Questions
5-8 MCQs per paper
Difficulty
Medium
Importance
High yield for Mining and Mechanical PSU streams
Overview
Drilling and Blasting covers the mechanical fragmentation of rock using explosives, a critical process in open-cast and underground mining operations. For PSU exams, this topic is essential because it bridges the gap between mechanical energy and geotechnical applications, frequently appearing in technical papers for ONGC, Coal India, and NTPC.
Types of Explosives and ANFO
Explosives are categorized based on their velocity of detonation and chemical composition. ANFO (Ammonium Nitrate-Fuel Oil) is the most widely used industrial explosive due to its cost-effectiveness and ease of handling in dry hole conditions.
- ANFO ratio is typically 94 percent Ammonium Nitrate to 6 percent Fuel Oil
- High explosives have high velocity of detonation (VOD)
- Low explosives undergo deflagration rather than detonation
- Slurry explosives are water-resistant and suitable for wet holes
- Emulsion explosives provide high gas pressure and water resistance
Blast Design: Burden and Spacing
Designing a blast pattern involves calculating the correct distance between holes (spacing) and the distance from the hole to the free face (burden). Incorrect ratios lead to flyrocks, oversized boulders, or poor fragmentation efficiency.
- Burden (B) is the distance from the charge to the nearest free face
- Spacing (S) is usually 1.25 times the burden for square patterns
- Staggered patterns improve fragmentation over square patterns
- Powder factor = Total weight of explosive / Total volume of rock blasted
- Specific charge is measured in kg/m3
Initiating Devices and Sequence
Initiation systems are responsible for triggering the explosion at precise intervals to ensure safe and controlled rock breakage. Understanding the difference between pyrotechnic and electronic delays is vital for exam accuracy.
- Detonating fuse (Cordtex) is used for initiating multiple holes simultaneously
- Non-electric initiation (Nonel) uses shock tubes for timing
- Electric detonators have instantaneous and delay variations
- Safety fuse burn rate is approximately 90-120 seconds per meter
- Blasting circuit resistance must be measured before firing
Controlled Blasting Techniques
Controlled blasting techniques are employed to protect the final pit wall from structural damage and reduce vibrations in the surrounding area. These methods focus on limiting the damage zone of the explosive energy.
- Pre-splitting involves firing a line of holes before the main blast
- Cushion blasting is performed after the main blast
- Smooth blasting uses light, decoupled charges
- Decking prevents excessive vibration near sensitive areas
- Peak particle velocity (PPV) is used to monitor vibration limits
Exam Tip
Focus on the relationship between Burden, Spacing, and Powder Factor; these constants appear in almost every numerical problem in PSU technical papers.
Common Mistakes
- Confusing the definitions of Burden and Spacing in MCQ diagrams.
- Neglecting the impact of water resistance on explosive selection, assuming ANFO is used universally.
- Forgetting that Powder Factor units are kg/m3 or kg/tonne, leading to calculation errors in numericals.
More Revision Notes
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