Drilling & Blasting | P&Q University Handbook

Explosives & Initiation Systems

Explosives are the toolset through which all blast energy is delivered, and how well they perform depends not just on the chemistry, but on the application.

In surface mining, most operations use bulk explosives tailored to their geology, bench height and desired fragmentation. Product selection must be driven by site conditions and performance requirements.

ANFO

ANFO (ammonium nitrate fuel oil) is a porous, prill-based blasting agent composed of 94 percent ammonium nitrate (AN) and 6 percent fuel oil (FO) by weight. It is classified as a non-ideal, oxygen-balanced explosive with a relatively low detonation pressure and velocity compared to emulsions or dynamite, but it remains one of the most cost-effective and widely used bulk products in dry-hole blasting applications.

Chemically, ANFO is a homogeneous mixture where the fuel oil is absorbed into the porous structure of the AN prills. When initiated with a high-energy booster, it undergoes detonation rather than deflagration, producing high volumes of gaseous detonation products – primarily nitrogen, carbon dioxide and water vapor. The detonation velocity of ANFO is typically around 3,200 to 4,500 meters per second, depending on confinement, charge diameter and porosity. Its gas energy output is its greatest advantage in fragmentation.

From a sensitivity standpoint, ANFO is insensitive to standard detonators and requires a properly coupled booster to achieve full detonation. Its critical diameter is relatively high – typically 1.5 to 2 in. in confined conditions, so reliable initiation cannot occur without a sufficient booster mass (often 0.5 to 1.0 lbs. of cast or pressed high explosive).

Water sensitivity is the primary limitation of ANFO. Even small amounts of water can dissolve the ammonium nitrate or disrupt the oil balance, leading to failure of the detonation train. ANFO is hygroscopic and absorbs moisture from the environment over time, which can degrade its sensitivity and energy output during extended sleep times. It should never be used in wet boreholes or where water inflow is expected post-loading.

Loading method

ANFO is auger-loaded or gravity-fed directly into the borehole from the top. Its free-flowing nature makes it fast and efficient for large-scale bench blasting in dry conditions.

However, because it does not fill irregular voids or adhere to the borehole walls, it lacks the coupling and column integrity seen in pumpable emulsions. Some field considerations include:

• Best used in dry, competent rock with short sleep times
• Requires high coupling and confinement to achieve full energy potential
• Highly economical on a per-pound basis, but the cost advantage disappears if holes are wet, voided or variable in condition.

Emulsion

Emulsions are high-density, water-resistant explosives composed of a supersaturated oxidizer phase (typically ammonium nitrate in water) dispersed as droplets within a continuous fuel matrix. This inverse emulsion is stabilized by emulsifying agents and microencapsulation, forming a cohesive, pumpable material that maintains uniformity and energy concentration throughout the borehole.

Modern emulsions have detonation velocities typically ranging from 4,500 to 5,800 meters per second, depending on density and confinement. Their high density, often between 1.15 and 1.35 grams per cubic centimeter, enables more explosive mass per foot of borehole compared to ANFO, resulting in greater total energy and improved coupling to the borehole wall.

One of emulsion’s primary advantages is its water resistance. The cohesive matrix prevents dissolution, allowing emulsions to maintain integrity in wet or partially flooded boreholes. When loaded via a pump system from the bottom up, emulsions form a continuous column, eliminating air gaps and promoting consistent detonation transfer. This makes them especially suitable for deep holes or variable ground conditions where water inflow may occur after loading.

Initiation requires a booster, typically 0.33 to 1.0 lbs. of high explosive. While emulsions are more sensitive than ANFO, they remain classified as secondary explosives and cannot be initiated reliably by detonator alone in their standard bulk form.

Still, specialized packaged versions can be made cap sensitive and treated as a high explosive. Their critical diameter is lower than that of ANFO, providing greater reliability in smaller boreholes or weaker confinement conditions.

Emulsions are often loaded using bulk delivery trucks equipped with onboard mixing and pumping systems. This method offers exceptional control over column height, loading density and pattern consistency – factors that directly influence fragmentation, heave and overall blast performance.

From a performance standpoint, emulsions allow for larger burdens and wider spacing due to their increased energy concentration per unit volume. However, this does not inherently equate to better fragmentation. Field studies and controlled trials have shown that gas energy and explosive distribution – not just VOD – are the primary drivers of breakage in most aggregate applications. While emulsions offer higher detonation pressures and improved confinement, they must still be paired with appropriate burden, timing and geology to deliver optimal results.

