Cutting-Load Characteristics of Excavation Machine Picks in Hydraulic-Precracked Coal–Rock

Hydraulic pre-fracturing is a rock-weakening technique applied in hard-rock excavation. To investigate the effects of hydraulic pre-fracturing on crack propagation in excavation roadwalls and on the cutting loads experienced by excavation machine picks when cutting precracked rock, a two-way fluid–solid couplinproach (CFD–DEM) was employed to simulate the three-dimensional crack propagation process of a rock face under hydraulic fracturing. The results indicate that crack propagation under hydraulic fracturing evolves through four stages: (1) initiation of the primary crack; (2) further development of the primary crack, accompanied by the emergence of fine subsidiary cracks; (3) retardation of the primary crack growth, concurrent with propagation of secondary cracks; and (4) further expansion of secondary cracks. The influences of borehole aperture and injection pressure on crack propagation were analyzed; within the investigated ranges, increasing either aperture or injection pressure produced a nonlinear increase in crack development. When the hydraulic-fracture borehole diameter increased from 85 mm to 100 mm, the number of broken bonds increased by 56.2%; when the injection pressure increased from 25 MPa to 40 MPa, the number of broken bonds increased by 153.9%. The cutting force experienced by picks when cutting precracked rock decreased by 9.05% compared with cutting intact (non-precracked) rock; after precracking, the mean forces in the Z and Y directions decreased by 11.46% and 7.20%, respectively, whereas the mean force in the X direction increased by 5.49%. The findings provide reference data for the practical implementation of hydraulic pre-fracturing in hard-rock excavation.