Decoupling in Blasting – Definition, Mechanism & Applications | DGMS Blasting Notes

                           🔹 1. Introduction

Decoupling is an important concept in controlled blasting, especially in underground coal mines, metal mines, and rock excavation where excessive damage must be avoided.
It involves placing an explosive charge smaller than the borehole diameter, allowing an air gap to exist between the explosive and borehole wall.This air gap significantly influences blast energy distribution, stress wave propagation, and crack formation — essential for safer, controlled, and efficient blasting.

🔹 2. Definition of DecouplingDecoupling:

A condition where the charge diameter (dch) is smaller than the borehole diameter (d), creating an air gap.Decoupling Ratio (RD):RD=ddchR_D = \frac{d}{d_{ch}}RD =dch d

• RD > 1 → Decoupled charge
  
• RD = 1 → Fully coupled
  
• Higher ratio = larger air gap
  

🔹 3. Mechanism of Decoupled Charge

When an explosive is decoupled:

A. Lower Pressure on Borehole Walls

Some detonation energy compresses the air gap before reaching rock → reducing borehole wall pressure.

B. Reduced Stress Wave Amplitude

Stress waves entering the rock are weaker than those from a fully coupled charge.

C. Fewer Radial Cracks

Tests confirm decoupled holes produce fewer cracks around the borehole.

D. Directional Fracturing

Air gap supports formation of a shaped charge jet, improving directional crack propagation—useful in coal seam gas drainage.

🔹 4. Influence of Decoupling Coefficient

• Stress on the hole wall increases first then decreases as decoupling ratio increases.
  
• There exists an optimal decoupling ratio = 1.67 – 2.0 where:
• Energy utilization is maximized
  
• Cracks are controlled
  
• Best results for gas extraction blasting
  

🔹 5. Applications of Decoupling

✔ Coal seam pre-splitting
✔ Controlled blasting in tunnels
✔ Crack direction control in gas drainage
✔ Blasting in weak/soft strata
✔ Overbreak reduction in metal mines

25 MCQs (Aligned + Answers + Explanations)
1) Decoupling refers to —

a) Using excess charge
b) Charge filling full borehole
c) Charge diameter smaller than borehole diameter
d) Reducing delay time
e) None
✅ Answer: c
2) Decoupling ratio (RD) is defined as —

a) dch / d
b) d / dch
c) d × dch
d) 1 / d
e) None
✅ Answer: b
3) A decoupling ratio > 1 indicates —

a) Coupled charge
b) Overcharged hole
c) Decoupled charge
d) Empty hole
e) None
✅ Answer: c
4) Decoupling creates an —

a) Water column
b) Chemical reaction
c) Air gap
d) Shock cavity
e) None
✅ Answer: c
5) Air gap reduces —

a) Velocity of detonation
b) Temperature
c) Pressure on borehole wall
d) Explosive quantity
e) Coal clearance
✅ Answer: c
6) Decoupling results in —

a) Higher crack density

b) Fewer cracks
c) No cracks
d) Delayed cracks
e) Water cracks
✅ Answer: b
7) Optimal decoupling ratio is —

a) 1.0–1.2
b) 1.67–2.0
c) 3.0–4.0
d) 5.0
e) 0.5
✅ Answer: b
8) Decoupled charges are mainly used for —

a) Maximum fragmentation
b) Primary blasting
c) Controlled blasting
d) Dragline blasting
e) None
✅ Answer: c
9) Air-gap blasting helps in —

a) Overbreak
b) Flyrock
c) Directional cracking
d) Charge failure
e) High VOD
✅ Answer: c
10) Decoupling reduces —

a) VOD
b) Explosive strength
c) Stress wave amplitude
d) Charge weight
e) Charging time
✅ Answer: c
11) Decoupling is recommended for —

a) Massive hard rock benches
b) Gas drainage blasting in coal
c) Tunnel cut blasting
d) Stemming removal
e) Detonating cord
✅ Answer: b
12) Large decoupling ratio leads to —

a) More cracks
b) Low energy transfer
c) Full coupling
d) Higher burden
e) None
✅ Answer: b
13) Decoupling ratio affects —

a) Explosive price
b) Blaster’s number
c) Energy transfer efficiency
d) Fuse length
e) Coal grade
✅ Answer: c
14) Which explosive benefits most in decoupled conditions?

a) ANFO
b) Cartridge explosives
c) Boosters
d) Water gels
e) None
✅ Answer: b
15) Air gap behaves like a —

a) Conductor
b) Cushion
c) Transformer
d) Capacitor
e) Vacuum
✅ Answer: b
16) Radial cracks in decoupling are —

a) More
b) Less
c) None
d) Random
e) Unlimited
✅ Answer: b
17) Decoupled holes reduce —

a) Safety
b) Production
c) Overbreak
d) Burden
e) None
✅ Answer: c
18) Decoupled charges require —

a) Less stemming
b) More stemming
c) No stemming
d) Steel casing
e) Extra primer
✅ Answer: b
19) Air gap effect improves —

a) Flyrock
b) Gas temperature
c) Crack control
d) Burden
e) Delay time
✅ Answer: c
20) Too high decoupling ratio leads to —

a) Explosive boost
b) Good fragmentation
c) Poor breakage
d) Slotted charge
e) Water logging
✅ Answer: c
21) Decoupled charge is mostly used for —

a) Coal benches
b) Draglines
c) Soft/weak strata
d) Oversized boulders
e) Gunpowder
✅ Answer: c
22) Air gap compresses during detonation creating —

a) Chemical gas
b) Nuclear energy
c) Fireball
d) Shock cushion
e) None
✅ Answer: d
23) Directional blasting in coal seams uses —

a) Fully coupled charge
b) Decoupled slotted cartridges
c) TNT
d) RDX
e) None
✅ Answer: b
24) Decoupling helps reduce —

a) Drilling burden
b) Explosive cost
c) Damage to nearby structures
d) Drill hole length
e) None
✅ Answer: c
25) DGMS approves decoupling for —

a) Production blasting
b) Stemming removal
c) Low-disturbance, controlled blasting
d) Blowout blasting
e) Fuse lighting
✅ Answer: c

                             🔚 Conclusion

Decoupling is a powerful technique in controlled blasting, reducing blast damage while improving directional fracturing and gas extraction efficiency.
Understanding the correct decoupling ratio, energy transfer, and stress wave behavior is crucial for DGMS-supervised blasting operations.

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