🔹 1. Introduction

Coupling plays a crucial role in determining how effectively explosive energy is transferred into the surrounding rock.
A charge is fully coupled when the explosive diameter equals the borehole diameter.
Coupling ensures maximum pressure transfer, stronger crack development, and higher rock breakage efficiency.

🔹 2. Definition of CouplingCoupling:

When the explosive diameter (dch) fills the entire borehole diameter (d), leaving no air gap.Coupling Ratio (Rc):RC=dchdR_C = \frac{d_{ch}}{d}RC =ddch

• Rc = 1 → Fully coupled
  
• Rc < 1 → Decoupled
  
• Rc > 1 → Overcharged (unsafe & prohibited by DGMS)
  

🔹 3. Characteristics of Fully Coupled Charge

A. Maximum Pressure on Borehole Walls

No air gap → full detonation pressure applied directly.

B. Higher Stress Wave Amplitude

More powerful P-waves and S-waves → stronger breakage.

C. More Radial & Tangential Cracks

Ideal for:

• Fragmentation blasting
  
• Hard rock excavation
  
• Bench blasting
  

D. No Energy Loss in Air Compression

Nearly full detonation energy enters the rock.

🔹 4. When to Use Fully Coupled Charges✔ Fragmentation blasting

✔ Hard rock development faces
✔ Production blasting in metal mines
✔ High-strength strata
✔ Massive rock blocks requiring full energy transfer
🔹 5. Comparison: Coupled vs. Decoupled Charges

Feature	Fully Coupled	Decoupled
Wall Pressure	High	Lower
Crack Formation	Many, large	Few, controlled
Stress Waves	Strong	Weak
Energy Loss	Minimal	Some lost compressing air
Applications	Hard rock blasting	Controlled blasting

25 MCQs (Aligned + Answers + Explanations)

1) A fully coupled charge occurs when —

a) Borehole > charge

b) Borehole < charge
c) Borehole = charge diameter
d) Only water filled
e) None
✅ Answer: c
2) Coupling ratio Rc is —

a) d / dch
b) dch / d
c) d × dch
d) d − dch
e) None
✅ Answer: b
3) Rc = 1 means —

a) Empty hole
b) Fully coupled charge
c) Decoupled
d) Slotted charge
e) Water stemming
✅ Answer: b
4) Fully coupled charge transfers —

a) Less energy
b) Moderate energy
c) Maximum energy
d) No energy
e) Compressed energy
✅ Answer: c
5) Fully coupled charges produce —

a) Fewer cracks
b) More radial cracks
c) No cracks
d) Water cracks
e) Hot cracks
✅ Answer: b
6) Best suited blasting for full coupling is —

a) Tunnel trim
b) Hard rock fragmentation
c) Coal gas drainage
d) Stope drainage
e) Cushion blasting
✅ Answer: b
7) Overcharging (Rc > 1) is —

a) Efficient
b) Cost-saving
c) Unsafe and not permitted by DGMS
d) Common in coal
e) Required for weak rock
✅ Answer: c
8) Fully coupled charges generate —

a) Weak stress waves
b) High amplitude stress waves
c) No shock
d) Delayed waves
e) Reverse waves
✅ Answer: b
9) Full coupling is ideal for —

a) Smooth blasting
b) Production blasting
c) Crack control
d) Gas drainage
e) None
✅ Answer: b
10) Fully coupled charging ensures —

a) Energy loss
b) Reduced breakage
c) Maximum borehole pressure
d) No fragmentation
e) None
✅ Answer: c
11) Coupling improves —a) Stemming
b) Energy transfer efficiency
c) Water sealing
d) Burden
e) Delay timing
✅ Answer: b
12) Fully coupled charges risk —

a) Underbreak
b) Overbreak
c) Trim blasting
d) Seam cracking
e) None
✅ Answer: b
13) Coupled charge is not preferred in —

a) Iron ore
b) Granite
c) Soft strata
d) Limestone
e) Bauxite
✅ Answer: c
14) Energy loss in fully coupled charges is —

a) High
b) Medium
c) Minimal
d) Zero
e) None
✅ Answer: c
15) Coupling ratio Rc < 1 means —

a) Coupled
b) Overcharged
c) Decoupled
d) Water stemming
e) None
✅ Answer: c
16) Fully coupled charges produce —

a) Smooth contour
b) High fragmentation
c) No breakage
d) Very few cracks
e) None
✅ Answer: b
17) Full coupling is avoided near —

a) Hard benches
b) Crusher
c) Conveyor belt
d) Sensitive structures
e) Ore passes
✅ Answer: d
18) Borehole pressure is directly proportional to —a) Cartridge color
b) Stemming
c) Hole depth
d) Coupling condition
e) None
✅ Answer: d
19) Fully coupled charges create —

a) Low impact
b) High strain energy in rock
c) No spalling
d) No waves
e) None
✅ Answer: b
20) Coupling is preferred when —

a) Smooth blasting needed
b) High fragmentation needed
c) Gas control needed
d) Soft rock
e) None
✅ Answer: b
21) Coupling ratio depends on —

a) Water depth
b) Explosive diameter vs. borehole diameter
c) Rock color
d) Bench height
e) None
✅ Answer: b
22) Fully coupled charges release —

a) Part of energy
b) Very little energy
c) Maximum energy into rock
d) Zero energy
e) None
✅ Answer: c
23) Coupled charges can cause —

a) Small cracks only
b) No breakage
c) Uncontrolled breakage
d) Water cracks
e) None
✅ Answer: c
24) Fully coupled charges are —

a) Good for smooth blasting
b) Bad for controlled blasting
c) Best for decoupling
d) Only used in coal
e) Required in waterlogged holes
✅ Answer: b
25) DGMS recommends coupling only when —a) Burden is small
b) Face is weak
c) High-fragmentation blasting is needed
d) Charging is done manually
e) Stemming is cement
✅ Answer: c

                             🔚 Conclusion

The coupling ratio determines how effectively explosive energy interacts with the rock mass.
Fully coupled charges maximize energy transfer and fragmentation, while decoupled charges offer controlled, safer, and more directional blasting.
DGMS guidelines require careful selection of charge diameter, borehole size, and desired blast results.

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