3.1 Drilling, Blasting & Explosives Engineering

3.1 Drilling, Blasting & Explosives Engineering

In rock excavation engineering, drilling and blasting constitute the primary unit operation responsible for fragmenting solid rock masses into sizes suitable for downstream loading, haulage, crushing, and mineral processing. The efficiency of the entire mining operation is directly tied to the quality, placement, and chemical energy release achieved during this initial stage.

1. Fundamentals of Rock Drilling Mechanics

Rock drilling systems transfer mechanical energy from a power source into the rock matrix to induce micro-fracturing and structural failure. The three primary methods of mechanical rock penetration are:

• Top-Hammer Percussive Drilling: The impact mechanism (piston) resides in the drill rig outside the hole. Impact energy is transmitted down the drill string through shank adapters and rods to the bit face. Top-hammer rigs exhibit high penetration rates in small-to-medium hole diameters (45 mm to 127 mm), but energy loss at rod joints limits their application depth and hole straightness in hard rock.
• Down-The-Hole (DTH) Percussive Drilling: The pneumatic or hydraulic hammer sits directly behind the drill bit at the bottom of the hole. Because impact energy acts directly on the bit without passing through rod joints, energy transfer remains constant regardless of hole depth. DTH drilling is widely applied in hard rock surface mining for medium-to-large hole diameters (110 mm to 203+ mm).
• Rotary Roller Cone Drilling: Penetration occurs under heavy pulldown force (thrust) and rotation without percussive impact. Tricone bits featuring tungsten carbide insert (TCI) buttons or steel teeth crush and shear the rock through indentation. Rotary drilling is the standard for large-scale, high-bench surface mines operating in soft-to-hard rock with blast hole diameters ranging from 175 mm to over 381 mm.

Bit flushing using compressed air, water mist, or drilling foam is critical to purge rock cuttings from the bit face, clear the annular space, and cool the drilling tools.

2. Blast Hole Pattern Geometry & Design Parameters

Optimal blast design distributes explosive energy evenly throughout the rock mass to achieve targeted fragmentation while minimizing backbreak, toe formation, and ground vibration. Key geometric parameters include:

• Burden ($B$): The distance from a blast hole to the nearest free face in the direction of intended rock displacement. Burden is the most critical parameter; if $B$ is excessive, explosive energy encounters too much resistance, producing severe ground vibration, backbreak, and tight toes. Empirical rules set $B \approx 25 \text{ to } 40 \times D$, where $D$ is hole diameter.
• Spacing ($S$): The distance between adjacent blast holes along a row. Spacing depends on burden, delay timing, and initiation sequence. For square patterns, $S = B$; for staggered patterns with instantaneous or delayed rows, $S = 1.15 \text{ to } 1.4 \times B$.
• Bench Height ($H$): Vertical distance of the bench face. The stiffness ratio ($H/B$) governs bench performance; higher stiffness ratios can improve burden relief in some designs, but no single ratio guarantees optimum fragmentation or low flyrock.
• Sub-Drilling ($J$): Additional hole depth drilled below the planned bench floor elevation ($J \approx 0.2 \text{ to } 0.5 \times B$). Subdrilling helps break the toe region; excessive or insufficient depth has cost, vibration, floor-damage, and toe consequences.
• Stemming Length ($T$): Inert material (typically angular crushed rock chippings, $0.10 \times D$ in size) placed in the upper uncharged section of the hole ($T \approx 0.7 \text{ to } 1.0 \times B$). Stemming helps confine gases and reduce premature venting, airblast, and flyrock, but material quality and length must be calibrated.

3. Explosive Thermochemistry & Physical Properties

An explosive is a chemical compound or mixture that undergoes rapid chemical decomposition upon detonation, producing high-temperature, high-pressure gases that perform mechanical work.

