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                          Introduction

In opencast mining, controlled blasting techniques such as pre-split, cushion, and line drilling are critical for maintaining slope stability, minimizing ground vibration, and preventing over-break.
DGMS emphasizes these methods under CMR 2017, Reg. 106–115 and various DGMS Technical Circulars. This topic is frequently asked in DGMS First-Class and Second-Class Manager (Coal/Metal) exams, both written and oral.

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🧱 1. What Is Controlled Blasting?

Controlled blasting refers to the use of special drilling and initiation patterns to control the extent of rock breakage and minimize damage to surrounding ground or structures.
Techniques include:

• Pre-split blasting
• Cushion blasting
• Line drilling
• Trim blasting

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🧩 2. Pre-Split Blasting Objective: 

To create a fracture plane along the final pit slope before production blasting. Design Parameters:

• Hole spacing: 8–12 times hole diameter.
• Charge per meter: 1–1.5 kg (depending on rock).
• Delay timing: 25–50 ms.
• Burden: 0.5 m (very small).
• Diameter: 100 mm (typical).

Advantages:

• Reduces back-break.
• Improves slope stability.
• Minimizes ground vibration.

DGMS Relevance: Essential for compliance under Reg. 106 (Blasting in Opencast Mines).

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🧤 3. Cushion & Line Drilling

• Cushion Blasting: Conducted after main blasting to smooth the final wall. A thin explosive charge with stemming creates a buffer zone.
• Line Drilling: A series of unloaded holes act as a plane of weakness. Used near sensitive installations.

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⚙️ 4. DGMS-Prescribed Safety Measures

1. Pre-split holes should be drilled before production holes.
2. Use non-electric delay detonators for precise sequencing.
3. Ensure adequate stemming (≥ 70 % of hole depth).
4. Maintain safe distances as per DGMS vibration control tables.
5. Record vibration data using seismographs.

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🧭 5. Case Example At NCL Nigahi Project (India), pre-split blasting reduced slope over-break from 6 m → 1.2 m, improving stability and reducing scaling work.

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⚡ Exam-Oriented Quick Notes

• Pre-split = fracture plane before main blast.
• Cushion = secondary control along the final wall.
• Line drilling = uncharged holes to define the wall.
• Stemming length ≥ 70 %.
• Delay between rows = 25–50 ms.
• DGMS Reg. 106–115 govern blasting safety.

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🎯 25 MCQs – Pre-Split & Controlled Blasting (DGMS Pattern) 

Q1. The main objective of pre-split blasting is to:
A. Increase fragmentation
B. Reduce vibration
C. Control final slope breakage
D. Increase powder factor
E. Reduce stemming length
Answer: C
Solution: Pre-split blasting defines the final wall and controls over-break.

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Q2. Cushion blasting is carried out:
A. Before pre-split holes
B. Along the final wall after production blasting
C. For surface blasting only
D. In underground stopes
E. Only in dragline benches
Answer: B
Solution: Cushion blasting smooths the wall post-production.

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Q3. Line drilling differs from pre-split by:
A. Use of smaller charge
B. Use of uncharged holes
C. Use of ANFO
D. Use of detonating fuse
E. Higher powder factor
Answer: B
Solution: Line drilling uses uncharged holes forming a fracture plane.

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Q4. Typical spacing between pre-split holes is:
A. 0.2 m
B. 0.5 m
C. 1–2 m
D. 8–12 × hole diameter
E. 20 m
Answer: D
Solution: Standard design is 8–12 times the hole diameter.

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Q5. DGMS Regulation covering blasting in opencast mines is:
A. Reg. 104
B. Reg. 106
C. Reg. 110
D. Reg. 121
E. Reg. 100
Answer: B
Solution: Reg. 106 of CMR 2017 covers blasting operations.

