Damage Effect and Deterioration Mechanism of Mechanical Properties of Fractured Coal–Rock Combined Body Under Water–Rock Interaction

The deterioration of mechanical properties and structural instability of coal–rock system under water–rock interaction seriously affect the efficient mining of coal and the safety of workers. The interlayer weak zone between coal seam and rock strata in coal–rock system is the main area of fracture distribution. The existence of fractures speeds the process of water–rock interaction, accounting for aggravating the damage of coal–rock system. Considering penetrating fractures and water–rock interaction, five kinds of coal–rock combined body with different fracture widths were prefabricated, and uniaxial compression tests under different soaking time were carried out. The research shows that: (1) The larger the fracture width and the longer the soaking time, the more obvious the compaction stage and the plastic stage, the less obvious the elastic stage, and the slower the damage. (2) With the increase of fracture width, the compressive strength, elastic modulus, pre-peak energy, post-peak energy and impact energy index decrease gradually, and there is a linear relationship between fracture width and each parameter. With the increase of soaking time, the compressive strength, elastic modulus, pre-peak energy, post-peak energy and impact energy index all decrease gradually, but there is a non-linear relationship between soaking time and each parameter. (3) The crack evolution law of coal–rock combined body under the influence of fracture width and soaking time is analyzed. It is found that the fracture width and soaking time have a certain influence on the wing crack, secondary inclined crack, secondary coplanar crack, inclined crack, secondary derivative crack, wing crack derivative crack, far field crack and flake phenomenon. (4) With the increase of fracture width and soaking time, the cohesion and internal friction angle decrease gradually. (5) Considering the influence of fracture width and soaking time, the D–P criterion is modified. Two parameters \(c{\,}_{(k,t)}\) $c_{(k,t)}^{}$ and \(\varphi {\,}_{(k,t)}\) $\phi _{(k,t)}^{}$ are introduced into the Drucker–Prager criterion, and the strength criterion considering fracture width and soaking time is constructed and verified. The error range of all samples is 2.48–9.30%, which indicates that the modified D–P strength criterion has high accuracy and good rationality. (6) The energy migration model of fractured coal–rock combined body is constructed, and the energy migration mechanism of intact and fractured coal–rock combined body is compared and analyzed. The two ends of the fracture are the main energy accumulation areas. When the fracture end of the coal component is destroyed, a part of the energy is transported from the coal component to the fracture end of the rock component, and released in the form of deformation or failure of the rock component. (7) The deterioration mechanism of mechanical properties of coal–rock combined body under water–rock interaction is analyzed. The physical, chemical and mechanical effects between water and coal–rock combined body lead to the damage inside the coal rock, and the continuous accumulation of damage leads to the macroscopic damage of the sample. The deterioration effect of mechanical properties under water–rock interaction is a process of continuous accumulation of damage.