Chapter Six - Blasting Safety

Precautions Before Loading

Blast Area Prior to Loading

Before any loading activities are started:

1. Mark the blast area clearly with flags, cones, or other readily identifiable markers
2. Remove all unnecessary equipment from the area
3. Clear all non-essential persons from the area
4. Control observers under a responsible person who will ensure they do not create a hazard by wandering
5. Disconnect any electrical power that might create a hazard

Electrical Hazards

When electric blasting is being used:

• If extraneous electricity is suspected, make appropriate checks with a blaster's multimeter or continuous ground current monitor
• Where extraneous electricity problems persist, use a nonelectric initiation system
• Turn off two-way radios in the near vicinity
• IME Pamphlet No. 20 gives safe transmitter distances as a function of transmitter type and power

Primer Safety

Cardinal Rule

Primers must be made up at the working face or as close to it as possible.

• Detonators and primer cartridges/cast primers should be brought in as separate components
• Preparation of primers at a remote location and their transportation presents undue hazard
• Exception: In large tunnel projects, use of an outside primer makeup facility is often considered safer than making primers at the face
• All unused primers should be dismantled before removing them from the job site
• Assembled primers containing detonators should never be stored

Making Primers

1. Use a nonsparking tool to punch the hole in the cartridge for cap placement
2. Keep the number of persons making primers to a minimum to ensure control
3. Check for electrical hazards if using electric caps
4. Ensure the cap is fully imbedded and attached so it will not be dislodged when tension is put on wires or tubes

Important Notes:

• A hard cartridge should not be rolled for softening - this destroys cartridge integrity and the cap may not stay fully imbedded
• A good nonsparking powder punch should make an adequate hole without rolling

Dangers of Cap Falling Out

1. The cap may be struck during loading or tamping and cause a premature detonation
2. The cap may fail to initiate the primer when activated

Small-Diameter Cartridges

When using electric caps with small-diameter explosive cartridges:

• Punch the cartridge at the end for cap insertion
• Fasten leg wires to the cartridge with a half hitch to remove possibility of tension on the cap

Large Cartridges and Cast Primers

• Larger cartridges may require punching the cap hole in the side
• With cast primers, pass the cap through the channel and into the cap well
• Leg wires may be taped to the cast primer for extra security

Other Detonator Types (Nonel, Primadet, Hercudet)

• Preparation is similar to electric blasting caps
• Because propagation through tubing may be hampered by sharp bends, tape the tubing to the cartridge rather than half hitching
• Consult manufacturer for recommendations

Detonating Cord to Primer Cartridge

• Secure with a tight knot, supplemented by half hitches
• With cast primers, pass cord through channel and tie a knot at the end to keep primer from slipping off
• Subsequent primers can be slid down the detonating cord

Cap and Fuse Primers

• Make a diagonal hole through the cartridge
• Pass cap and fuse through this hole and into the second hole made for cap emplacement
• Alternatively, place cap into a single diagonally placed side hole and tie fuse to cartridge with string
• With fuse that will withstand a 180-degree bend, end priming may be used
• Cast primers are not normally used with cap and fuse

Borehole Loading

Pre-Loading Checks

Before loading begins:

• Double check for unnecessary personnel and equipment
• Double check for possible electrical hazards if using electric caps
• Check each borehole for proper depth to prevent excessive column buildup

If an electrical storm approaches at any time when explosives are present, the area must be vacated, regardless of whether electric detonators are being used.

Weather reports, lightning detectors, or static from AM radio receivers may serve as warning of approaching electrical storms.

Borehole Depth

• Holes too deep: Should be partially backfilled (loading excessive depth wastes explosive and increases ground vibrations)
• Holes too short: May require cleaning or redrilling

Explosive Column Buildup

Using a weighted tape, check column buildup frequently during loading:

Stemming length: As a general rule, 14 to 28 borehole diameters

Loading Small-Diameter Cartridges

1. Use a nonsparking tamping pole
2. Do not use a cushion stick - the primer should be the first cartridge in the hole
3. The base of the cap should point toward the collar
4. Never slit the primer cartridge
5. Push primer into place firmly - never tamp vigorously
6. Load 2-3 cartridges, slit them, place as a column, and tamp firmly
7. Load remaining cartridges, slit and tamp firmly
8. Never do excessive tamping
9. Take care not to damage detonator leg wires or tubes

