Image credit: KAZ Minerals
Introduction
Presplitting is a widely used controlled blasting technique in open-pit mining, aimed at minimizing damage to the remaining rock formation and ensuring greater stability (Pomasoncco-Najarro et al., 2022). By creating a pre-defined fracture line before the main blast, presplitting helps to prevent overbreak and unnecessary rock damage, which is especially crucial when dealing with less competent or weak rock masses.
What is presplitting?
Presplitting involves drilling a row of closely spaced holes along the perimeter of the intended final excavation line, which are lightly charged with explosives (Samhudi, 1991). These holes are detonated before the main production blast to create a continuous crack in the rock mass. This crack, or presplit, acts as a buffer that absorbs the energy of the main blast, preventing it from extending beyond the desired excavation limits and reducing overbreak.
What is the importance of presplitting?
The primary objective of presplitting is to enhance the stability of the remaining rock face by controlling the fracturing process. This is particularly important in sensitive areas where stability is critical, such as near final pit walls, roads, or infrastructure. Key benefits of presplitting include:
- Minimizing overbreak: By pre-fracturing the rock mass, presplitting reduces the risk of overbreak, ensuring that the remaining rock is undisturbed and structurally stable.
- Improved wall stability: The presplit crack helps to prevent damage to the final pit walls, which is essential for maintaining long-term slope stability and preventing rock falls.
- Reduced vibration: Presplitting limits the energy transmitted to the surrounding rock during the main blast, reducing vibration and potential damage to nearby structures.
- Enhanced control in weak rock masses: In less competent rock masses, where the natural tendency for uncontrolled fracturing is high, presplitting allows for better control of the blast-induced damage.
What are design considerations for presplitting?
Effective presplitting requires careful design and planning to achieve the desired outcomes. The key elements of presplitting design include hole spacing, burden, charge distribution, and delay timing. Below is a breakdown of the main factors to consider:
Hole Spacing
The spacing between presplit holes is one of the most critical design parameters. Hole spacing must be optimized based on the characteristics of the rock mass, including its strength and jointing. Generally, presplit hole spacing ranges from 0.75 to 2.5 meters, depending on the competency of the rock (Gyebuni et al., 2024).
- Competent rock mass: Wider hole spacing can be used in strong, competent rock, as it is more resistant to overbreak.
- Weak rock mass: Closer hole spacing is required in weaker or fractured rock masses to maintain continuous fracture lines and ensure that the presplit crack forms effectively.
Burden and Stemming
The burden, or distance from the presplit holes to the nearest production holes, must be calculated to ensure that the presplit fracture acts as an effective boundary. Inadequate burden may lead to excessive overbreak, while too much burden may cause poor fragmentation. Stemming is typically used to confine the explosive energy within the presplit holes, ensuring that the energy is directed toward the formation of the crack. Proper stemming materials and lengths should be selected to maximize efficiency.
Explosive Charge Distribution
Presplit holes are lightly charged compared to production blast holes. The goal is to generate just enough energy to create the fracture line without causing excessive rock movement or damage. Low-density explosives or decoupled charges are commonly used to minimize energy output. The charge weight is carefully controlled to match the characteristics of the rock and the spacing of the holes. Common explosives used for presplitting include:
- Decoupled explosives: These are explosives that are smaller in diameter than the drill hole, reducing the energy transmitted to the surrounding rock.
- Non-explosive materials: In some cases, non-explosive materials like expanding cement can be used in particularly sensitive areas.
Timing and Delays
Timing is crucial in presplitting to ensure that the presplit crack forms before the production blast occurs. Delays are used to separate the detonation of the presplit holes from the main blast, typically by a few milliseconds. Proper timing ensures that the presplit crack can act as a buffer to absorb the shockwaves from the production blast. In some cases, electronic detonators are used to provide more precise control over the timing of the blast sequence, ensuring optimal fracture propagation and minimizing overbreak.
Credits to Blessing Olamide Taiwo
For further reading
Gyebuni, R., Kunkyin-Saadaari, F., & Mensah-Kane, D. (2024). Pit Slope Configuration for Open Pit Mining–A Case Study. American Journal of Science, 9(2), 96-132.
Pomasoncco-Najarro, A., Trujillo-Valerio, C., Arauzo-Gallardo, L., Raymundo, C., Quispe, G., & Dominguez, F. (2022). Pre-split blasting design to reduce costs and improve safety in underground mining. Energy Reports, 8, 1208-1225.
Samhudi, S. (1991). The charge requirements for rock fracturing by explosives with reference to presplit blasting (Doctoral dissertation, UNSW Sydney).
Taiwo, B. O., Fissha, Y., Palangio, T., Palangio, A., Ikeda, H., Cheepurupalli, N. R., … & Kawamura, Y. (2023). Assessment of charge initiation techniques effect on blast fragmentation and environmental safety: an application of WipFrag Software. Mining, 3(3), 532-551.