Some operational advantages of emulsions include:

• Water resistance. Makes emulsions ideal for wet or mixed conditions
• High density. Delivers more energy per foot, reducing pattern hole count
• Pumpable form. Ensures complete column coupling and consistent loading
• Improved economy. Done in mixed dry/wet shots by eliminating product switching.

Meanwhile, some field considerations to make include these:

• Site-specific evaluation is needed to determine whether wider patterns offset higher cost
• Despite greater energy, fragmentation gains are not automatic
• Emulsions are generally more supply chain stable and widely supported in bulk systems

Heavy ANFO/blends

Heavy ANFO refers to any blend of ANFO and emulsion, typically mixed at the point of loading using bulk delivery trucks. These blends combine the low-cost gas energy of ANFO with the water resistance, density and improved energy confinement of emulsions.

Common ratios range from 20/80 to 70/30 (emulsion/ANFO by weight), with formulations tailored to site-specific ground conditions and water presence.

Chemically, heavy ANFO is not a true emulsion; it is a heterogeneous mixture of two explosives with differing physical properties. ANFO serves as the gas-generating component, while the emulsion adds density, water resistance and a degree of shock energy. The result is a hybrid explosive that improves performance over straight ANFO – particularly in partially wet or unconsolidated boreholes, while maintaining lower cost than full emulsion systems.

Detonation velocity for heavy ANFO blends typically falls between 3,800 and 5,200 meters per second, depending on mix ratio, density and confinement. These blends retain enough density (generally 1.0 to 1.25 grams per cubic centimeter) to support more reliable column loading, reduced slumping and better confinement in the borehole.

The primary advantage of heavy ANFO is versatility. It allows operations to apply a single explosive formulation across a pattern with mixed moisture conditions, reducing the need for separate product logistics or hole-by-hole selection.

For example, in patterns where 10 to 30 percent of holes contain water, heavy ANFO may outperform ANFO in consistency and economics without requiring a full transition to emulsion. This efficiency in logistics and cost control is one reason heavy ANFO is widely used in modern quarry operations.

Still, heavy ANFO is often difficult to obtain proper oxygen balance with, and as a result, is more prone to errors and quality control issues.

Additionally, due to the impossibility of properly mixing these products, heavy ANFO often results in the generation of toxic gases, such as nitrous oxides, in larger quantities than a properly manufactured ANFO or emulsion.

Loading method

Heavy ANFO is typically delivered and mixed on bulk trucks, where emulsion and ANFO are combined prior to discharge into the borehole. Some systems premix the blend, while others dynamically blend at the point of delivery, offering flexibility for field adjustment.

Some performance characteristics to consider:

• Higher density and water resistance than ANFO
• Retains good gas energy characteristics for effective fragmentation
• Lower cost than a standard emulsion product

Additionally, some field considerations to make include these:

• Blends must be homogenous; separation of components during loading can lead to inconsistent detonation.
• Not suitable for fully flooded boreholes; water resistance is improved but not absolute.
• Booster placement and column integrity remain critical to full detonation.

Primer and booster systems

Primers and boosters serve a single, critical purpose: to ensure the reliable initiation of the main explosive column. This process is governed by initiation energy thresholds and detonation transfer efficiency – both of which depend on precise placement, correct product selection and effective coupling.

A primer is a high-energy initiating unit, typically consisting of a detonator (cap) and a high-velocity secondary explosive charge – often a cast or pressed composite of TNT (trinitrotoluene) and PETN (pentaerythritol tetranitrate), or in some systems, RDX (cyclotrimethylene trinitramine). These compositions are selected for their high detonation velocities (6,900 to 8,400 meters per second) and sensitivity to initiation stimuli, ensuring the cap can initiate the booster, which, in turn, initiates the column.

The primer is inserted in direct contact with the main charge, whether ANFO, emulsion or a blend, to transmit a shock-induced detonation front. This is not a low-order deflagration or sympathetic event.

True detonation is required to initiate the column. Explosives like ANFO, with relatively low sensitivity and a high critical diameter, cannot be reliably initiated by a cap alone. They require a high-density, high-energy booster to create a stable detonation wave across the column cross-section.

From a scientific standpoint, the booster acts as an intermediate charge to bridge the energy gap between the cap and the column. It must release sufficient shock energy and pressure to exceed the main explosive’s critical initiation threshold. Insufficient booster mass, poor coupling (air gaps) or misplacement (i.e., not positioned at the base of the column) can result in partial detonation, low-order firing or misfire.

