Chapter 13: Priming and Loading Explosives

Learning Objectives

Describe what priming is and how it works.
Explain best practices for adequate priming to ensure detonation of charges.
Discuss the principles of priming.
Describe how to prime a detonator-sensitive explosive cartridge.
Describe how to prime a cast booster.
Explain the process and principles of loading.
Describe the process of decking.
Identify precautions when working around loaded holes.
Describe the process of pneumatic loading and the safety precautions required.
Describe bulk emulsion and ANFO loading and best practices for each method.
Identify requirements for successful tamping.
Describe stemming, its purpose, its importance, and how to use it.

Overview

Blasting operations involve preparing and placing explosive charges. Combining a charge with a detonator is known as priming. Placing a primed charge into a blast hole before detonation is known as loading. A blaster is expected to know the procedures, requirements, and restrictions for priming and loading explosive charges in any situation.

Priming the Charge

Priming a charge brings together — for the first time — an explosive and a detonator. Safe procedures for priming a charge are determined by the conditions, the application, the explosives, and the initiation system.

Primers

A primer is a detonator-sensitive explosive, such as a stick of dynamite or a cast booster, that has a detonator inserted. The primer is used in the initiation chain to increase the effects of the detonator and cause the remainder of the explosive column to detonate. The detonator needs to be properly installed in the explosive for the pairing to be considered a primer.

The definition of a primer depends on how the explosive is used, not on the explosive's form, composition, or sensitivity. For example, a cast booster on its own is considered an explosive. When placed without a detonator into a blast hole filled with emulsion, the cast booster is considered a booster. When a detonator is inserted, the cast booster becomes a primer.

Priming Considerations

To ensure detonation, the explosive charge must be adequately primed:

The velocity of detonation (VOD) of the primer must equal or exceed the VOD of the explosives in the main charge.
The primer should have a detonation pressure equal to or greater than that of the explosives in the rest of the column.

Warning: Do not under-prime a charge. This may result in a misfire, improper breakage, and excessive fumes.

Under normal conditions, most high explosives are reliably initiated by a high-strength detonator:

Modern dynamites have high sensitivity to initiation, even in cold temperatures.
Water-gel and emulsion explosives tend to become less sensitive in colder temperatures.
Blasting agents have low sensitivity to initiation.

For these reasons, the blaster should use a high-strength primer to ensure proper detonation. Be sure to follow the manufacturer's technical data sheet recommendations when priming an explosive charge, especially in longer blast holes.

What is detonation pressure? Detonation pressure is a measure of the amount of force exerted by an explosive. It is calculated using the explosive's VOD as well as its density. The higher the density and/or VOD, the higher the detonation pressure.

Principles of Priming

The following principles apply to preparing all primers:

Do not prepare primers in a magazine or near large quantities of explosives.
Do not prepare a primer until immediately before placing the explosives.
Do not prepare more primers than immediately needed.
Do not punch a hole in an explosive that is very hard or frozen.
Protect the detonator from abrasion, impact, and other harm.
Point the detonator in the direction of the main explosive charge.
Never force or attempt to force a detonator into an explosive.
Make sure the detonator is fully inserted in the explosive.
Secure the detonator to the explosive so that no tension is placed on the leg wires, shock tube, plastic tubing, or detonating cord at the point of entry into the detonator.
Do not tie a half hitch with shock tube, as it can pinch or damage the tube.
Do not slit, drop, twist, or tamp a primer.

Priming with an Explosive Cartridge

Explosive cartridges are most commonly used in priming. They can contain dynamites, emulsions, or water gels. Cartridges are designed to maintain rigidity while being loaded into drill holes and to compress readily when tamped. Cartridges are made of heavy paper or plastic tubing.

Note: Many blasters refer to a cartridge simply as a "primer." This is accurate only when a high-strength detonator has been inserted into a detonator-sensitive explosive.

How to Prime a Detonator-Sensitive Explosive Cartridge

Use only:
- A cartridge of good structural integrity, capable of being initiated by the detonator.
- A "powder punch" made of wood, plastic, or non-sparking metal such as brass or copper.
Use the powder punch to form a hole so the detonator can be fully inserted. The hole may be punched in the end of the cartridge, but take care to ensure it does not exit through the side.
Insert the detonator so it is completely buried in the centre of the cartridge with the base or "business" (closed) end pointed toward the bulk of the cartridge.
If the primer could become caught up in the hole during loading, ensure that the detonator is securely fastened to the cartridge by:
- Taping any protruding leg wires, detonating cord, or shock tube to the cartridge, or
- If using an electric detonator, tying a half hitch in the leg wires around the middle of the cartridge.

