Blasting Misfires: What Every Blaster Needs to Know
By Petr Explosives Group & Practical Explosives Training School (PETS)
If you’ve spent any time in the field, you know that misfires are one of the most serious problems you’ll ever face on a blast site. A misfire — when a charge fails to fully detonate after initiation — isn’t just a production headache. It’s a genuine safety emergency that can injure drill crews, equipment operators, and anyone else who enters the blast area afterward.
The good news? Most misfires are preventable. Understanding why they happen is the first step to making sure they don’t happen on your watch.
1. Poor Priming — Getting the Foundation Right
Think of the primer as the heart of your blast hole. Its entire job is to transfer detonator energy into the explosive column. Get it wrong, and nothing else matters.
What goes wrong:
- Detonator placed outside the primer cartridge
- Poor contact between primer and explosive
- Wrong primer orientation or insufficient booster size
- Primers damaged or separated during loading
In ANFO columns, especially, a weak or misplaced primer near the bottom of the hole can mean the difference between a clean blast and a misfired hole.
What to do:
- Secure detonators firmly inside primers before loading
- Lower primers carefully using loading lines — never drop them into deep holes
- Verify orientation every time
- Use the booster size your manufacturer recommends
- Inspect your initiation system before stemming, not after
Field lesson: A quarry crew once dropped heavy emulsion cartridges into deep boreholes, kinking shock tubes and separating primers from the explosive column. Six holes misfired. A simple loading line would have prevented all of it.
2. Dead Pressing — When Pressure Works Against You
This one surprises many blasters. Explosives need internal void spaces to propagate detonation properly. Compress them too hard — through hydrostatic pressure in deep wet holes or heavy mechanical loading — and they lose that sensitivity. The result is a “dead pressed” column that simply won’t go.
Watch out for:
- Deep underwater or flooded borehole conditions
- Over-compaction during loading
- Pumped emulsions pushed beyond their rated depth
- Collapsed boreholes are squeezing the column
What to do:
- Always use explosives rated for the depth and water conditions you’re working in
- Follow manufacturer depth limits — they exist for a reason
- Space your boosters properly in deep columns
- Don’t rush the loading proces
3. Water and Wet Holes — ANFO’s Worst Enemy
Standard ANFO and water simply don’t mix. Ammonium nitrate is hygroscopic — it absorbs moisture quickly — and even a small amount of water in the hole can dissolve or desensitize your explosive column before you ever fire the shot.
What water does to your blast:
- Dissolves and desensitizes ANFO
- Causes primers to separate
- Floats shock tube systems out of position
- Weakens borehole walls, leading to collapse
What to do:
- Dewater holes before loading whenever possible
- Switch to water-resistant emulsions or water gels in wet conditions
- Use borehole liners where appropriate
- Load wet holes as close to firing time as possible
- Never leave loaded wet holes to sit overnight if you can help it
Field lesson: A surface coal mine loaded dry ANFO into partially flooded holes during spring runoff. Water migrated up into the explosive columns overnight. The result was multiple misfires the next morning.
4. Damaged Shock Tube — Small Damage, Big Consequences
Shock tube is tough, but it’s not indestructible. A single kink, crush point, or cut is all it takes to break the initiation signal before it reaches your primer.
Common causes of damage:
- Equipment rolling over surface connections
- Sharp rock edges cutting through the tube during stemming
- Excessive pulling tension during tie-ins
- Improper routing across the blast pattern
What to do:
- Keep equipment away from surface connections — enforce blast area discipline
- Inspect every tie-in before you fire
- Route trunk lines away from traffic and sharp edges
- Never pull the shock tube tight across the corners or rocks
Field lesson: A loader bucket crossed surface connections during stemming on a construction blast, crushing several shock tube lines. Multiple holes went dead. One walk-around before firing would have caught it.
5. Excessive Sleep Time — Don’t Let Your Blast Sit Too Long
Sleep time is the window between loading and detonation. The longer a loaded hole sits, the more opportunities there are for things to go wrong — water migration, chemical degradation, borehole movement, shock tube damage.
Risks of long sleep times:
- Water is working its way into the explosive column
- ANFO slowly desensitizing
- Ground shifts damaging initiation systems
- Borehole walls are collapsing onto the column
What to do:
- Follow your manufacturer’s recommended sleep time limits
- Load as close to firing time as your schedule allows
- Re-inspect your pattern after any unexpected delay
- In wet or unstable ground, treat sleep time as your enemy
6. Incorrect Stemming — It’s Not Just About Confinement
Stemming does more than confine blast energy — done wrong, it can also destroy your initiation system before the shot fires. Oversized angular rock is one of the most common culprits for cut-off holes.
Common stemming mistakes:
- Using coarse, angular rock that cuts through downlines
- Over-compacting stemming onto the shock tube
- Too little stemming depth, causing premature venting
- Using wet or clay-heavy material
What to do:
- Use properly sized crushed stone or stemming aggregate
- Place stemming carefully around downlines, not on top of them
- Follow the recommended stemming column lengths for your hole size
- Never rush the stemming process
Field lesson: Angular stemming rock crushed shock tube downlines in several holes during loading. The initiation signal never made it to the primers. Proper stemming material and a careful hand would have saved those holes.
When a Misfire Happens: Know the Rules
No matter how well you prepare, misfires can still occur. When they do, your job is to follow procedure — and the law.
Regulatory agencies, including ATF, MSHA, OSHA, and your state blasting authority, have specific requirements covering:
- Securing and guarding the blast area after a misfire
- Mandatory wait times before re-entry
- Documentation and notification requirements
- Safe re-firing or disposal procedures
Never improvise a misfire response. Know your site’s blasting plan and applicable regulations before you ever load the first hole.
The Bottom Line
Every misfire on this list is preventable. Proper training, careful loading habits, quality initiation systems, and strong field discipline will protect your crew and your shot.
At Petr Explosives Group and the Practical Explosives Training School (PETS), we believe safe blasting isn’t luck — it’s the result of understanding the science, respecting the process, and building good habits in the field.
Safe blasting is never accidental — it is engineered.
Petr Explosives Group provides advanced explosives consulting, technical training, vibration analysis, underwater blasting expertise, and blaster education through PETS. Topics include surface, underground, and underwater blasting, detonation physics, vibration control, misfire prevention, initiation systems, and quarry optimization.
Learn more at www.petrexplosivesgroup.com
