Experimental study on fracture evolution mechanism of complex underground stacked coal rock structure

Understanding the fracture evolution mechanism of underground complex stacked coal rock structures is of great significance for preventing and controlling coal wall spalling, scientifically using coal–rock fractures for coalbed methane extraction and pre-breaking coal–rock separation. In this study, the fracture evolution characteristics of coal–rock–coal (CRC) composite structure under uniaxial compression were monitored and studied by using three-dimensional digital image correlation (3D-DIC) and RA–AF evaluation system. The fracture mechanisms of the CRC composites were revealed based on experimental results and theoretical analyses. The results show that compressive strength and elastic modulus of the CRC composites increase with the increase of rock layers number, the decrease of rock layers spacing and the distribution of rock layers approaching the central axis. The difference of rock layers distribution of CRC composites, there are three fracture modes: extending from the center to both ends, the continuous fracture expansion of ‘coal–rock–coal,’ and the jumping fracture of coal seam. The fracture of the CRC composite structure is determined by the stress redistribution and energy release dominated by the rock layers distribution and interface effects of coal and rock.