Priming with a Cast Booster

A cast booster is a manufactured unit of high explosive designed to initiate an explosive charge. A cast booster achieves this by boosting or amplifying the shock wave from a detonator.

A cast booster is a cylinder of pentolite high explosive (a mixture of TNT and PETN). Some cast boosters also contain other explosives such as RDX. Cast boosters have a high VOD and very high detonation pressures. Some cast boosters have a detonator well for easily inserting and holding the detonator. Larger cast boosters may also have a hollow core (or "tunnel") through which a detonator may be passed.

How to Prime a Cast Booster

Use only a booster with a detonator well that allows the detonator to be easily and completely inserted.
Thread the detonator through the tunnel.
Insert the detonator into the bottom of the detonator well.
If the primer could become caught up in the hole during loading, securely fasten the detonator to the cast booster by:
- Taping any protruding cord line, leg wire, or tubing to the booster, or
- Tying a half hitch in the electric detonator leg wires around the middle of the charge.

Important: Never try to enlarge the detonator well in the cast booster.

Loading

Drilled holes must be examined to ensure they are clear of obstructions that could hinder loading the explosives. Where blast holes may collapse (e.g., due to groundwater or unstable ground), tubular inserts may be used to keep the holes open.

Removing Obstructions

Techniques for removing mud and rocks from a hole include:

Blowing out the mud and rock with a blow pipe using compressed air
Removing the obstruction using a "scraper" or spoon (a rod with a dished end)
Pushing the obstruction to the bottom of the hole with a drill steel (rod) or loading pole

Primer Placement

In general, primers are best placed in the lower part of the explosive column, normally at or below grade. If charges are separated by inert material, primers should be placed in each charge section to ensure reliable detonation. Follow manufacturers' recommendations for priming locations.

Lower each charge into the hole gradually. Dropping charges into the hole can lead to premature detonation. Follow the progress of each charge with a loading pole to ensure it doesn't become lodged in the wrong location. If an explosive is not in direct contact with a primer, the explosive may fail to detonate.

Other Loading Principles

Inspect the blast site for overhead hazards and safe working conditions before loading.
Check each blast hole to ensure it is safe for loading.
Where two-inch-diameter or smaller cartridges are used, use the first cartridge in the blast hole as the primer cartridge.
It must be possible to place the primer easily and safely in a blast hole without damage to the detonator, leg wire, or tubing.
Position the primer so that the base or "business" (closed) end of the detonator is oriented toward the main part of the explosive column.
Ensure direct contact between the primer and the additional explosives.
Never force explosives into a blast hole.
Never load a blast hole containing hot or burning material. Temperatures above 65°C (150°F) could be dangerous.
Do not stack more explosives than needed near working areas during loading.
Do not drop large-diameter, rigid cartridges — 102 mm (4 in.) or larger — directly on a primer.

Decking

Decking is a method of creating inert (unloaded) zones within blast holes to enhance explosive performance or limit the weight of explosives used per hole. This is usually done with stemming or wooden spacers (air decking). The term "deck" (or "deck charge") refers to an explosive charge separated from other charges by inert zones.

Each deck requires a separate primer that is initiated separately.

Uses for Decking

In some cases, the blaster can use decking to do one or more of the following:

Protect a weak zone (e.g., a mud seam or a joint) from explosive energy.
Create a zone without explosives to cushion the back wall.
Separate charges to improve energy distribution.
Reduce unnecessary use of explosives.

When decking is used, the blaster should provide enough distance between charges to prevent them from interfering with each other and causing misfires.

Precautions Around Loaded Holes

The blast site, especially loaded holes, should be clearly identified. This can be done using individual markers such as cones (drill plugs), with warning or caution tape, or with other highly visible indicators.

Blasters must ensure that no vehicle or other equipment is moved over a loaded hole.

Equipment and vehicles not required in the blasting operation should be kept out of the blast site. Mobile equipment is only permitted near explosives when under the direct control of the blaster of record and only to assist the blaster in:

Excavating
Placing fill or stemming materials
Placing blasting mats

Pneumatic Loading

Pneumatic loading uses compressed air to place explosives into a drilled hole. The process requires a pneumatic loading machine and a special hose. The loading hose can be up to about 30 m (33 yd.) long in some cases.

