mechanical properties of coal seams. To reveal the failure modes and energy evolution law of underground coal during the mining process, conventional uniaxial and uniaxial cyclic loading tests were carried out on three types of samples: coal, rock, and coal-rock combinations. The results show that the samples mainly behave with three failure modes (shear slip, tensile splitting, and fracture), and all the coal sections in the coal-rock combinations fail, whereas most rock sections remain intact. The compressive strength of the coal-rock combination is higher than coal and much smaller than rock. Compared with the conventional uniaxial loading condition, both the maximum deformation before failure and Young’s modulus under the cyclic loading condition are greater, and the latter increases quadratically with the cycle index. The energy densities are also calculated, and their variations are analysed in detail. The results show that with increasing cycle index, both the elastic energy stored in the sample and the dissipated energy increase in a quadratic function, and the failure process becomes more intense. This research reveals the failure modes, deformation characteristics, and energy evolution of the coal-rock combination under different loading conditions, which can provide strong support for controlling underground surrounding rocks of the coal face and roadway in coalmines.
Highlights
In the production process of an underground coal mine, production activities such as roadway excavation and coal face mining will cause strata movement and stress redistribution around the stope. us, a mining stress field is generated, and the stress changes continuously with mining activity [1,2,3,4]
Its superposition with the original rock stress acts on surrounding coal seams, the roof, and floor strata, which is a typical cyclic loading condition [5,6,7]
Yang et al carried out uniaxial cyclic loading tests on coal and rock. ey found under cyclic loading, the destruction of samples is divided into three stages: the beginning stage, the stable deformation stage, and the accelerating deformation stage [11,12,13]
Summary
In the production process of an underground coal mine, production activities such as roadway excavation and coal face mining will cause strata movement and stress redistribution around the stope. us, a mining stress field is generated, and the stress changes continuously with mining activity [1,2,3,4]. In the production process of an underground coal mine, production activities such as roadway excavation and coal face mining will cause strata movement and stress redistribution around the stope. Its superposition with the original rock stress acts on surrounding coal seams, the roof, and floor strata, which is a typical cyclic loading condition [5,6,7]. Scholars and engineers have carried out a significant amount of research on the law of deformation, failure, and energy evolution of coal and rock under cyclic loading. Found that the fragments of coal and rock under cyclic loading are mostly irregular, wedge-shaped, and blockshaped, with obvious shape characteristics [8]. Yang et al carried out uniaxial cyclic loading tests on coal and rock. Yang et al examined the mechanical damage characteristics of sandstone subjected to cyclic loading under triaxial compression condition. Yang et al examined the mechanical damage characteristics of sandstone subjected to cyclic loading under triaxial compression condition. e result showed that the higher the unloading stress level, the more the work done on the coal and rock mass during
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