Water-bearing effect on mechanical properties and interface failure mode of coal–rock combination samples

The coal and rock mass in water-rich mines with underground waterproof coal-and-rock pillars will be inevitably eroded by groundwater. The mechanical properties and interface failure mode of coal–rock mass under high-moisture conditions directly affect the mine stability and operation safety. This study performed uniaxial compression tests of samples from coal, rock, and their combination with different water contents, the evolution law of mechanical properties and acoustic emission (AE) damage characteristics of coal–rock combinations (C-RC) with different water contents were studied. The C-RC interface failure modes at different water contents were clarified. The results showed that the deterioration of average peak strength of coal, rock, and C-RC was obvious with increased water content. The deterioration degree in the descending order was as follows: C-RC > coal mass > rock mass. The maximum AE ringing number first increased rapidly and then sharply dropped, while the AE cumulative ringing number dropped slowly and then rapidly, reflecting the internal crack propagation patterns in C-RC. The failure mechanism of water-bearing C-RC was related to coal–rock strength ratio and the water-bearing effect of coal–rock interface. When the water-bearing state of C-RC changed from dry to saturated, the macro-failure mode showed the law of only coal fracture (0%)—minor fracture at the coal–rock interface (4%)—both coal and rock fracture (8%)—only coal fracture (12%). With increased water content, the water-bearing effect of C-RC interface gradually prevailed in the C-RC failure pattern.