Burden and Spacing Calculation in Drilling & Blasting – Complete Practical Guide
In drilling and blasting, two parameters decide almost everything about the success of a blast: Burden and Spacing. If these two are wrong, even the best explosive and perfect timing cannot save the blast. You will get either poor fragmentation, tight muckpile, excessive throw, high vibration, or wastage of explosive.
This detailed guide explains how to calculate burden and spacing correctly for open-cast mines and quarries in India, with practical formulas, rules of thumb, worked examples, and field tips.
Correct burden and spacing are the foundation of every successful blast
Why Burden and Spacing are the Most Critical Parameters
Burden and spacing control:
- Fragmentation size
- Muckpile shape and looseness
- Throw distance and fly-rock risk
- Ground vibration
- Powder factor (explosive consumption)
- Digging rate of excavators
- Overall cost of drilling and blasting
Many mines in India still use fixed burden and spacing for years without checking actual rock conditions or hole diameter. This is one of the biggest reasons for high blasting cost.
Clear Definitions
Burden (B)
The distance between the free face and the first row of blast holes, or the distance between two consecutive rows of holes. It is measured perpendicular to the free face.
Spacing (S)
The distance between two adjacent holes in the same row, measured parallel to the free face.
Hole Diameter (D)
The diameter of the drilled hole (commonly 100 mm, 115 mm, 150 mm, 165 mm, 200 mm in Indian mines).
Basic Rules of Thumb for Burden Calculation
The most widely used practical method in the field is based on hole diameter.
Burden (B) = K × Hole Diameter (D)
Where K is a factor that usually ranges from 25 to 40.
| Rock Condition | Recommended K Factor | Approximate Burden for 115 mm hole |
|---|---|---|
| Soft / Well-jointed rock | 30 – 40 | 3.5 – 4.6 m |
| Medium hard rock | 28 – 35 | 3.2 – 4.0 m |
| Hard / Massive rock | 25 – 30 | 2.9 – 3.5 m |
| Very hard rock (Granite, BHJ) | 22 – 28 | 2.5 – 3.2 m |
Quick Field Formula (most commonly used in India):
- For 100–115 mm holes → Burden = 3.0 to 4.0 m
- For 150–165 mm holes → Burden = 4.0 to 5.5 m
- For 200 mm holes → Burden = 5.5 to 7.0 m
How to Calculate Spacing
Once burden is fixed, spacing is calculated as:
Spacing (S) = 1.15 to 1.50 × Burden
Common practice in Indian mines:
- Soft rock / good fragmentation required → S = 1.25 to 1.40 × B
- Hard rock → S = 1.15 to 1.30 × B
- When using electronic detonators and precise timing → S can be increased slightly
Most common ratios used:
- Spacing = 1.25 × Burden (very popular)
- Spacing = 1.30 × Burden
- Spacing = 1.15 × Burden (for hard rock)
More Accurate Empirical Formulas
For better accuracy, some engineers use these formulas:
1. Langefors Formula (simplified)
Burden (m) = 0.012 × Diameter (mm) + 0.6 to 0.9 (adjustment for rock)
2. Practical Formula used in many Indian projects
Burden (m) = 0.3 to 0.4 × √(Hole Diameter in mm)
3. Based on Bench Height
As a check: Burden should generally not exceed 0.5 to 0.6 times the bench height for good results.
Step-by-Step Method to Decide Burden and Spacing
- Know your hole diameter clearly.
- Assess rock type (soft / medium / hard) and jointing.
- Select suitable K factor from the table above.
- Calculate Burden = K × D (in metres).
- Decide Spacing ratio (usually 1.25 × B).
- Check against bench height (B should preferably be less than 0.6 × Bench Height).
- Adjust slightly after first few trial blasts based on fragmentation and muckpile.