• Velocity of Detonation (VOD): The speed at which the detonation wave travels through the explosive column (typically 3,000 to 6,500 m/s for commercial explosives). High VOD explosives produce strong shock waves suited for hard, dense rock.
• Explosive Density ($\rho_e$): Ranges from 0.8 g/cm³ (loose ANFO) to 1.35 g/cm³ (heavy emulsions). Higher density concentrates energy within the blast hole.
• Detonation Pressure ($P_d$): The pressure generated immediately behind the detonation front in the Chapman-Jouguet plane: An ideal estimate makes detonation pressure proportional to explosive density and the square of VOD; unit consistency, measured VOD, confinement, and the equation of state govern the numeric result.
• Oxygen Balance (OB): The degree to which an explosive contains sufficient oxygen to completely oxidize its carbon and hydrogen into $\text{CO}_2$ and $\text{H}_2\text{O}$. Zero theoretical oxygen balance is a formulation target; it does not eliminate toxic fumes under field loading and detonation conditions. For standard ANFO, the stoichiometric ratio is 94.3% Ammonium Nitrate ($\text{NH}_4\text{NO}_3$) and 5.7% Fuel Oil No. 2: $3\text{NH}_4\text{NO}_3 + \text{CH}_2 \rightarrow 3\text{N}_2 + 7\text{H}_2\text{O} + \text{CO}_2$ Deficiency in fuel oil (positive OB) generates toxic brown nitrogen oxides ($\text{NO}_x$), whereas excess fuel oil (negative OB) generates deadly carbon monoxide ($\text{CO}$).

4. Commercial Explosive Formulations

• ANFO (Ammonium Nitrate / Fuel Oil): The workhorse of mining explosives due to low cost and ease of bulk handling. However, ANFO has zero water resistance and low density (~0.8 g/cm³).
• Emulsion Explosives: Water-in-oil emulsions containing microscopic droplets of ammonium nitrate solution emulsified in a continuous oil/wax matrix. Emulsions exhibit excellent water resistance, high density (1.15–1.30 g/cm³), and high VOD (4,500–6,000 m/s).
• Heavy ANFO: Physical blends of ANFO prills and bulk emulsion (e.g., 70% ANFO / 30% emulsion up to 30% ANFO / 70% emulsion). Blends increase bulk density, energy output, and water resistance compared to pure ANFO.
• Dynamite & Cast Boosters: High-velocity nitroglycerin or PETN/RDX-based cast boosters used as primers to reliably detonate cap-insensitive blasting agents like ANFO and emulsions.

5. Initiation Systems & Timing Dynamics

Initiation systems detonate explosive charges safely and in a precise sequence:

• Electric Detonators: Triggered by electric current; susceptible to stray currents, radio frequency hazards, and lightning.
• Non-Electric (Nonel) Shock Tube Systems: Use plastic tubes lined with reactive HMX/aluminum powder that transmit a low-energy shock wave (~2,000 m/s) without firing the tube itself. Insensitive to electrical hazards.
• Electronic Programmable Detonators: Contain integrated microchips allowing delay programming from 0 to 20,000 ms with $\pm 1 \text{ ms}$ accuracy. Accurate programmable timing improves control of relief and charge per delay, but site response and timing scatter must be measured and destructive wave cancellation cannot be assumed.

6. Powder Factor & Kuz-Ram Fragmentation Modeling

Powder Factor ($PF$) quantifies explosive consumption per unit mass or volume of rock: $PF = \frac{\text{Mass of Explosive (kg)}}{\text{Volume of Rock Broken (m}^3\text{)}} \quad \text{or} \quad \frac{\text{kg}}{\text{tonne}}$ Published powder factors are only starting references; trial blasts, confinement, geology, explosive properties, fragmentation, damage, vibration, fumes, and cost establish the site value.

The Kuz-Ram Model predicts mean fragment size ($x_{50}$) based on rock mass properties, explosive strength, and blast geometry: $x_{50} = A \cdot K^{-0.8} \cdot Q^{1/6} \cdot \left(\frac{115}{RWS}\right)^{19/30}$ where $A$ is the rock factor, $K$ is powder factor (kg/m³), $Q$ is mass of explosive per hole (kg), and $RWS$ is relative weight strength.

7. Ground Vibration Control & Environmental Impacts

Blasting converts a fraction of chemical energy into seismic ground vibrations. Peak Particle Velocity (PPV) measures structural damage risk: $PPV = K \cdot (SD)^{-\beta} = K \cdot \left(\frac{R}{\sqrt{W}}\right)^{-\beta}$ where $R$ is distance to structure (m), $W$ is the effective explosive charge per delay used by the adopted standard or calibrated site model (kg); an 8-ms grouping is one convention, not a universal rule, $SD$ is Scaled Distance, and $K, \beta$ are site constants. Delay selection controls charge per delay, burden relief, fragmentation, displacement, and vibration timing, but it does not guarantee destructive wave interference. Use site-specific vibration records and seed-wave or signature-hole analysis where appropriate rather than assuming cancellation.