Experimental Study on Directional Rock Fracture by Energy-Gathering Cutting under Dynamic Impact

This study addresses the challenge of excavating through heterogeneous tuffaceous sandstone formations in tunnel construction by proposing a novel energy-gathering slotting rock-breaking technique based on dynamic impact. Using self-developed geotechnical dynamic impact testing system, cylindrical tuffaceous sandstone specimens ( $\varnothing$100 mm×50 mm) were prepared with 10 mm thick polyurethane pads adhered to one end. Radially arranged holes of 3, 6, and 9 mm in diameter were drilled into the pads, each fitted with six corresponding energy-gathering nails. Seven groups of tests were conducted under impact air pressures ranging from 0.35 MPa to 0.65 MPa to investigate the effects of varying impact energy and nail diameter on directional rock fracturing performance. The results show that as the impact pressure increases, the peak stress and energy absorption of the specimens rise significantly, with fracture patterns transitioning from primarily intergranular to transgranular cracking. The 3 mm nails were prone to local crushing and failed to produce effective through-cutting cracks, while the 9 mm nails caused blocky or pulverized failure under high pressure. In contrast, the 6 mm nails consistently induced stable, continuous, and directional fractures under various pressures, producing more transgranular cracks, and demonstrating excellent energy utilization efficiency. Scanning electron microscopy confirmed the strain-rate effect of impact: cracks were predominantly intergranular under low strain rates (low impact forces), and became transgranular under high strain rates. This technique leverages the compressive-reflective-tensile stress chain mechanism inherent in dynamic fracture mechanics to achieve controlled, directional rock breaking without explosives or liquid media. By properly matching impact parameters and nail diameters, this method can efficiently guide crack propagation along predetermined paths in deep, heterogeneous rock masses, offering a promising strategy for controlling over- and under-excavation in complex geological tunneling conditions.