Mechanical properties and damage characteristics of coal-rock combined samples under water-rock interaction

Water-rock interaction is common seen in underground mining engineering activities. In order to study the mechanical properties, damage characteristics and deterioration mechanism of coal-rock combined samples under water-rock interaction, axial compression tests were carried out on FM, GM and FMG combined samples under different immersion time. The main results are as follows: ①Water content of the sample increases with the increase of immersion time, and reaches saturated after 20, 5−15 is the water absorption period of the sample, 15−20 is the near-saturation period, and 20−25 is the saturation period.②The compressive strength, elastic modulus, pre-peak energy and burst energy index of the three types of samples have obvious degradation effect under the interaction of water and rock, and the parameter values decrease with the increase of immersion time. The steep drop region of pre-peak energy is observed in the 5−15 d of immersion, and the slow drop region of pre-peak energy is observed in the 15−25 d of immersion. Immersion time 5−15 d, the burst energy index deterioration is obvious, after 15 d, the deterioration effect is weak; Sample of immersion of 5−10 d is of weak burst proneness, Sample of immersion of 15−25 d is of no burst proneness.③With the increase of immersion time, the cumulative number of acoustic emission of the sample decreases significantly. Based on the damage theory, the damage model of the sample with respect to immersion time was constructed. The model reveals the relationship between the damage variable and the immersion time, and reflects the behavior that damage degree of the sample increases with the increase of the immersion time. ④The calculation formula of fake-void ratio is derived, and the fake-void ratio of the combined body is as follows: FM combined body < FMG combined body < GM combined body. The relationship between the fake-void ratio of composite and compressive strength was analyzed. the larger the fake-void ratio, the smaller the compressive strength. ⑤With respect to the deterioration effect, the deterioration mechanism of coal-rock combination under water-rock interaction was analyzed from physical, chemical and mechanical aspects.⑥The failure characteristics and deformation of coal-rock pillar under the influence of water in a coal mining area of Hegang mining area were analyzed. The strength and stability of coal-rock mass under the water-rock interaction have obvious deterioration effect, and this deterioration effect is more obvious at the early immersion stage. With the increase of immersion time, the deterioration effect is gradually weakened. This is consistent with the conclusions of the tests.