Research on Optimization of Radial Uncoupled Charge Parameters Based on the Mechanical Characteristics of Step-Type Partitioning

Technical Summary: Optimization of Radial Uncoupled Charge Parameters Based on the Mechanical Characteristics of Step-Type Partitioning

Problem Statement
In open-pit granite mining, traditional fully coupled charge structures often result in excessive energy concentration at the borehole wall. This creates a "crushing zone" that consumes significant energy, leading to poor fragmentation efficiency, high rates of fine particles (fines), and uneven block size distribution. While decoupled charging (inserting a medium between the explosive and borehole wall) is known to attenuate shock waves and reduce fines, traditional methods using rock debris or air gaps often suffer from imprecise energy distribution, mixing issues, and a lack of adaptability to the varying stress constraints found in different sections of a blast bench (high constraint at the bottom vs. low constraint at the top). The specific challenge addressed is how to optimize radial uncoupled charge parameters using mixed emulsion explosives to control the fines generation rate while ensuring uniform fragmentation across varying rock weathering conditions.

Methodology
The study employed a three-pronged approach combining theoretical analysis, physical simulation, and industrial field trials at a granite mine:

1. Theoretical & Comparative Analysis: The authors analyzed the energy transfer mechanisms of coupled versus radially uncoupled charges, focusing on peak detonation stress attenuation and the role of the annular gap in energy release.
2. Transparent Tube Simulations: A 4.0-meter transparent polycarbonate tube (140 mm diameter) was used to simulate vertical blast holes. Three specific tests were conducted to evaluate a self-developed variable-diameter radial uncoupled spacer:
   • Free-fall embedding: Assessing spacer stability and embedding depth into rock slag.
   • Charging and backfill mixing: Evaluating explosive-segregation and mixing with rock debris.
   • Sinking behavior: Observing spacer stability during primary and secondary explosive charging.
3. Field Industrial Trials: Comparative blasting tests were conducted on two granite benches with different weathering degrees: weakly weathered (+30 m) and moderately weathered (+45 m).
   • Control Group: Used traditional rock-slag interval charging.
   • Test Groups: Utilized the new variable-diameter spacer to create radially uncoupled structures.
     • Scheme I: 125 mm diameter at the bottom (4 m) and 110 mm diameter in the upper section (9 m).
     • Scheme II: A three-stage structure with 125 mm (bottom, 4 m), 110 mm (middle, 4 m), and 90 mm (upper, 5 m).
   • Data Collection: Post-blast analysis included image analysis and sieve testing to determine fragment size distribution, fines ratio (<5 mm), boulder ratio (>1000 mm), and muckpile morphology.

Key Contributions

• Development of a Variable-Diameter Spacer: The study introduced a self-developed mechanical spacer capable of creating an axially continuous, radially uncoupled charge structure with different diameters along the borehole depth. This allows for precise control of the decoupling coefficient in specific zones.
• Identification of Lithological Adaptation Boundaries: The research established that the optimal decoupling coefficient is not universal but depends on rock weathering. It identified that weakly weathered granite (hard, low jointing) is highly sensitive to initial impact energy, whereas moderately weathered granite (pre-existing fractures) can utilize the "gas wedge effect" to compensate for lower initial energy.
• Optimized Gradient Energy Distribution: The study demonstrated that matching the charge structure to the "strong bottom, weak top" stress gradient of a blast bench is critical. A structure providing moderate energy enhancement at the bottom and moderate weakening at the top yields superior results compared to uniform or overly aggressive decoupling.

Results

• Simulation Findings: The transparent tube tests confirmed that the mechanical spacer prevents the mixing of explosives with rock slag (a common issue with traditional rock-debris spacing) and maintains structural stability during charging.
• Field Trial Outcomes (Scheme I):
  • Explosive Consumption: Reduced unit explosive consumption by over 16% (16.3% for weakly weathered, 16.2% for moderately weathered) compared to the control group.
  • Fragmentation: Achieved a uniform block size distribution. In weakly weathered granite, the proportion of fines (<1 cm) dropped from 6.84% (control) to 3.41%. In moderately weathered granite, fines dropped from 8.93% to 5.63%.
  • Muckpile Quality: Produced a moderately loose muckpile favorable for excavation and loading, with no significant accumulation of large boulders or "umbrella rock."
• Field Trial Outcomes (Scheme II - Excessive Decoupling):
  • While Scheme II (upper decoupling coefficient ~1.55) further reduced explosive consumption (by ~26%), it resulted in insufficient energy for the upper sections of weakly weathered granite. This led to a significant increase in large blocks (3.91%) and the formation of "umbrella rock" along the crest.
  • Moderately weathered granite showed better tolerance to this higher decoupling coefficient due to its pre-existing fractures, though large block rates still increased.

Significance and Claims
The paper claims that the proposed radial uncoupled charge structure, specifically the configuration with a 125 mm bottom charge and 110 mm middle/upper spacers, offers a technically superior solution for granite blasting. By effectively controlling the "peak reduction and gas extension" energy distribution mechanism, this method:

1. Reduces Fines: Significantly lowers the pulverization rate, addressing the issue of over-crushing common in traditional coupled charges.
2. Improves Efficiency: Reduces specific explosive consumption by more than 16% while maintaining or improving fragmentation quality.
3. Enhances Adaptability: Provides a technical reference for optimizing blasting parameters based on rock weathering degrees, ensuring that energy supply matches the specific mechanical constraints of the rock mass.

The authors conclude that this approach effectively balances the trade-off between fines generation and boulder formation, offering a viable path toward efficient, low-consumption blasting operations in granite quarries.