Question 1 of 7: Explosives Chemistry, Sensitivity, Timing and Blast Effects

Question 1: Explosives Chemistry, Sensitivity, Timing and Blast Effects (40 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

1. Oxygen balance, and the OB of 94% AN / 6% FO

Given. AN = NH₄NO₃ (MW = 14×2+1×4+16×3 = 80 g/mol); FO = CH₂ per repeat unit (MW = 12+1×2 = 14 g/mol); mixture 94% AN / 6% FO by mass.

The oxygen balance (OB) of an explosive compound C_aH_bN_cO_d is the mass percentage of oxygen left over (positive) or still required (negative) once every carbon atom is oxidised to CO₂ and every hydrogen atom to H₂O:

$$OB\%=\frac{1600}{MW}\left(d-2a-\frac{b}{2}\right)$$

1. Component balances. AN (a=0,b=4,d=3): $$OB_{AN}=\frac{1600}{80}\left(3-0-2\right)=+20.0\%$$ FO (a=1,b=2,d=0): $$OB_{FO}=\frac{1600}{14}\left(0-2-1\right)=-342.9\%$$
2. Mass-weighted mixture balance (OB is additive on a mass basis for a physical blend): $$OB_{mix}=0.94(+20.0\%)+0.06(-342.9\%)=18.8\%-20.6\%=\boxed{-1.8\%}$$

The industry-standard stoichiometric (OB=0) ANFO recipe is 94.5% AN / 5.5% FO by mass; the 94/6 mix in this question is very close to balanced, only marginally fuel-rich, so its fume signature leans very slightly toward CO rather than NO_x.

2. Parameters controlling the ideal (maximum) velocity of detonation

The ideal VOD is the steady-state detonation velocity an explosive reaches once its charge diameter is large enough that no further increase in diameter raises the velocity (the "infinite-diameter" limit). It is controlled by: the explosive's chemical composition and energy content (oxidiser/fuel ratio, i.e. oxygen balance); density – VOD generally increases with loading density up to the point of dead-pressing; particle/prill size and uniformity of the oxidiser and fuel, which set reaction-zone kinetics; degree of confinement (ideal VOD is normally quoted unconfined, but full confinement modestly raises the measured value); temperature of the explosive; and, for emulsions/slurries, the type and concentration of the sensitiser (microballoons, gas bubbles, or chemical gassing agent).

3. Parameters controlling critical diameter

Critical diameter is the smallest charge diameter at which stable detonation can still propagate. It is controlled by: composition (oxidiser/fuel combination – emulsions and gelatines have much smaller critical diameters than dry prilled ANFO); particle size of the oxidiser (finer AN prills/grain give a smaller critical diameter because more reactive surface area is available per unit volume); density (higher density generally raises critical diameter, until dead-pressing occurs); degree of confinement (confinement lowers the critical diameter – an unconfined column needs a larger diameter than the same product confined in rock); sensitiser type and content (chemical or physical sensitisers lower critical diameter); and temperature/moisture (water ingress into ANFO raises its critical diameter sharply, sometimes to the point of complete desensitisation).

4. Expected VOD and detonation pressure for three charge configurations

Given. (a) ANFO prills, D=75 mm, ρ=0.8 g/cm³; (b) ANFO prills, D=311 mm, ρ=0.8 g/cm³; (c) Emulsion, ρ=1.1 g/cm³, D=40 mm.

Approach. No VOD-vs-diameter formula is supplied for this sub-question (unlike Q5's explicit attenuation law), so representative values are read from published ANFO/emulsion VOD-diameter curves (Dyno Nobel/Orica technical data, reproduced in Hustrulid and the ISEE Blasters' Handbook): unconfined prilled ANFO has a critical diameter of roughly 75-100 mm and an ideal (infinite-diameter) VOD near 4500-4700 m/s for ρ=0.8 g/cm³; packaged emulsion has a much smaller critical diameter (10-25 mm) so a 40 mm charge is already well above critical. Detonation pressure follows the standard ideal-detonation approximation $$P_d=\frac{\rho\,VOD^2}{4}\ \ (\text{GPa},\ \rho\ \text{in g/cm}^3,\ VOD\ \text{in km/s})$$

1. (a) D=75 mm – just above ANFO's own critical diameter. VOD is low and unstable at this diameter, roughly 2200 m/s: $$P_d=\frac{0.8\times2.2^2}{4}=\boxed{0.97\ \text{GPa}}$$
2. (b) D=311 mm – far above critical diameter. VOD approaches its ideal/limiting value, roughly 4500 m/s: $$P_d=\frac{0.8\times4.5^2}{4}=\boxed{4.05\ \text{GPa}}$$
3. (c) Emulsion, D=40 mm, well above its own small critical diameter. VOD is near its own ideal value for this density, roughly 5000 m/s: $$P_d=\frac{1.1\times5.0^2}{4}=\boxed{6.88\ \text{GPa}}$$

5. When to use a 30/70 vs. a 70/30 Emulsion/ANFO blend

(a) 30% Emulsion / 70% ANFO is used in largely dry holes where only limited water resistance or a modest density/energy boost over straight ANFO is needed – e.g. holes with minor seepage, or where the emulsion is added mainly to raise bulk density and VOD for slightly better fragmentation in moderately hard rock, while keeping overall product cost close to that of plain ANFO.

(b) 70% Emulsion / 30% ANFO is used in wet or standing-water holes where full water resistance is essential (the emulsion phase coats and protects the AN prills from dissolution), and/or where higher density, VOD and detonation pressure are needed for harder, more competent rock or larger burdens – typical applications include deep wet blastholes below the water table, underwater/marine blasting, and any design where ANFO alone would desensitise or wash out before firing.

