Experimental Investigation on the Rock-Breaking Efficiency of High-Pressure Jet Drill Rod Perforation in Sandstone Roof

Against the backdrop of global energy transition and carbon neutrality goals, the demand for efficient, safe, and sustainable exploitation of coal resources has grown increasingly urgent. High-pressure jet rock-breaking technology has emerged as a key enabling solution for directional rock mass weakening, gob-side entry retention, and other critical coal mine engineering projects. However, existing theoretical and experimental studies on jet rock breaking have struggled to fully reveal the evolutionary mechanism of rock fragmentation during down-the-hole jet drilling operations. To directly observe the rock-breaking effect of the jet drill rod, this study established a similarity simulation test system to investigate the rock-breaking efficiency of high-pressure jet drill rods in sandstone roof strata under simulated in situ conditions. The results indicate that the fluid transport process during jet rock breaking can be divided into four distinct stages: initial jet formation, early rock breaking, intermediate rock breaking, and stable rock breaking. Correspondingly, the sandstone surface damage morphology evolves through five sequential phases: surface particle detachment, microfracturing of the borehole wall, steady penetration depth increase, positive feedback of energy accumulation for rock fragmentation, and large-scale surface instability. As jet pressure increases, the damage mode transitions from localized minor damage to large-scale disintegration, with a critical rock-breaking pressure of approximately 3.25 MPa. A longer duration of pure water jet impact enables cumulative energy to drive damage propagation, expanding from localized boreholes to full-surface collapse. The influence of target standoff distance exhibits a nonmonotonic trend, first increasing and then decreasing, with an optimal value of 2 mm; beyond this threshold, jet diffusion leads to diminished damage efficiency. The nozzle diameter exerts a strong initial influence on sandstone surface damage that gradually weakens, with 1.2 mm identified as the optimal size; larger diameters cause rapid dissipation of jet energy. A more oblique incident angle results in more intense and extensive shear damage. The employment of high-abrasion-resistant abrasives triggers large-scale crushing, with prolonged abrasive impact time exacerbating surface damage and achieving a drilling efficiency more than 2.5 times that of pure water jets. Compared with multiple linear regression, neural network models demonstrate superior performance in capturing the nonlinearity of sandstone surface damage, providing more accurate predictions with smaller deviations between the predicted and experimental values. This study offers important theoretical guidance for the green and efficient exploitation of coal resources.