Detailed Practical Examples
Example 1: Limestone Mine (115 mm holes)
- Hole diameter = 115 mm
- Rock = Medium hard limestone
- Selected K = 32
Burden = 32 × 0.115 = 3.68 m ≈ 3.7 m
Spacing = 1.25 × 3.7 = 4.6 m
Example 2: Iron Ore Mine (150 mm holes)
- Hole diameter = 150 mm
- Rock = Medium to hard iron ore
- Selected K = 30
Burden = 30 × 0.150 = 4.5 m
Spacing = 1.25 × 4.5 = 5.6 m
Example 3: Hard BHJ / Banded Hematite (165 mm holes)
- Hole diameter = 165 mm
- Rock = Very hard
- Selected K = 26
Burden = 26 × 0.165 = 4.3 m
Spacing = 1.20 × 4.3 = 5.2 m
Example 4: Coal Overburden (200 mm holes)
- Hole diameter = 200 mm
- Rock = Medium hard sandstone/shale
- Selected K = 33
Burden = 33 × 0.200 = 6.6 m
Spacing = 1.30 × 6.6 = 8.6 m
Example 5: Granite Quarry (100 mm holes)
- Hole diameter = 100 mm
- Rock = Hard granite
- Selected K = 25
Burden = 25 × 0.100 = 2.5 m
Spacing = 1.20 × 2.5 = 3.0 m
Relationship of Burden & Spacing with Other Parameters
- Higher burden → higher charge per hole required
- Larger spacing → fewer holes → lower drilling cost but risk of poor breakage
- Burden and spacing together decide the volume per hole (Volume = B × S × H)
- Powder factor is directly affected by B and S
Factors That Influence Burden and Spacing Selection
- Hole diameter
- Rock strength and toughness
- Joint and bedding pattern
- Type of explosive and its energy
- Bench height
- Desired fragment size
- Presence of water
- Vibration restrictions
- Free face condition
Common Mistakes Made in Indian Mines
- Using the same burden and spacing for soft and hard rock
- Not measuring actual burden after drilling (planned vs actual difference is common)
- Keeping spacing too high to reduce number of holes
- Ignoring hole deviation while deciding burden
- Not reducing burden when rock becomes harder
- Copying burden-spacing from another mine without checking diameter and rock type
Best Field Practices
- Always measure actual burden and spacing with a tape after drilling is finished
- Keep a record of planned vs actual burden/spacing for every blast
- Start with slightly conservative (lower) burden and increase gradually after observing results
- In multi-row blasts, keep the back-row burden 5–10% higher than front row
- Review burden and spacing every time hole diameter or rock type changes
- Use blast design software or simple Excel sheet for consistency
Quick Reference Table for Common Hole Diameters
| Hole Diameter | Soft Rock Burden | Medium Rock Burden | Hard Rock Burden | Typical Spacing Ratio |
|---|---|---|---|---|
| 100 mm | 3.2 – 3.8 m | 2.9 – 3.4 m | 2.5 – 3.0 m | 1.20 – 1.35 |
| 115 mm | 3.6 – 4.4 m | 3.3 – 3.9 m | 2.9 – 3.5 m | 1.20 – 1.35 |
| 150 mm | 4.8 – 5.8 m | 4.3 – 5.1 m | 3.8 – 4.5 m | 1.20 – 1.30 |
| 165 mm | 5.2 – 6.3 m | 4.7 – 5.6 m | 4.2 – 4.9 m | 1.20 – 1.30 |
| 200 mm | 6.5 – 7.8 m | 5.8 – 6.8 m | 5.2 – 6.0 m | 1.25 – 1.35 |
Conclusion
Burden and spacing are not fixed numbers that can be copied from one mine to another. They must be calculated based on hole diameter, rock condition, and desired results. The simple rule “Burden = 25 to 40 times hole diameter” combined with Spacing = 1.15 to 1.40 times Burden works extremely well in most Indian open-cast mines when applied with proper judgement.
Measure actual values after drilling, observe the blast results carefully, and keep refining. This disciplined approach will reduce your powder factor, improve fragmentation, and lower overall drilling and blasting cost.
If you want a ready-to-use Excel sheet for burden-spacing calculation, or detailed examples for any specific hole diameter and rock type, leave a comment. I will prepare it for you.
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