6. Peak Particle Velocity vs. Peak Vector Sum

Peak Particle Velocity (PPV) is the maximum value reached by a single recorded vibration component (typically the transverse, radial/longitudinal, or vertical channel of a triaxial geophone), read independently at that component's own peak time. Peak Vector Sum (PVS) is the vector resultant of all three orthogonal components computed at each instant in time, $$PVS(t)=\sqrt{L(t)^2+T(t)^2+V(t)^2}$$ with the reported PVS being the maximum of this resultant time history. Because PVS combines components that are genuinely simultaneous, $$PVS \le \sqrt{PPV_L^2+PPV_T^2+PPV_V^2}$$ (the right-hand "pseudo vector sum" of the three independent peaks, which generally overstates the true resultant since the three components rarely peak at exactly the same instant). Regulatory blasting limits are most often written in terms of the single-component PPV precisely because it is simpler to measure and is not sensitive to instrument timing/axis alignment, whereas PVS is sometimes used internally as a more conservative structural-response metric.

7. Air-decking

Air-decking is the practice of leaving a controlled air gap between an explosive charge and the stemming column, or between two separate charge decks within the same hole, rather than loading the explosive as one continuous column. The gap spreads and delays the pressure pulse acting on the surrounding rock (the confined detonation gases must first expand into and compress the air gap before loading the borehole wall over that interval), extending the effective duration of loading at a lower peak pressure for the same total explosive mass. Two examples: (1) a top air-deck between the charge column and the stemming plug, which reduces stemming ejection/flyrock and cuts overbreak in the collar zone while still delivering adequate fragmentation near the top of the bench, at a reduced powder factor; (2) deck separation in a long perimeter/trim hole (smooth blasting or presplitting), where two or more short charge segments separated by stemmed air gaps are spaced along the hole length to control the average linear charge concentration and limit borehole pressure below the crushing threshold, protecting the final wall from overbreak while still covering the full hole length.

8. Selecting an in-hole delay: 100 ms vs. 375 ms vs. 1000 ms

Given. Surface (NONEL) timing: 17 ms between holes in the same row, 3×17=51 ms between rows; candidate in-hole delays: 100, 375, 1000 ms.

Approach. Pyrotechnic (NONEL) delay elements have a manufacturing timing tolerance ("scatter") that is roughly proportional to the nominal delay period – typically on the order of a few percent of the nominal value, growing in absolute terms as the nominal delay grows. For the designed firing sequence (17 ms/hole, 51 ms/row) to be preserved, the absolute scatter of the chosen in-hole delay must stay well below these small surface increments; otherwise a later-designed hole can fire before, or simultaneously with, an earlier one, causing charge overlap (excess vibration/airblast from effectively larger per-delay charge) or out-of-sequence firing (poor relief, flyrock, cut-offs).

1. Estimate absolute scatter at each candidate delay (typical in-hole pyrotechnic tolerance ≈3-5% of nominal): 100 ms → ±3-5 ms; 375 ms → ±11-19 ms; 1000 ms → ±30-50 ms.
2. Compare against the design increments (17 ms/hole, 51 ms/row).

Use the 100 ms in-hole delay. It is the only candidate whose expected timing scatter stays comfortably below the 17 ms/51 ms surface design increments, so the intended hole-by-hole and row-by-row firing sequence (and the associated per-delay charge control for vibration/airblast) is reliably preserved; the 375 ms and especially the 1000 ms delays risk enough absolute scatter to overlap or reverse the designed sequence between adjacent holes and rows.

9. Steps to reduce ground vibration and air blast

• Reduce the maximum charge weight per delay (add more delay periods, deck the charge).
• Increase the number of delays/decks so fewer holes detonate simultaneously.
• Use adequate, well-confined stemming (angular crushed rock, correct length) – poor stemming is a leading cause of excess airblast.
• Avoid excess confinement (correct burden/spacing for the hole diameter and rock mass).
• Select delay timing/sequence to avoid constructive interference between adjacent holes.
• Orient the free face and initiation sequence away from vibration/airblast-sensitive structures.
• Use pre-splitting or smooth (trim) blasting at the perimeter to decouple the blast from the remaining rock mass.
• Avoid exposed detonating cord on the surface (a major, avoidable airblast source); prefer NONEL/electronic surface initiation.
• Reduce explosive VOD/energy near sensitive receivers if vibration limits are being approached.
• Avoid blasting during temperature inversions or with a following wind toward sensitive receivers (both increase airblast propagation).
• Monitor with seismographs/microphones and adjust burden, spacing, charge per delay and stemming based on measured results (site-specific attenuation law).

10. Effect of blasting on downstream operations

Blast fragmentation and muckpile condition affect essentially every downstream step: loading (dig rate, shovel/loader fill factor and cycle time depend directly on fragment size and muckpile looseness/shape), hauling (truck payload efficiency and tyre/structure wear are worse with oversize or poorly fragmented rock), secondary breakage (oversize boulders require costly, slow secondary blasting or hydraulic hammering), and pit-wall/backbreak stability (poor perimeter control damages the remaining rock mass and future slope stability). Of these, the operation most affected is downstream crushing and grinding (comminution): comminution is consistently the single most energy-intensive step in the mine-to-mill chain (commonly 50%+ of total site process energy in hard-rock operations), and the primary crusher/mill feed size distribution is set directly by blast fragmentation. Even a modest improvement in blast fragmentation (finer, more uniform run-of-mine size) yields disproportionately large reductions in downstream crushing/grinding energy and increases mill throughput – this fragmentation-to-comminution linkage is the foundation of the modern "mine-to-mill" optimisation philosophy, which deliberately shifts breakage work from the (comparatively cheap, energy-efficient) blast into the rock, away from the (expensive, energy-intensive) mill.