Loading Large-Diameter Blastholes

• Cartridges may be dropped down the hole
• However, lower the primer cartridge and one or two cartridges above it to prevent damage
• "Wet bags" of ANFO should not be dropped (may break package and cause desensitization)
• Check column rise frequently to prevent overloading

Extraneous Electricity and Pneumatic Loading

When pneumatically loading blastholes with pressure pots or venturi loaders over electric blasting cap leg wires:

1. Properly ground the loader to prevent static electricity buildup
2. Do not ground to pipes, air lines, rails, or other fixtures that conduct stray current
3. Do not use plastic liners when pneumatically loading small blastholes (increases static buildup)
4. Use a semiconductive loading hose with:
   • Minimum resistance: 1,000 ohms per foot and 10,000 ohms total
   • Maximum total resistance: 2,000,000 ohms

Such a hose permits static charge to bleed off but will not allow stray currents to enter the borehole.

Where extraneous electricity is a problem, or where it is illegal to load pneumatically over leg wires, use a nonelectric initiation system. This does not entirely eliminate the hazard, so safeguards should still be followed.

Hooking Up the Shot

Crew Size

• Keep the crew size to an absolute minimum
• A single person should be in charge of final checkout to ensure the hookup plan has been properly followed

Electric Blasting Circuits

The series circuit is the easiest, safest, and surest:

• For multiple shots or excessive number of caps, use a parallel series circuit
• Make sure each series has the same resistance
• A twisted loop is the best connection for two light gauge wires
• Coil or remove excessive wire between holes for neatness and visual inspection
• Ensure bare connections do not touch each other or the ground (avoid short circuits, current leakage, or pickup of extraneous currents)

After Hooking Up Electric Circuits

1. Check for continuity and proper resistance with a blasting multimeter or galvanometer
2. Shunt the circuit until ready for final hookup
3. Keep the lead wire shunted at the shot firer's location until ready to fire

Firing Electric Blasts

Recommended: Use a blasting machine for firing all shots

If using a powerline:

• Must be specifically dedicated to blasting
• Must be equipped with safeguards against over-energizing caps and resulting arcing

Batteries should never be used for firing electrical blasting circuits because their output is unpredictable and may cause only a portion of the round to fire.

Parallel circuits are less desirable because they require high current and cannot be checked for shorts or broken wires. If powerline firing or straight parallel circuits are necessary, consult the cap manufacturer.

Firing with Detonating Cord

1. Make sure knots are tight and secure
2. Avoid tight lines and severe angles between lines
3. Do not permit the cord circuit to cross itself
4. Lay out circuit so each hole can be initiated by at least two paths from the detonator
5. Carefully check the circuit visually after hookup is complete
6. Do not connect the initiating cap until it is time to blast

Firing with Cap and Fuse

• Use Ignitacord for multiple hole blasts (strongly recommended)
• Principal cause of fuse accidents: Trying to light too many fuses at one time
• Secondary causes: Wet or deteriorated fuse, insufficient or improper lighting equipment

When using Ignitacord:

• All fuses should be the same length
• The path of the Ignitacord determines the delay sequence
• Ignitacord should not cross itself (cross-lighting is possible)

Regulations:

• At least two persons must be present when lighting fuses
• If lighting fuses individually, no person shall light more than 15 fuses
• MSHA specifies burning times depending on number of fuses lit
• Test burning speed frequently (nominal: 40 seconds/foot or 130 seconds/meter)
• All fuses must be burning inside the hole before the first hole detonates

Accident rates show that fuse blasting is inherently more hazardous than other initiation methods. Many incidents occur with highly experienced miners. Wherever practical, fuse blasting should be replaced by an alternative initiation system.

Newer Initiation Systems

For systems such as Hercudet, Detaline, and Nonel:

• Seek advice on proper hookup procedure from the manufacturer or distributor
• Certain aspects are still evolving and recommended procedures change from time to time

Shot Firing

More people are injured and killed during the shot firing operation than any other phase of blasting.

