Stress wave propagation and crushing mechanism of soft-hard composite coal under water-jet impact load.

Composite coal seams have the characteristics of large differences in coal quality, difficulty in coalbed methane extraction, and poor permeability. In this work, the smooth particle hydrodynamics (SPH) method is employed to simulate the crushing process of composite coal impacted by water jet. The simulation results indicate that when the stress wave collides with the interface of soft and hard coal, the high stress zone caused by jet impact load is mainly concentrated at the interface. The reflection and attenuation of jet impact load on the contact surface restrain the crack from continuing to expand. Due to the blockage of the contact surface, the propagation energy of effective stress wave in composite coal decreases, especially in the coal particles at the lower end of the contact surface. When the soft coal above the contact surface is broken, the transmission of the barrier stress wave is reduced, and the effective stress rises again until it falls. Therefore, the crushing of composite coal caused by water jet impact can be considered as a cyclic loading and unloading process. This study can lay a theoretical foundation for the crushing mechanism of composite coal by water-jet.