Abstract
The cyclic loading causes the strength change of porous rocks, which can be frequently encountered in underground coal mining. In order to quantify the cumulative damage of porous coal, multilevel uniaxial cyclic compressive tests were carried out considering different loading frequencies. The results show that under cyclic loading, Young’s modulus of coal shows a first drastic increase and then decrease trend in terms of the number of loading-unloading cycles, while the Poisson’s ratio gradually increased at lower peak stress amplitudes and then increased sharply with peak cyclic stress amplitude until losing load bearing capacity, following a stepwise rising trend. At a higher loading frequency, volumetric compressibility-dilatancy transition is shifted to an earlier time. The loading/unloading response ratio (LURR) was used to evaluate the damage of coal under cyclic loading. It is found that LURR is an effective parameter to evaluate the damage of coaly rock under cyclic loading and is also useful in distinguishing different deformation mechanism at different loading stages. The results also show that the damage of coal can initiate at the early stage of cyclic loading at a low loading frequency; however, the increase in frequency of cyclic loading can delay the damage time, requiring a relatively higher level of peak cyclic stress amplitude.
Highlights
Ground control has been deemed as one of most important issues in many rock engineering applications, and its prerequisite is to evaluate and characterize the geotechnical property evolution of porous rocks, such as coal and shale [1, 2]
7 coal specimens were used for the compressive tests, in which coals samples, C1, C2, and C3, were selected to measure the uniaxial compressive strength (UCS), while coal samples, C7, C8, C9, and C10, were used for cyclic loading tests
A relatively larger amount of irreversible deformation occurred in the first cyclic loading
Summary
Ground control has been deemed as one of most important issues in many rock engineering applications, and its prerequisite is to evaluate and characterize the geotechnical property evolution of porous rocks, such as coal and shale [1, 2]. Different from monotonic load, cyclic loading is time dependent in a repeated manner and usually expressed by superposition of a mean stress σmean and a periodically time-dependent cyclic amplitude Δσ in mathematics [3]. The cyclic loading at different loading rates and frequencies can degrade the strength properties of rocks [4,5,6,7]. It is unfavorable to the stability of rock structures in underground coal mining. The cyclic loading can be induced by the back-and-forth coal cutting, periodic overlying roof strata weighting, drilling, and blasting. Quantification of coal damage under cyclic loading, especially in the early stages of microcrack initiation and propagation, is important for a proper understanding of its cyclic behavior [12]
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