Lessons on the Effect of Fractures on Rock Fragmentation Rock fragmentation during blasting is strongly influenced by the interaction between stress waves and geological structures. Weak or less stiff zones, such as joints, bedding planes, or other discontinuities, reflect the incoming shock wave during detonation. This reflected energy increases damage on the opposite side of the discontinuity, often producing coarser fragmentation in those areas. Conversely, stronger and stiffer rock units transmit stress waves more efficiently. Blasting produces three primary damage zones within the rock mass: Crushed Zone This forms immediately around the borehole where the explosive shock wave exceeds the rock’s dynamic compressive strength, pulverizing the rock. Fracture Zone As the stress wave travels outward, the rock yields when the induced tensile stresses surpass the dynamic tensile strength. This creates radial and circumferential fractures extending several hole diameters from the blast hole (Ding et al., 2022). Spalling Zone The spalling zone develops when stress waves encounter a free face. The wave reflects back as a tensile wave, and if this reflected tensile stress exceeds the rock’s tensile strength, slabbing or thin “tile-like” breakage occurs (Zhang, 2016). The size and intensity of these zones depend on explosive type, energy characteristics, and rock mass properties. Influence of Geological Structures and Impedance The impedance mismatch between intact rock and geological structures also significantly affects the transmission and distribution of stress waves. When stress waves pass through materials with different densities or stiffness, their speed and amplitude change. This alters fragmentation patterns, influences damage zone extent, and affects material throw. Numerical and Field Evidence Numerical and field studies by Magreth Dotto Ph.D., P.Eng. and Yashar Pourrahimian provide valuable insight into damage distribution in jointed rock masses under blast loading: Their LS-DYNA numerical model for 51 mm holes shows the crushed zone radius extends 87.71 mm, approximately 1.72 times the borehole radius. Using peak particle velocity (PPV) criteria, the fracture zone extends to 3.02 m, or 59.2 times the hole radius. Field trials confirmed these results, with a crushing radius of 93.09 mm and a fracture radius of 3.1 m. PPV measurements showed a significant drop from 102 m/s near the hole to 2.35 m/s beyond the fracture zone indicating the rapid attenuation of energy after fracturing. Key Takeaway for Blasting Engineers A critical lesson from these findings is that the fracture zone generated around each blast hole must remain within the hole’s burden and spacing. If the induced fracture radius exceeds or become lesser than these design parameters, fragmentation becomes inconsistent and inefficient. Understanding the fracture zone radius is essential for designing burden, spacing, and energy distribution that deliver optimal fragmentation.