Pneumatic loading of explosives such as ANFO moves the prill material through the loading hose at relatively high velocity. The purpose is to break down or crush the material in the blast hole and increase the overall packing density. Pneumatically loaded ANFO densities can increase to between 0.95 and 1.00 g/cc.

Safety Precautions

When pneumatically loading a blasting agent (such as ANFO), use only a semi-conductive hose designed for this purpose. This type of hose drains away static electricity, which can increase the risk of accidental detonation. Static electricity may be produced by the flow of the blasting agent through the hose. Hole liners made of plastic can also contribute to the buildup of static electricity during loading.

During the loading operation, the pneumatic loading machine must be effectively grounded. This means connecting the machine to a metal stake driven into the ground or rock surface. The grounding cable must not be attached to rails, pipes, or other conductors.

Important: Electric or electronic detonators must not be placed in a hole before it is loaded pneumatically, unless written permission has first been obtained from WorkSafeBC.

Bulk Loading

Bulk loading refers to filling a drill hole with either bulk emulsion or bulk ANFO (or a similar type of explosive, such as water-resistant ANFO).

Bulk Emulsion Loading

In bulk emulsion loading, a loading hose is inserted in the hole and emulsion is loaded from the bottom up. The bulk emulsion is under pressure. As loading progresses, the hose is retracted so it doesn't become embedded in the emulsion.

Since bulk emulsion can be loaded into wet holes, floating primers (primers that move out of their intended location) are a concern. An experienced hose handler can prevent floating. Putting some weight on the primer can help ensure it stays in place.

Follow the manufacturer's recommendations concerning the position of the hose in the blast hole to avoid trapping water. Bulk loading into wet holes should not be done via the top-down method of ANFO loading.

Bulk ANFO Loading

Bulk ANFO loading is similar to pneumatic loading but done with a larger-diameter hose and at much lower pressure. The ANFO prills are usually moved to the hose by an auger screw from the bin on the vehicle (top-down loading).

This type of loading usually does not result in floating primers because the drill hole needs to be dry to load any ANFO product.

Monitor the Height of the Explosive Column

For both types of bulk loading, the blaster and the bulk truck operator must continuously monitor explosive column rise. This is done by dipping a weighted tape measure into the blast hole and measuring the amount of explosives pumped. These steps help avoid losing product in seams and overloading blast holes.

Tamping

To help ensure a successful blast, explosives loaded into a blast hole should fill the hole's diameter as much as possible. The difference in size between the cartridge diameter and the blast-hole diameter is called the annular space.

For an effective blast, explosives may be compressed into a blast hole with a tamping rod to remove as much annular space as possible. This ensures contact with the other explosives. Tamping should be done frequently during the loading process — ideally after each cartridge, or at minimum after every second cartridge.

Tamping Requirements

Tamping rods should be made of wood or plastic and have non-sparking metal fittings.
Devices made of a ferrous metal, such as iron or steel, must not be used.
Successful tamping requires steady pressure. Excessive force or impact could damage the explosives or cause premature detonation.
Undue pressure on a primer can result in an accidental explosion.

In an "up hole" (one that is drilled or angled upward), cartridges can be held in place by means of a plastic drill-hole plug known as a "shuttlecock."

Other Tamping Principles

Do not tamp a primer or explosive product that has been removed from its cartridge.
Do not kink or damage detonating cord, shock tube, plastic tubing, coupling devices, or wires of detonators when tamping.

Stemming the Collar

The final stage of loading explosives into a blast hole is stemming the collar (i.e., the unloaded portion) of the hole. The purpose of stemming is to maintain detonation gases and pressures for as long as possible in order to do useful work. Stemming also helps to reduce noise and air blast from detonation.

The type and quantity of stemming, as well as how it is used, play critical roles in any blast operation:

Stemming that is too small for the hole tends to rifle (blow out at the time of detonation).
Stemming that is too large blocks the hole, creating large pockets into which the explosive gases escape.

In general, crushed rock is the best stemming, as it compacts well and acts as a plug to contain the explosive gases.

Stemming Procedures

When stemming the collar, do the following:

Pour slowly enough to avoid clogging the hole with stemming buildup.
Ensure the leg wires, cord, or tubing is held to one side to avoid damaging it or obstructing the stemming.
Make sure the stemming materials are the correct size for the diameter of the hole.
Make sure the stemming has settled before moving on to the next hole.
In small-diameter holes, gently moving the downline from side to side will indicate whether the stemming has settled.
Make note of how much stemming has been poured and if the amount is consistent with other holes of the same depth.

Stope Blasting