Influence of diameter of empty hole on the fragmentation effect of parallel cut blasting under confining pressure

Cut blasting is the key technology for roadway blasting excavation. High confining pressure has obvious inhibition effect on cut blasting effect, while empty hole is of great significance to improve cut blasting efficiency, but the relevant research is not sufficient so far. To study the crushing effect of the cut blasting with different empty hole diameters under the confining pressure, cubic cement mortar test blocks whose side length was 600 mm were made with sand cement ratio of 2:1. The test blocks were subjected to equal bi-axial confining pressure through an electro-hydraulic servo loading system, and the parallel cut blasting cement mortar physical model test was carried out. After the blasting fragmentation was screened and statistically analyzed, the fragmentation distribution and average particle size of model fragments under different empty hole diameters were analyzed and investigated. With fractal features of fragments characterized by fractal dimension, the fragmentation degree was quantitatively analyzed. ANSYS/LS–DYNA software was used to establish numerical models based on the parameters of the test model, and the stress contour at different times with different hole diameters was analyzed. The research result shows that, when two empty holes are charged with different charge of 2 g and 3 g, respectively, the maximum fractal dimension of the model fragment can be attained with empty holes in diameter of 24 mm and 16 mm, respectively, with the particle size of the fragment mainly concentrated in the middle segment between 19 to 75 mm, and the largest total mass of the fragment is 3.128 kg and 3.615 kg, respectively; When the sizes of empty holes are different, the stress concentration effect and the reflected tensile wave produced near the empty hole are also different, which brings the difference of cut blasting fragmentation effect and can effectively benefit from the empty hole effect. The peak value and action range of stress wave near the empty hole are bigger, with a long action time, suggesting a better crushing effect; the diameter of the empty hole is too small, the stress around the empty hole is small. An oversized empty hole will lead to the overflow of the stress wave from the hole. The compressive stress is small after the superposition of stress concentration, and the action area of tensile stress is also small. The proportion of large rock pieces will be high with extremely large or small empty hole. When charged with 3 g, the fractal dimension difference between empty hole model fragments of 32 mm and 16 mm is greater than the difference of model fragments between 16 mm and without empty hole, indicating that the influence on the fragmentation of rock is greater when the empty hole diameter is too large. When the empty hole diameter is the same as 16 mm, the average particle size of the fragments in the model charged with 3 g is less than that in the model charged with 2 g, with the greater fractal dimension and total mass of the fragments than that in the test model charged with 2 g. This shows that an increased charge can contribute to rock fragmentation and the volume of the cut cavity with the same diameter of empty holes.