How to Calculate Number of Blast Holes – Complete Practical Field Guide
Every blasting supervisor or mining engineer has faced this question at the start of the shift:
“How many blast holes do I need for this blast?”
The number of blast holes is not a random figure. It decides your total drilling metres, explosive consumption, charging time, blast size, and finally the cost. Calculate too few holes and you get big boulders and tight muckpile. Calculate too many and you simply burn money on extra drilling and extra explosive.
In this detailed guide, I will explain exactly how to calculate the number of blast holes in a practical, field-oriented way — the same way it is done in good Indian open-cast mines.
Correct calculation of number of blast holes saves both drilling cost and explosive
Why the Number of Holes Matters So Much
The number of holes controls almost everything in a blast:
- Total drilling metres (and drilling cost)
- Total explosive quantity required
- Time required for charging and stemming
- Size of the blast (important near villages or other mines)
- Fragmentation quality
- Matching of loader and dumper capacity
- Ground vibration (more holes usually mean more charge per delay if not timed properly)
Many mines keep drilling the same number of holes every day without proper calculation. Over a year, this habit costs a lot of money.
Two Reliable Methods Used in the Field
There are two methods that experienced engineers use:
1. Area / Pattern Method
This is the most common and practical method. You work with the length and width of the block and the burden-spacing pattern.
2. Volume Method
Useful when you already know how many tonnes or cubic metres you want to blast.
Both methods should give nearly the same answer if applied correctly.
Method 1: Area / Pattern Method (Most Used in Daily Work)
This is the method you will use 90% of the time.
Step-by-step process:
- Measure the actual length of the free face (or the length of the marked blast block).
- Decide how much width you want to take in the burden direction (how many rows).
- Fix your Burden (B) and Spacing (S) according to hole diameter and rock type.
- Calculate holes in one row and number of rows.
Formulas you should remember:
Number of holes in one row = (Length of face ÷ Spacing) + 1
(The +1 is important because holes are placed at both ends of the row.)
Number of rows = Width of blast ÷ Burden
(Always round it properly so that the full width is covered.)
Total number of holes = Holes in one row × Number of rows
Method 2: Volume Method
Volume broken by one hole = Burden × Spacing × Bench Height
Total number of holes = Total rock volume to be blasted ÷ Volume per hole
This method is very useful when the production target is fixed in tonnes.
Detailed Practical Examples from Indian Mines
Example 1: Limestone Mine (Cement Plant)
- Length of free face = 90 m
- Width to be blasted = 20 m
- Burden = 3.6 m
- Spacing = 4.2 m
- Bench height = 10 m
Holes in one row = (90 ÷ 4.2) + 1 = 21.4 + 1 ≈ 22.4 → Take 23 holes
Number of rows = 20 ÷ 3.6 ≈ 5.55 → Take 6 rows
Total holes = 23 × 6 = 138 holes
Volume check:
Volume per hole = 3.6 × 4.2 × 10 = 151.2 m³
Total volume = 90 × 20 × 10 = 18,000 m³
Number of holes = 18,000 ÷ 151.2 ≈ 119 holes
(You can see the difference because we rounded up the rows. In practice we accept this and adjust charge if needed.)
Example 2: Iron Ore Mine
- Length = 110 m
- Width = 22 m
- Burden = 4.5 m
- Spacing = 5.2 m
Holes per row = (110 ÷ 5.2) + 1 = 21.15 + 1 ≈ 22.15 → 23 holes
Number of rows = 22 ÷ 4.5 ≈ 4.89 → 5 rows
Total holes = 23 × 5 = 115 holes
Example 3: Coal Overburden
- Length = 160 m
- Width = 28 m
- Burden = 5.8 m
- Spacing = 6.5 m
Holes per row = (160 ÷ 6.5) + 1 ≈ 24.6 + 1 = 25.6 → 26 holes
Number of rows = 28 ÷ 5.8 ≈ 4.83 → 5 rows
Total holes = 26 × 5 = 130 holes
Example 4: When Tonnage Target is Fixed
You need to blast 40,000 tonnes of iron ore.
- Bulk density = 2.85 t/m³
- Volume = 40,000 ÷ 2.85 ≈ 14,035 m³
- Burden = 4.3 m
- Spacing = 5.0 m
- Bench height = 12 m
Volume per hole = 4.3 × 5.0 × 12 = 258 m³
Number of holes = 14,035 ÷ 258 ≈ 54 holes
Staggered (Triangular) Pattern
In staggered pattern the calculation of total number of holes remains almost the same. Only the position of holes in the second, fourth, sixth rows etc. is shifted by half spacing.
Many engineers prefer staggered pattern because it gives slightly better fragmentation distribution.
What to Do When the Face is Irregular
In real mines the free face is rarely a perfect straight line. In such cases:
- Divide the face into two or three sections
- Calculate holes separately for each section
- Or take average length and proceed
- Always mark the holes on the ground before starting drilling
Important Field Adjustments
- Always round up the number of rows if the decimal is more than 0.3–0.4, otherwise the last portion will remain under-burdened.
- In multi-row blasts, sometimes one extra buffer row is added at the back.
- When hole deviation is high (deep holes), keep a small safety margin in spacing.
- Near sensitive structures, deliberately reduce the total number of holes or use more delays so that maximum charge per delay stays within limit.
Link Between Number of Holes and Other Parameters
- More holes → more drilling metres → higher drilling cost
- More holes → more explosive (if charge per hole is same)
- Number of holes × Volume per hole = Total blast volume
- Number of holes also decides how many delays you will need
Common Mistakes That Cost Money
- Forgetting the “+1” while calculating holes in a row
- Taking number of rows as 4.7 and drilling only 4 rows
- Using design burden and spacing instead of actual measured values
- Not measuring the actual length of the marked block
- Calculating only on length and ignoring width
- Copying previous day’s hole numbers without fresh calculation
Best Practices Followed in Good Mines
- Measure length and width with tape after the block is marked
- Decide burden and spacing first, then calculate number of holes
- Make a simple sketch showing rows and holes before drilling starts
- Record planned number of holes vs actual holes drilled
- Review the calculation if rock type or hole diameter changes
- For big blasts, double-check the calculation with both Area and Volume methods
Quick Formula Card for Field Use
- Holes in one row = (Length ÷ Spacing) + 1
- Number of rows = Width ÷ Burden
- Total Holes = Holes per row × Number of rows
Alternative:
Total Holes = Total Volume ÷ (B × S × H)
Conclusion
Calculating the number of blast holes is simple mathematics, but it needs discipline. Once you start measuring the block properly and applying the formulas consistently, you will see better control over drilling metres, explosive consumption, and blast results.
Do not drill on guesswork. Take five minutes, measure the length and width, apply the formula, and then start drilling. This small habit separates average blasting practice from good blasting practice.
If you want a ready-to-use Excel sheet where you just enter Length, Width, Burden, Spacing and it gives you the number of holes automatically, or if you need examples for irregular faces and staggered patterns, leave a comment. I will prepare it for you.
0 Comments