This is usually due to:

• Inadequate guarding
• Improper signalling
• Other unsafe practices permitting persons to be too close

The blaster must take positive steps to ensure that no one, including the blaster and crew, is in the area of potential flyrock at the time of detonation.

Just Prior to Blasting

1. Allow adequate time to inspect the blast area for last minute problems
2. Establish a fail-safe system to prevent inadvertent firing (safeguard the key or handle)
3. While proceeding to the shot firing location, ensure all connections are intact

Guarding

• Ensure enough guards to seal off the area and protect persons from entering
• Block access to the blast zone 5-10 minutes before the blast
• Guards should proceed outward from the blast area, clearing all personnel
• Guards should position beyond range of flyrock, concussion, and toxic gases
• Once sealed off, guards must permit no one to pass unless they first inform the shot firer

Warning Persons of an Impending Blast

• Sound a warning siren with audible range of about 0.5 miles
• Signs or audible warnings alone are not dependable - they may not be understood and do not clearly delineate the hazardous area
• Many underground mines use check-in/check-out procedures to ensure no one strays into the blast area

Important:

• Tell guards if more than one blast is to be fired
• After all blasts, guards must receive an audible or visual all-clear signal before allowing persons to pass
• If in doubt, keep the area secure until doubt is removed

Precautions for Shot Firer

1. Choose a safe firing location with adequate distance and/or cover for protection
2. Maintain two-way visual or audible contact with guards if possible
3. On surface blasts, the firing location should command a good view of the surrounding area

Firing Procedure:

1. Alert guards to seal off the area
2. Receive positive response from each guard
3. Immediately before firing, alert guards again
4. If response is positive, fire the shot
5. If the shot fails to fire, maintain security while correcting the problem
6. Continue guarding procedure until the shot is fired

When Contact with Guards is Impractical

Particularly underground:

• Guards must clear and secure the area
• Maintain security until all shots have been fired or until relieved by the blaster
• This may mean guarding for an extended period

Night Blasting at Surface Mines

Blasting at night at surface mines is especially hazardous because of lack of visibility and should be done only in an emergency.

Postshot Safety

Flyrock and Reentry

• Allow at least 15 seconds for all flyrock to drop
• Even after flyrock subsides, hazards of toxic gases and loose rock exist
• Do not reenter until toxic gases have dispersed:
  • Surface blasting: May be a minute
  • Poorly ventilated underground opening: An hour or more

Waiting Periods for Misfires

The practice of blasting between shifts is recommended because it avoids or minimizes guarding problems and allows gases to clear before reentry.

Reentering the Blast Site

The first person reentering should:

1. Inspect for loose rock posing hazard to personnel
2. Danger off the area until loose rock is addressed
3. Check for misfires

Indicators of Misfire:

• Loose explosives or detonating cord in the muckpile
• Area that has not broken or pulled properly
• Unusual shape of muckpile
• Unusually long bootleg

A misfire is not always obvious - a trained eye is often required to detect one.

Do not permit others into the blast area until certain no hazards exist.

Disposing of Misfires

Removing a Misfired Charge

The best method is to remove the undetonated charge by:

• Water flushing
• Air pressure

Horizontal or shallow holes are most amenable to this technique.

Visually inspect the hole using a light source to ensure all of the charge has been removed.

Refiring a Misfired Charge

Where removal is too difficult, detonate the charge:

1. If leg wires, tubes, or detonating cord are intact: Reconnect and fire
2. If reconnection not possible: Remove stemming, insert new primer at top of powder column, and refire

Caution: When refiring misfired holes where much of the burden has been removed:

• Excessive flyrock is likely
• Guard the area accordingly

Digging Out a Misfired Charge

If neither removal nor refiring is feasible:

1. Flood the hole with water to desensitize any non-water-resistant explosive
2. Carefully dig out the rock surrounding the misfire
3. Have an observer present to guide the excavator

Drilling Adjacent to a Misfire

The practice of drilling and shooting a hole adjacent to the misfire has been used, but can be extremely hazardous.

People have been killed when the new hole intersected the misfired hole and detonated it.

All previously described techniques are preferable to drilling an adjacent hole.

MSHA metal-nonmetal regulations prohibit drilling a hole where there is a danger of intersecting a charged or misfired hole.