In mining and quarrying, the blast hole diameter is a fundamental design parameter that acts as the “anchor” for almost all other geometric variables. It dictates the distribution of explosive energy within the rock mass, directly impacting the quality of fragmentation and the subsequent costs of processing that rock.
Influence on fragmentation
Blast hole diameter is the primary factor in determining the distribution of energy.
- Small diameters: using smaller holes requires a tighter “pattern” (smaller burden and spacing). This leads to a more even distribution of explosive energy throughout the rock mass. Because the energy sources are closer together, there is a higher probability of intersecting natural fractures and creating new ones, resulting in finer and more uniform fragmentation.
- Large diameters: large holes allow for wider patterns. While this reduces drilling costs, it often creates “dead zones” between holes where the explosive energy is insufficient to break the rock effectively. This typically results in coarser fragmentation and a higher percentage of “boulders” (oversized rock).
- The crushed zone: immediately around the blast hole, the rock is pulverized into “fines.” The radius of this crushed zone is directly proportional to the hole radius (typically 2–5 times the radius) [1]. Therefore, larger holes inherently generate more unwanted fines in the immediate vicinity of the hole.
Influence on powder factor
The Powder Factor is the ratio of the weight of explosives used to the volume (or mass) of rock broken (kg/m3 or kg/t) [2].
- Geometric Scaling: As hole diameter increases, the volume of the hole (and thus the weight of the explosive column) increases by the square of the radius ().
- Efficiency Trade-off: While a larger hole holds more explosive, it also requires a larger burden (the distance to the nearest free face) and spacing to avoid wasting energy.
- Optimization: To achieve the same fragmentation result, a larger hole diameter usually requires a higher powder factor because the energy is less efficiently distributed compared to many small holes spread across the same area. However, in soft rock or massive formations with few joints, larger holes may allow for a lower powder factor to achieve acceptable (though coarser) results.
Impact on downstream crushing efficiency
Blasting is often referred to as “primary crushing.”[3] The efficiency of the actual mechanical crusher depends entirely on the “feed” it receives from the blast [4].
- Throughput: Finer fragmentation from smaller-diameter, high-energy blasts increases crusher throughput [5]. When the rock is pre-fractured and smaller, the crusher spends less time on each cycle and experiences fewer “bridging” events (where large rocks block the intake).
- Energy Consumption: Smaller feed sizes significantly reduce the specific energy consumption (kWh/t) of the primary crusher [5]. In some “Mine-to-Mill” studies, increasing the powder factor in the pit (using optimized hole diameters) reduced total energy costs because the savings at the mill far outweighed the extra cost of explosives.
- Wear and Tear: Coarse fragmentation from large-diameter holes leads to higher impact loads on crusher liners and mantles, increasing maintenance costs and reducing the operational life of the equipment.
Ultimately, the optimal blast hole diameter must balance drilling capability, rock mass characteristics, bench height, and production targets. A systematic evaluation using blast trials and fragmentation analysis is essential to achieve consistent performance across the mining value chain.
Reference
[1] Y. Pourrahimian, “The Influence of Explosive and Rock Mass Properties on Blast Damage in a Single-Hole Blasting,” Feb. 06, 2024, Preprints: 2024020351. doi: 10.20944/preprints202402.0351.v1.
[2] S. Prasad, B. Choudhary, and A. Mishra, “Effect of Blast Design Parameters on Blast Induced Rock Fragmentation Size – A Case Study,” Jan. 2017.
[3] “Blast Fragmentation Measurements in Open Pits | Open-pit Mining.” Accessed: Jan. 15, 2026. [Online]. Available: https://www.amcconsultants.com/blast-fragmentation-measurements-in-open-pits
[4] A. Nikkhah, A. B. Vakylabad, A. Hassanzadeh, T. Niedoba, and A. Surowiak, “An Evaluation on the Impact of Ore Fragmented by Blasting on Mining Performance,” Minerals, vol. 12, no. 2, p. 258, Feb. 2022, doi: 10.3390/min12020258.
[5] H. Losaladjome Mboyo, B. Huo, F. K. Mulenga, P. Mabe Fogang, and J. Kalenga Kaunde Kasongo, “Assessing the Impact of Surface Blast Design Parameters on the Performance of a Comminution Circuit Processing a Copper-Bearing Ore,” Minerals, vol. 14, no. 12, p. 1226, Dec. 2024, doi: 10.3390/min14121226.
