Assessment of firing patterns on moderately strong and weak sandstone cover rocks in a surface mine

• 3 firing patterns on 2 sandstone bench types in a major coal mine were studied.
• Firing patterns influenced the fragmentation and other operations.
• Skewed V and straight V patterns were better in moderately strong strata.
• Diagonal firing patterns provided good results in weak sandstone.
• Geological anomalies and bench stiffness also influenced the fragmentation.

The present paper highlights the application of three different types of firing pattern on two different types of sandstone overburden benches of a large coal mine in India. The study reveals that firing pattern influences the effective spacing to burden ratio at the time of detonation. By changing the firing pattern, effective burden (Be) changes and the ratio of effective spacing (Se) to effective burden (Be) during the firing is also subject to change in comparison to drilled spacing to burden ratio (S/B). Hence, the Se/Be ratio vis-à-vis firing pattern must be used to design the blast rounds. The results have been derived on the digital image analysis based fragment size characterization, excavator performance and evaluation of powder factor values on blasted muck piles. Skewed V-type firing yielded the best results in comparison to straight V-type and normally practiced diagonal firing pattern on the moderately strong sandstone benches, whereas, the existing diagonal firing pattern appears to be satisfactory on weak (weathered and gritty) sandstone benches.

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