Understanding the Fundamentals of Blasting and Fragmentation – Part 1

Introduction

Blasting is a critical operation in mining, quarrying, and construction that involves the controlled detonation of explosives to break rock into manageable fragments. The fundamental principle behind blasting lies in understanding the interaction between explosive energy and rock mechanics, particularly the propagation of shock waves and the subsequent formation of fractures.

The Shock-Wave Theory of Blasting

The shock-wave theory provides a framework for understanding how explosive energy transforms into mechanical work, breaking the rock. As mentioned by Hino, (1956), when an explosive charge detonates, it generates an intense shock wave accompanied by a rapid release of gas and energy.

This energy produces two primary effects:

1. Crushed Zone Formation:

Near the explosive charge, the rock undergoes intense compressive stress, exceeding its compressive strength. This creates a crushed zone, a region where the rock is pulverized into fine fragments. However, because rocks generally have a high compressive strength, this crushed zone is limited to the immediate vicinity of the charge.

Figure 1 )Shadab Far et al., 2019)

2. Shock Wave Propagation:

Beyond the crushed zone, a high-pressure shock wave propagates outward as a compressive wave. This wave does not immediately cause rock breakage but transfers energy through the rock mass.

At the first free face (a boundary with no external constraint, such as the surface of a bench or tunnel wall), the compressive wave reflects as a tensile wave. In rock mechanics, this transition is crucial because rocks are significantly weaker under tensile stress than under compressive stress. As the tensile wave interacts with the rock, fractures form when the effective tension the difference between the reflected tensile wave and any residual compression exceeds the rock’s tensile strength (Himanshu et al., 2024).

Thickness of the First Slab and Fragmentation

The initial fracture caused by the tensile wave occurs at a distance from the free face known as the thickness of the first slab (Hino, 1956). This distance is critical because:

• It determines the size of the initial fragment.
• Other fragment dimensions are generally proportional to this thickness.

If the remaining compressive wave retains sufficient energy after the detachment of the first slab, it continues propagating outward (See Figure 2). This process repeats at newly created free faces, producing successive layers of fractures and reducing the rock into smaller fragments. The cycle continues until the energy of the compressive wave diminishes below the tensile strength of the rock.

The interaction between these phenomena: shock wave propagation, energy dissipation, and rock strength, governs the fragmentation process. Understanding these principles allows blasting engineers to optimize blast designs to achieve desired fragment sizes, minimize blast induced issues like ground vibration, flyrock, overbreak, and ensure efficient downstream operations.

Factors Affecting Shock-Wave Propagation and Fragmentation

Several factors influence the effectiveness of a blast and the resulting fragmentation:

1. Explosive Properties

• The energy content, detonation velocity, and confinement of explosives significantly affect the shock wave’s intensity and duration.

2. Rock Properties

• Variations in rock strength, density, and structure (e.g., joints, fractures, and bedding planes) influence the propagation of shock and tensile waves.

3. Blast Design Parameters:

• Hole diameter, spacing, burden, and the placement of charges determine the distribution of energy and the resulting fragmentation.

4. Free Face Orientation:

• The presence and orientation of free faces play a pivotal role in enabling tensile wave reflection and fracture initiation.

5. Energy Distribution:

• Proper distribution of explosive energy ensures uniform fragmentation and minimizes the generation of oversize boulders or fines.

Importance of Fragmentation in Mining Operations

Effective fragmentation is essential for the efficiency and cost-effectiveness of mining operations. Well-fragmented rock facilitates:

• Reduced loading and hauling costs.
• Improved crusher throughput and efficiency.
• Lower energy consumption in downstream processing.
• Enhanced safety by minimizing the occurrence of hazardous oversize boulders.

Importance of Assessing Blast Performance and Output

WipFrag, a state-of-the-art image analysis software, revolutionizes the assessment of blast performance and fragmentation. By analyzing images of fragmented rock, WipFrag provides precise and actionable insights into the quality of a blast. Here’s how WipFrag enhances blasting operations:

1. Particle Size distribution (PSD) Analysis:

• WipFrag generates PSD curves that quantify the size range of rock fragments, enabling operators to evaluate whether the fragmentation meets specifications.

2. Specification Envelope Assessment

• The software allows users to define specification envelopes for crusher-compatible fragmentation. Deviations from these envelopes highlight areas for improvement in blast design.

3. Boulder Identification and Counting:

• WipFrag’s advanced algorithms detect and count oversize boulders, providing critical data for optimizing explosive placement and burden.

4. Image Merging and Orthomosaic Integration:

• The capability to merge multiple images ensures comprehensive analysis of large muck piles. Integration with drone orthomosaics enables wide-area assessment of blast results.

5. Continuous Improvement:

• By comparing fragmentation results across blasts, WipFrag supports continuous improvement in blasting practices, reducing costs and improving efficiency.

6. Real-Time Analysis:

• Integration with systems like Solo 6 and Reflex 6 facilitates real-time monitoring and analysis, ensuring immediate feedback for decision-making.

Figure 3

Figure 3 showcases results obtained from the WipFrag software, illustrating its capabilities in fragmentation analysis.

• Figure 3a presents the GIS-integrated on-site fragmentation assessment. This feature, embedded within WipFrag, allows users to visualize blast results spatially. The red sections of the GIS map highlight areas with poor blast outcomes, whereas lighter colors like blue and green represent zones with favorable fragmentation.
• Figure 3c displays the Particle Size Distribution (PSD) curves comparing three different blasts. The yellow envelope outlines the production specification of the case study mine, serving as a benchmark. WipFrag enables each mine to define their Key Performance Indicator (KPI) sizes and utilize them for ongoing assessments. This facilitates the evaluation of blast improvements over successive rounds.
• Additionally, the PSD curves feature size classifications and flag specific sizes that deviate from mine production requirements, ensuring precise monitoring and alignment with operational goals.

This comprehensive analysis provided by WipFrag aids in identifying areas of improvement, optimizing blasting strategies, and enhancing overall mining efficiency.

Conclusion

Blasting and fragmentation are complex processes driven by the interaction of explosive energy, rock mechanics, and blast design parameters. Understanding these fundamentals is essential for optimizing operations and achieving desired outcomes. WipFrag software plays a pivotal role in this optimization by providing detailed and accurate fragmentation analysis, enabling operators to assess performance, identify areas for improvement, and implement data-driven strategies for continuous enhancement. With tools like WipFrag, the mining industry can achieve safer, more efficient, and cost-effective blasting operations (download software here https://wipware.com/get-wipfrag/).

References

Hino, K. (1956). Fragmentation of rock through blasting and shock wave theory of blasting. In ARMA US Rock Mechanics/Geomechanics Symposium (pp. ARMA-56). ARMA.

Himanshu, V. K., Bhagat, N. K., Vishwakarma, A. K., & Mishra, A. K. (2024). Principles and Practices of Rock Blasting. CRC Press.

Shadab Far, M., Wang, Y., & Dallo, Y. A. (2019). Reliability analysis of the induced damage for single-hole rock blasting. Georisk: Assessment and Management of Risk for Engineered Systems and Geohazards, 13(1), 82-98.