Experimental study on compressive strength characteristics of coal-rock combinations influenced by number of bedding surfaces

The layering effect of the coal seam roof poses major influence on the mechanical behavior of the coal-rock system. Herein, the coal seam roof was simplified into coal-rock combinations with different number of bedding planes in the rock unit, and on this basis, uniaxial compression tests were conducted. Then, the stress-strain characteristics, the evolution law of surface strain field and the acoustic emission characteristics during the test were collected and analyzed using the stress monitoring system, the digital image correlation (DIC) method, and the acoustic emission (AE) system. The test shows that: the stress-strain process of the coal-rock combinations can be divided into 4 stages: the fracture compaction stage, linearly increasing stage, unstable failure stage, and post-peak stage. Besides, progressive failures were found to first occur in the coal units, including coal ejection, coal-rock combinations peeling, ejection of peeled coals, and toppling failure. Based on the acoustic emission characteristics, the coal-rock combinations showed a more obvious phenomenon of compaction, small step, and post-peak stress fluctuation during the uniaxial compression process, due to the internal cooperative deformation failure caused by an increasing heterogeneity degree of the combinations. Meanwhile, obvious stress concentration occurs at the “coal-rock” interface in the combination. This is because the physical-mechanical properties of material therein vary greatly, and lateral restraint will be resulted from the adhesive in the bedding surface. Thus, the uniaxial compression of rock unit is changed to the triaxial tension-compression, while that of the coal unit is changed to the triaxial compression. Hence, the “coal-rock” interface is more prone to failure, where the acoustic emission signal is relatively concentrated, and it is more likely to form strain concentration. Therefore, it could be concluded from the study that the coal-rock combination has the equivalent elastic modulus decreased, the integrity weakened, the carrying capacity reduced, and the uniaxial compression strength in a decreasing trend with the increasing number of bedding surfaces in rock units. Generally, the results could provide some guidance for the prevention and control of dynamic disasters, such as rock burst in roadway.