Introduction
The stiffness ratio, a term introduced by Calvin Konya in the mid-1960s, plays a significant role in determining the effectiveness of blast design, particularly regarding energy distribution and fragmentation outcomes. Defined as the ratio of the bench height to the burden, the stiffness ratio influences how explosive energy is distributed within the rock mass, impacting both the movement of the muckpile and the degree of fragmentation.
Energy Distribution Based on Stiffness Ratio
The stiffness ratio directly impacts how the energy from the blast is distributed within the rock mass.
- Low Stiffness Ratio (≈1): Cratering Effect: At a stiffness ratio of 1, the energy distribution becomes inefficient, with a high concentration of energy directed vertically. This leads to a cratering effect, where the blast force ejects rock upwards in a violent blowout, rather than breaking it down horizontally or evenly distributing the explosive energy throughout the rock mass.
- Optimal Stiffness Ratio (≈2.6): Balanced Energy Distribution: As the stiffness ratio increases toward an optimal range of around 2.6, energy distribution becomes more efficient. The blast energy is transferred in such a way that it promotes controlled fracturing of the rock, minimizing both vertical ejection and excessive horizontal movement. At this range, fragmentation is generally at its most efficient, with better energy transfer to the rock mass resulting in a uniform fragmentation size that is ideal for downstream processing.
- High Stiffness Ratio (>4): Borehole Effect and Flexural Failure: At stiffness ratios greater than 4, the borehole effect becomes dominant. Energy distribution is directed more horizontally within the rock mass, resulting in flexural failure of the rock and horizontal displacement. When the stiffness ratio is too high, there is a risk of underutilizing the explosive energy, which can cause uneven breakage and require secondary blasting.
Conclusion
The stiffness ratio is a key factor in blast design, influencing both energy distribution and fragmentation outcomes. Optimal fragmentation is achieved at a stiffness ratio of around 2.6, where energy distribution is balanced, and the rock mass is efficiently broken. Lower stiffness ratios result in cratering, leading to inefficient energy use and poor fragmentation, while higher ratios emphasize the borehole effect and require careful control.