Booster performance is defined by:

• Density (typically 1.6 to 1.7 grams per cubic centimeter for cast TNT-based boosters)
• Detonation velocity (7,000 to 8,000 meters to second)
• Critical diameter (to ensure full detonation transmission)

The long-circulated concern of “overdriving” explosives, where a primer of too high energy improves the performance of the column, has been shown to be largely irrelevant in practical field conditions. While localized overdriving effects may occur in extremely short columns or tight decks, they do not materially affect full-column detonation in quarry-scale holes.

Proper priming must ensure

• The detonator is fully seated and secured in the booster cavity
• The booster is in firm, continuous contact with the base of the main explosive column
• The column is loaded to maintain confinement around the booster to promote energy transfer
• No gaps or slumps exist between the primer and the main charge

In short, a primer either works or it doesn’t. And when it doesn’t, no downstream adjustment can recover the lost energy or control. Field operations must treat primer selection, booster mass and placement with the same scrutiny given to the rest of the design. Reliable initiation is not an art; it is engineered, calculated and verified.

Electric detonators

Once the standard in surface blasting, electric detonators are now mostly used to start non-electric systems.

Traditional electric caps used pyrotechnic delays triggered by a current sent from a blasting machine. While still accurate when properly used, they are more vulnerable to stray current and static, requiring strict control measures. They are rarely used alone in modern quarries.

Non-electric (shock tube) systems. Non-electric systems use a thin, plastic tube filled with a reactive powder – typically a mix of HMX and aluminum – that transmits a shock wave when initiated. The wave initiates a pyrotechnic delay housed in the in-hole detonator. These systems are widely used for their reliability, ease of use and resistance to electrical hazards.

However, delay timing scatter is a known issue. Standard shock tube systems can have 10 to 25 percent variability in delay times, which can affect fragmentation and throw uniformity – especially in large or tightly timed patterns.

Electronic detonator. Electronic systems use programmable microchips and capacitors embedded in the detonator. These are assigned precise delay times – often to the millisecond – using a handheld device or blast control system. The benefits are threefold:

1. Accuracy. Delay scatter is virtually eliminated.
2. Flexibility. Timing can be fine-tuned for vibration control or fragmentation goals.
3. Safety. Built-in testing features allow confirmation of circuit continuity and system integrity before firing.
   

Electronic detonators are not necessary for every blast, but they offer performance and control advantages – especially in complex shots or sites requiring tight vibration management. While current research continues to define optimal delay sequences, electronic systems give operators the ability to experiment and refine timing with a level of precision that shock tube systems cannot match.

Their value is most evident in:

• Close-proximity blasting where timing affects vibration
• Complex benches with irregular timing needs
• High-consequence or production-critical shots

Electric Detonators

Electric detonators, commonly referred to as electric blasting caps, are one of the oldest forms of commercial initiation and, while largely replaced by non-electric systems in surface blasting, still play a role in modern quarry operations.

Electric detonators, commonly referred to as electric blasting caps, are one of the oldest forms of commercial initiation and, while largely replaced by non-electric systems in surface blasting, still play a role in modern quarry operations.

Their primary use today is as starter devices for initiating non-electric blast patterns or for firing standalone shots where controlled delay sequencing is not required.

An electric cap consists of a bridge wire, primary charge and pyrotechnic delay column – all housed within a metallic shell. When a voltage is applied across the leg wires, current flows through the bridge wire, heating it to ignition temperature. This triggers the pyrotechnic delay, which burns for a set time before initiating the base charge – usually PETN or RDX – which then detonates a booster or column charge.

Historically, electric systems were used to build sequential delay patterns, with each cap assigned a different delay period. These systems are highly accurate in timing but susceptible to stray current, static electricity and electromagnetic interference (EMI). This sensitivity made them vulnerable in high-noise environments or in proximity to high-voltage equipment, leading to safety concerns.

Despite being largely displaced by shock tube systems, electric caps remain useful in certain applications due to their low cost, simplicity and timing reliability. When used as a starter system – especially in conjunction with a trunkline delay – they provide a safe and reliable method to initiate the rest of the non-electric pattern.

Non-electric detonators

Non-electric detonators, commonly known as shock tube systems or non-electric blasting caps, are the most widely used initiation systems in surface aggregate blasting today.

Their popularity is driven by reliability, resistance to electrical hazards and ease of deployment across a range of blast sizes and bench geometries. They offer consistent, field-proven performance with lower susceptibility to misfires from static discharge or stray current, while still allowing some flexibility in delay sequencing and pattern design.