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Q6. The typical charge per meter of pre-split hole is:
A. 0.5–1.5 kg
B. 2–5 kg
C. 6–8 kg
D. 10 kg
E. 0.1 kg
Answer: A

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Q7. Stemming in pre-split holes should be at least:
A. 30 %
B. 50 %
C. 70 %
D. 90 %
E. None
Answer: C

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Q8. Controlled blasting helps in:
A. Increasing slope angle
B. Decreasing bench height
C. Slope stability and vibration reduction
D. Increasing ANFO use
E. Reducing burden
Answer: C

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Q9. Delay time between rows in pre-split blasting is:
A. 1 ms
B. 5 ms
C. 25–50 ms
D. 100 ms
E. None
Answer: C

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Q10. DGMS Circular on vibration control refers to:
A. Tech Circular 7/2018
B. Tech Circular 2/2010
C. Tech Circular 6/2005
D. Rescue Circular 2019
E. All of the above
Answer: B

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Q11. Vibration is measured in:
A. ppm
B. mm/s
C. dB
D. Lux
E. Hz
Answer: B

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Q12. In pre-split, the plane of weakness is created:
A. Before main blasting
B. After production blast
C. During excavation
D. Using ANFO slurry
E. Only mechanically
Answer: A

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Q13. Back-break occurs when:
A. Excessive burden is used
B. Delays are short
C. Rock mass is weak
D. Controlled blasting is absent
E. All of the above
Answer: E

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Q14. DGMS recommends monitoring of:
A. PPV (Peak Particle Velocity)
B. Sound intensity
C. Flyrock length
D. Bench width
E. Powder factor
Answer: A

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Q15. Ideal PPV limit for residential areas is:
A. 100 mm/s
B. 50 mm/s
C. 25 mm/s
D. 10 mm/s
E. 5 mm/s
Answer: D

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Q16. Flyrock distance depends on:
A. Burden & spacing
B. Charge per delay
C. Rock jointing
D. Hole depth
E. All of the above
Answer: E

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Q17. Line drilling is used primarily to:
A. Reduce ground vibration
B. Form a smooth wall
C. Prevent over-break
D. All of the above
E. None
Answer: D

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Q18. Which explosive is most commonly used in pre-split blasting?
A. Gelatine
B. ANFO
C. Emulsion
D. Detonating cord only
E. Black powder
Answer: C

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Q19. Flyrock is most dangerous in:
A. Underground mines
B. Opencast blasting
C. Stemming operations
D. Drilling stage
E. None
Answer: B

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Q20. A safe vibration level for equipment foundations is about:
A. 50 mm/s
B. 25 mm/s
C. 19 mm/s
D. 12.5 mm/s
E. 5 mm/s
Answer: C

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Q21. The purpose of delay detonators in pre-split blasting is to:
A. Create simultaneous firing
B. Provide sequential firing to reduce vibration
C. Increase fragmentation
D. Reduce stemming
E. None
Answer: B

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Q22. DGMS recommends recording seismograph data for blasts within:
A. 100 m
B. 300 m
C. 500 m
D. 1000 m
E. 1500 m
Answer: C

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Q23. Excessive spacing between holes leads to:
A. Smooth wall
B. Back-break
C. Lesser throw
D. Reduced burden
E. All of the above
Answer: B

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Q24. Controlled blasting minimizes:
A. Bench height
B. Ground vibration
C. Ore dilution
D. Explosive strength
E. Drill diameter
Answer: B

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Q25. DGMS mandates blast log books to be maintained for:
A. One month
B. Six months
C. Twelve months
D. Five years
E. Indefinite
Answer: D
Solution: Records must be preserved for five years.

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                     🧩 Conclusion 

Controlled blasting—especially pre-split and cushion methods—is a cornerstone of modern opencast mine safety.
DGMS regulations require detailed design, monitoring, and vibration control to ensure slope stability and protect workers and nearby structures.
Mastering these techniques is vital for DGMS oral and written exams as well as for real-world mine management.

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