Background In practical engineering, the waterproof coal pillar usually suffers from different degrees of dynamic load from the overlying strata, and its internal water distribution is non-uniform. Although the traditional “water content” index can reflect the overall water content, it is difficult to describe the non-uniform characteristics of the soaking space. Therefore, it is of great significance to clarify the dynamic failure mechanism of coal and rock mass under different immersion conditions for stability zoning control.

Methods On this basis, the coal–rock combined bodies under three different forms of immersion, namely: complete immersion, unilateral immersion, and non-immersion were prepared. These specimens were then subjected to split hopkinson pressure bar testing combined with high-speed photography, digital image correlation (DIC), and CT scanning, supplemented by FLAC–PFC3D coupled numerical simulations. Thereafter, the evolutionary characteristics of stress–strain, crack propagation, energy distribution, and force chain structure of these specimens under impact pressure of 0.3 MPa, 0.5 MPa, and 0.7 MPa were compared from macroscopic and mesoscopic angles.

Results and Conclusions (1) Due to inertial confinement and strain-rate strengthening, the peak stress and elastic modulus of the specimens increase with impact pressure, but are reduced by water-induced softening. (2) The softening of water immersion makes the crack tip more likely to turn, forming a tortuous composite tensile-shear path, while the dry coal keeps brittle tensile fracture. With the increase of impact pressure, the main crack will gradually approach the interface, the damage degree will evolve from micro to macro, and the crack morphology will develop from single to composite. (3) Under the same immersion condition, the change of medium and high impact pressure has little effect on the proportion of reflection energy, dissipation energy and transmission energy. On the contrary, under the same impact pressure, different immersion states significantly affect the energy proportion of each part. (4) The immersion effect promotes the degeneration of the force chain, resulting in the transformation of the impact pressure-driven system from uniform energy consumption to local load carrying, and the failure mode of coal–rock combined bodies will change from tensile failure to shear failure with the increase of the impact pressure and the decrease of the immersion volume. The research results can provide a theoretical basis for the stability zoning control of dynamic load waterproof coal pillar.