Influence of In-situ Stress on Distribution Characteristics of Rock Blasting Fragmentation

During blasting excavation of deep rock masses under high in-situ stress, the rock masses are often difficult to be fully broken, frequently resulting in large fragments. To understand this problem more deeply, blast-induced rock cracks under various in-situ stress conditions are simulated by using a dynamic finite element method. The fractal dimension theory and the image recognition method are introduced to investigate the influence of in-situ stress on the distribution characteristics of rock blasting fragmentation. The project cases that were blasted at different depths and in situ stress levels are also employed to study this problem. The results show that the generation of large fragments during blasting in deep rock masses owes to the inhibiting effect of in-situ stress on the growth of blast-induced cracks in the far field of blastholes, and the orienting effect of non-hydrostatic in-situ stress on the propagation of blast-induced cracks. As the in-situ stress level increases, the average size, maximum size, nonuniform coefficient and large block rate of rock fragmentation increase significantly. The size of the smaller fragments is little affected by in-situ stress as these fragments are generated in the vicinity of blastholes and the propagation of blasting cracks in this zone is almost unaffected by in-situ stress. When the maximum and minimum principal in-situ stresses perpendicular to the blasthole axis differ greatly, it is adverse to rock fragmentation by blasting. In comparison, a better rock fragmentation is achieved when the lateral coefficient of the principal in-situ stresses is about 0.75.