A Practical Lesson from PETS Training
During one of our Practical Explosives Training School (PETS) classes, we discussed blast design fundamentals and worked through several real-world blasting examples. During the discussion, I noticed that many of our blasters had difficulty determining how many drill holes are required for a blast job. This is a very common challenge, especially for newer blasters learning to translate a blast plan into a practical drilling pattern.
Calculating the correct number of drill holes is a critical part of blast design, because it directly affects:
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Fragmentation quality
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Explosive energy distribution
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Ground vibration
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Flyrock control
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Overall blasting cost
Understanding this calculation allows a blaster to design safer, more efficient, and more predictable blasts.
Why the Number of Drill Holes Matters
Each drill hole represents a source of explosive energy within the rock mass. If the number of holes is incorrect, the energy distribution across the blast area will also be incorrect.
If too many holes are drilled:
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Drilling costs increase
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Explosive consumption increases
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Ground vibration and airblast may increase
If too few holes are drilled:
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The rock may not break properly
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Large boulders may remain
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Secondary blasting may be required
Proper blast design ensures that explosive energy is evenly distributed across the rock volume being blasted.
Key Parameters Required
To determine the number of drill holes needed, several blast design parameters must be known:
• Blast length (L) – length of the blast area
• Blast width (W) – width of the blast area
• Burden (B) – distance from the blast hole to the free face
• Spacing (S) – distance between holes within a row
These parameters define the drilling pattern.
How to Estimate the Number of Drill Holes for a Blast
Two Practical Methods Used by Blasters
During one of our Practical Explosives Training School (PETS) classes, we discussed blast design and worked through several real blasting examples.
One challenge that many blasters mentioned was determining how many drill holes are required for a blasting job, especially when some design parameters are still unknown.
In practice, blasters typically estimate the number of drill holes using two different methods:
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Using hole diameter to estimate burden and spacing
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Using known burden and spacing values
Both approaches are useful depending on the information available during the blast planning process.
Method 1 — Estimating Holes Using Drill Hole Diameter
When the burden and spacing are not yet known, blasters often estimate them from the drill hole diameter.
A common rule used in blasting engineering is:
Burden ≈ 25–35 × hole diameter
A simple rule used in many queries is:
Burden ≈ 30 × hole diameter
Spacing is usually slightly larger than burden:
Spacing ≈ 1.2 – 1.5 × Burden
Many blast designs use approximately:
Spacing ≈ 1.3 × Burden
Example
Suppose we have the following conditions:
Blast length = 120 ft
Blast width = 80 ft
Hole diameter = 4 inches
Step 1 — Estimate Burden
Convert hole diameter to feet:
4 in = 0.33 ft
Burden ≈ 30 × D
Burden ≈ 10 ft
Step 2 — Estimate Spacing
Spacing ≈ 1.3 × Burden
Spacing ≈ 13 ft
Step 3 — Area Broken by Each Hole
Area per hole:
Area = Burden × Spacing
Area = 10 × 13 = 130 ft²
Step 4 — Total Blast Area
Blast area:
Area = Length × Width
Area = 120 × 80 = 9600 ft²
Step 5 — Number of Holes
Number of holes: Number = Total Area ÷ Area per Hole
Number = 9600 ÷ 130 ≈ 74 holes. So the blast would require approximately 70–75 drill holes.
Method 2 — Calculating Holes from Known Burden and Spacing
If the burden and spacing are already known, the calculation becomes much simpler.
First, calculate the total blast area. Blast Area = Length × Width
Next, calculate the area controlled by each hole. Hole Area = Burden × Spacing
Finally, divide the total blast area by the area per hole. Number of Holes = Blast Area ÷ Hole Area
Example
Blast length = 120 ft ,Blast width = 80 ft, Burden = 10 ft, Spacing = 12 ft
Step 1 — Blast Area: 120 × 80 = 9600 ft²
Step 2 — Area per Hole: 10 × 12 = 120 ft²
Step 3 — Number of Holes 9600 ÷ 120 = 80 holes. In this case, the blast requires about 80 drill holes.
Why These Calculations Matter
Determining the correct number of drill holes is a fundamental step in blast design because it affects:
• explosive energy distribution
• fragmentation quality
• ground vibration
• flyrock control
• drilling and blasting cost
Too many holes increase drilling and explosive costs.
Too few holes can lead to poor fragmentation and secondary blasting.
Proper blast design ensures that explosive energy is evenly distributed across the rock mass.
Final Thoughts from PETS Training
In our PETS blasting classes, we emphasize that blast design is both an engineering calculation and a practical field skill.
Understanding how to estimate drill holes using hole diameter, burden, and spacing relationships allows blasters to quickly develop safe and effective blast layouts.
Blast smarter. Break rock better. Train with the professionals at PETS.
Visit:
petrexplosivesgroup.com