At the core of the system is a shock tube – a thin, hollow plastic tube (typically 3 to 4 mm in diameter) with a thin coating of reactive powder on the interior surface. This powder is usually a fine mixture of HMX and aluminum, and when initiated by a percussion starter (like a blasting cap or igniter), it transmits a low-energy detonation-like shockwave down the length of the tube. This wave produces no flame or external blast, making the system inherently safer to handle.

Each in-hole non-electric detonator includes:

• A delay element (pyrotechnic)
• A detonator capsule (high explosive)
• A connector block to join multiple tubes or attach to trunkline delays

Non-electric detonators are available in millisecond (MS) and long-period (LP) delays, with color-coded tubing and tags indicating specific delay times. The system allows blasters to create controlled timing sequences by assigning different delay intervals to rows, holes or decks.

Timing scatter and limitations

While highly effective, non-electric systems are limited by timing variability, known as delay scatter. The pyrotechnic delay elements inside each detonator can vary by ±10 to 25 percent from their rated time due to manufacturing tolerances and environmental factors.

For example, a 500-millisecond delay cap may actually fire anywhere between 375 and 625 milliseconds. This variability can lead to overlapping hole firings or inconsistent burden relief, which may result in uneven fragmentation, poor muckpile shape or elevated vibration.

Despite this, in aggregate quarry applications where blast timing windows are wide and tolerance for timing error is greater, non-electric systems remain the standard. They offer a strong balance of control, simplicity and safety for typical production blasting needs.

Their system advantages include:

• Immune to stray current and EMI
• Safe and simple to handle, store and transport
• No electrical hook-up required
• Easily audited visually for connections and tie-ins
• Compatible with a wide range of boosters and column charges

Field use and best practices

• Always verify shock tube connections are secure – especially in cold or wet conditions
• Anchor tubes properly to prevent movement during stemming or loading
• Store and handle tubes carefully, as physical damage can interrupt signal transmission

Electronic detonators

Electronic detonators represent the most advanced initiation systems available in commercial blasting today.

Built for precision, flexibility and enhanced safety, these systems incorporate programmable microchips, onboard capacitors and digital communication protocols to control exact detonation timing down to the millisecond. In high-risk blasting environments or where fragmentation, vibration control or timing flexibility are critical, electronic detonators offer unmatched performance.

Each detonator contains:

• A programmable circuit board (embedded microcontroller)
• A firing capacitor (charged before initiation)
• A diagnostic resistor (for verification and feedback)
• A high-energy initiator (usually a semiconductor bridge or exploding foil initiator)
• A high explosive base charge (i.e., PETN or RDX)

These components are housed in a sealed metal shell and connected via a twisted-pair copper wire or other durable signal line, allowing robust communication from the blasting console to each individual detonator. Unlike shock tube or electric caps, which are limited to factory-set delays, electronic detonators are field-programmable using handheld or PC-based units, giving the blaster complete control over the delay timing of each hole.

Timing accuracy and cap scatter reduction

The most significant advantage of electronic detonators is timing accuracy.

Traditional pyrotechnic-based systems (i.e., shock tube, electric) have cap scatter ranging from ±10 to 25 percent, depending on environmental conditions and delay length. Electronic systems reduce that scatter to ±0.5 milliseconds or less. This allows for precise sequencing of hole firing, which improves burden relief, fragmentation uniformity, muckpile consistency and airblast control.

This timing control also enables:

• Vibration minimization through exact wavefront separation
• Customized heave profiles for specific muckpile movement
• Decked charge firing with exact interval control between decks

System diagnostics and safety

Before firing, electronic detonators undergo a two-way verification sequence, checking for:

• Line integrity and continuity
• Detonator presence
• Stored energy status (capacitor charged or not)
• Programming accuracy

This diagnostic loop significantly reduces the risk of misfire and improves confidence in shot performance. Additionally, because firing energy is not applied until all checks are passed and confirmed, electronic systems are among the safest initiation platforms in terms of premature initiation risk.

Operational considerations

• Requires a programming unit and a blasting control device
• Higher upfront hardware and detonator cost
• Must be handled and programmed by trained personnel
• Programming errors or damaged communication lines can lead to delays in setup or failure to fire

Value justification

Electronic detonators offer the greatest value when used for:

• Complex bench geometries requiring non-linear timing
• Vibration-sensitive environments (i.e., near structures, utilities or regulated zones)
• Optimization experiments for fragmentation or heave control
• High-precision, large-scale production blasts

Their ability to assign exact timing to every hole transforms the blast from a mechanical sequence into a digitally synchronized detonation event. While electronic systems may not be economically justified for every quarry, their strategic application can significantly elevate performance where timing precision matters most.

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