Why Do You Care about Stemming Length Calculation: Good & Bad Practices
Petr Explosives Group | Technical Blog
Stemming Done Right: A Blaster’s Guide to Better Confinement
Proper stemming is one of the most important things you can do on a shot. Get it right, and you’ll keep the energy in the hole longer, get better fragmentation, reduce flyrock, and cut down on airblast complaints. Get it wrong, and you’re throwing rocks and making noise for no reason. Proper stemming length converts explosive energy into productive rock breakage rather than wasted energy, flyrock, and environmental complaints. When you correctly calculate and apply stemming, you improve fragmentation, safety, and overall blast economics.
!!!!Here’s a straight-to-the-point comparison of the most common stemming design methods used on surface blasts.!!!
Key Equations for Stemming Length
1. Primary Formula (Crushed Rock Stemming)
T = 0.7 × B
- Where:
- T = Stemming length (m or ft)
- B = Burden (m or ft)
This is the most widely recommended rule for angular crushed rock.
2. Alternative for( Drill Cuttings )
T = 1.0 × B (or up to 1.2 × B)
Drill cuttings provide poorer interlocking, so more length is required.
3. Hole Diameter Rule (Common for Smaller Holes)
T = 20 × D to 30 × D
- Where:
- D = Hole diameter (in the same units as T, usually meters)
4. Particle Size Recommendation Particle size = D / 10 to D / 25 (angular, well-graded material)
Comparison of Common Stemming Design Methods
| Method | Formula | Best Used When | Advantages | Limitations |
|---|---|---|---|---|
| Crushed Rock Stemming | T = 0.7 × B | You have a good angular stone | Excellent gas confinement, less flyrock, better fragmentation | Needs quality material available |
| Drill Cuttings Stemming | T = 1.0 × B to 1.2 × B | Crushed stone is not available | Cheap and easy | Poor confinement, more flyrock & airblast |
| Hole Diameter Rule | T = 20D to 30D | Quick field checks | Super fast to calculate | Ignores burden and geology |
| Particle Size Guideline | Size = D/10 to D/25 | Choosing your stemming material | Better interlocking & confinement | – |
Practical Examples
Example Calculation (6″ Hole, 10 ft Burden)
Method 1: Crushed Rock (Preferred)
T = 0.7 × Burden
- T = 0.7 × 10 = 7 ft
- Recommended: 7 ft of good angular crushed stone
Method 2: Drill Cuttings
T = 1.0 × B to 1.2 × B
- T = 10 ft to 12 ft
- Recommended: 10–12 ft
Method 3: Hole Diameter Rule
T = 20D to 30D (D = 0.5 ft)
- T = 10 ft to 15 ft
- Recommended: 10–15 ft
Method 4: Proper Particle Size (6″ hole)
Particle Size = D/10 to D/25
- = 0.24″ to 0.60″
- Recommended: ¼” to ⅝” angular crushed stone
Good Practices vs Bad Practices
| Aspect | Good Practice | Bad Practice | Consequences of Bad Practice |
|---|---|---|---|
| Material | Angular crushed rock (D/10–D/25) | Fine drill cuttings, sand, or rounded gravel | Poor interlocking → early venting, flyrock, airblast |
| Length | 0.7 × Burden (crushed rock) | Too short (< 0.6 × B) or too long (> 1.2 × B) | Short: Flyrock & poor fragmentation Long: Boulders in collar zone |
| Placement | Placed in small lifts and tamped/compacted | Dumped in large volumes at once | Bridging, voids, reduced confinement |
| Particle Grading | Well-graded (mix of sizes) | All fines or all large pieces | Voids or easy ejection |
| Wet Holes | Dewater if possible; use water-resistant stemming | Stemming directly into the heavy water column | Stemming sinks or becomes ineffective |
| Inspection | Measure and record the actual stemming height | Visual estimate only | Inconsistent results and safety risks |
| Documentation | Record in Trial Log (material, length, performance) | No records kept | Cannot optimize or defend in audits |
Quick Reference Table for Common Hole Sizes
| Hole Diameter | 20D | 25D | 30D | Recommended Particle Size |
|---|---|---|---|---|
| 3″ | 5 ft | 6.3 ft | 7.5 ft | 0.12 – 0.30 in |
| 4″ | 6.7 ft | 8.3 ft | 10 ft | 0.16 – 0.40 in |
| 5″ | 8.3 ft | 10.4 ft | 12.5 ft | 0.20 – 0.50 in |
| 6″ | 10 ft | 12.5 ft | 15 ft | 0.24 – 0.60 in |
| 6¾” | 11.3 ft | 14.1 ft | 16.9 ft | 0.27 – 0.68 in |
| 7⅞” | 13.1 ft | 16.4 ft | 19.7 ft | 0.32 – 0.79 in |
| 9⅞” | 16.5 ft | 20.6 ft | 24.7 ft | 0.40 – 0.99 in |
| 12¼” | 20.4 ft | 25.5 ft | 30.6 ft | 0.49 – 1.23 in |
Recommendations from Petr Explosives Group
- Use angular crushed stone whenever you can get it.
- The 0.7 × Burden rule is the go-to for most modern surface blasts.
- Running drill cuttings? Add extra length (go 1.0B–1.2B).
- Use the 20D–30D rule as a quick field sanity check (rule as a cross-check)
- Particle size matters just as much as stemming length.
- Conduct test blasts when changing hole diameter, burden, or stemming material.
- Include stemming details in every post-blast report and Trial Log.
- Train crews on proper placement technique — small lifts + compaction.
- Bad stemming causes more flyrock than being a little light on powder.
- Too much stemming = poor collar breakage and big boulders on top.
Blaster’s Rule of Thumb
Good Stemming = Good Fragmentation
Short Stemming → Flyrock & Airblast
Long Stemming → Boulders & Poor Collar Break
Load it right, stem it right, and the shot will thank you. Stay safe out there! 💥
Visual Summary of Good Stemming
- Optimal: A firm plug that holds until gases have done maximum work on the rock.
- Too Short: Gases vent upward → dramatic flyrock and noise.
- Too Long: Energy wasted in the upper zone → poor top fragmentation and large boulders.
Have questions about stemming optimization for your specific quarry or mine? Contact the Petr Explosives Group technical team for site-specific calculations and field support.
