24-MMP-B2 Rock Fragmentation · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams, 09-Mmp-B2 Rock Fragmentation, December 2016, 3 hours, closed book (one double-sided aid sheet permitted). Question 1 plus four (4) of Questions 2-6 constitute a complete paper; every question (1-6) is answered in full as a complete study resource.
Reference texts: Persson, Holmberg & Lee, Rock Blasting and Explosives Engineering; C.J. Konya & E.J. Walter, Rock Blasting and Overbreak Control (FHWA); ISEE, Blasters' Handbook, 18th ed.; W. Hustrulid, Blasting Principles for Open Pit Mining; SME Mining Engineering Handbook, 3rd ed., Ch. Drilling and Blasting; W.I. Duvall & C.F. Fogelson, USBM RI 5514 (cratering theory); D.E. Siskind et al., USBM RI 8507 (vibration/airblast).
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.
Given. Air-bubble-sensitized emulsion, borehole diameter 165 mm, length 17 m, adequate priming (a full-strength primer/booster at the collar-most initiation point).
Find. The expected shape of VOD as a function of depth along the column.
Because the diameter (165 mm) is well above the critical diameter of a properly gassed bubble-sensitized emulsion, VOD rises rapidly from the primer's initiation velocity over a short run-up (build-up) zone, typically the first 1–2 m of column, as the reaction front accelerates from the primer's shock toward the explosive's own steady-state detonation velocity. Once run-up is complete the VOD reaches a plateau close to the emulsion's ideal (steady-state) VOD – for a 165 mm bubble emulsion this is typically in the 4.5–5.0 km/s range – and stays essentially constant (small measurement scatter only) for the bulk of the 17 m column, since the hole is uniformly primed, well above critical diameter throughout, and the emulsion's density/sensitisation is assumed uniform. A gassed emulsion's VOD can drift slightly with depth if hydrostatic pressure from the column above compresses and shrinks the sensitizing bubbles in the lower part of a long, wet hole (raising local density and hence critical diameter) – this is why performance in very deep/wet holes is monitored rather than assumed constant.
Fumes form when the detonation reaction departs from ideal, fully-balanced combustion. CO results from an oxygen-deficient mixture (negative oxygen balance) – not enough oxygen is available to fully oxidise the fuel's carbon to CO₂, common with excess fuel oil, water contamination, poor mixing, sub-critical diameter/poor confinement, or a low/unstable VOD reaction that does not go to completion. NOx results from an oxygen-rich mixture (positive oxygen balance) – the free oxygen reacts with the AN's nitrogen instead of the nitrogen staying as inert N₂, common with too little fuel oil or a formulation error. Poor blast design (inadequate stemming/confinement, wet holes, old/deteriorated product) promotes both by driving incomplete or off-design reaction.
Non-electric (shock-tube/pyrotechnic) detonators use a pyrotechnic delay element whose burn time has real scatter (a few percent of nominal, growing with delay number) – successive detonations in a delay sequence can therefore overlap or even fire out of designed order if two nominal delays are close enough that their scatter bands intersect, and a nearby detonation's shock can prematurely (shock-)initiate an as-yet-unfired unit, causing a "cut-off." Electronic detonators use a programmable timing chip and are essentially immune to burn-rate scatter (accuracy of ±0.01–0.1 ms vs. several percent for pyrotechnic units), so delay sequences fire in the exact designed order even with very short (sub-10 ms) intervals, giving tighter vibration/airblast control and virtually eliminating scatter-driven cut-offs – at higher cost and requiring a compatible blasting machine/logger.
Flyrock results whenever excess explosive energy is directed toward a free surface with too little rock mass/confinement to absorb it as fragmentation and displacement, so the surplus converts to kinetic energy of ejected fragments. Common causes: excessive powder factor / overcharging relative to the available burden; insufficient burden (drilling error, poor as-built pattern, or hole deviation reducing the true burden locally); inadequate or poor-quality stemming (allows early gas venting/cratering at the collar); structural weaknesses or voids (joints, mud seams, old blast-damaged rock) intersecting the charge and venting energy along an unplanned path; poor timing/cut-offs redirecting energy into an already-relieved or wrongly-confined face; and face burden variability on an irregular or previously-blasted face.
Powder factor (specific charge, kg explosive per m³ or tonne of rock) directly controls fragmentation size and muckpile looseness/diggability, which cascade through every downstream process: a low powder factor gives coarse, poorly-loosened muck that increases loader/shovel dig time and bucket-fill factor loss, increases secondary breakage (blasting or hydraulic hammer) cost, and can oversize crusher feed (bridging, increased crusher wear and reduced throughput). A high powder factor over-fragments (wasting explosive cost and increasing fines, which can hurt heap-leach permeability or concentrator grinding-circuit throughput if too many ultrafines are produced), while also increasing vibration/airblast/flyrock risk and muckpile heave/swell (affecting dig efficiency and truck loading geometry). The optimum powder factor therefore balances drilling+blasting cost against downstream loading, crushing/grinding and hauling cost – a classic "mine-to-mill" total-cost optimisation, not a fragmentation-only one.
Final wall quality (minimal overbreak/backbreak, a stable, planar face) is controlled by: final-row/perimeter hole spacing and burden (tight spacing relative to diameter, per smooth-blasting/presplit guidelines); decoupled, reduced-energy charge in perimeter holes (vs. full production charge); drilling accuracy and hole parallelism (deviation destroys the intended uniform burden along the wall); stemming length/quality on the perimeter row; timing (perimeter row fired with adequate relief already established, or presplit fired ahead of the main blast with no relief at all); and rock structure (joint orientation/spacing relative to the wall – structurally-controlled overbreak cannot be fully eliminated by blast design alone).
Primarily by digital image analysis of the muckpile or a conveyor stream (e.g., WipFrag, Split-Desktop/Split-Online) – photographs are segmented into individual fragments and converted to a size distribution (X50, X80, etc.) via a delineation algorithm and a shape/scale calibration. Secondary methods include manual/mechanical screening (sieving) of a representative sample (accurate but slow, and impractical at production scale), and simple visual/photographic comparison against a reference size chart for quick field estimates.
At the muckpile face immediately after the blast (primary QA/QC of the blast design itself); on the excavator bucket/truck bed or crusher feed conveyor (to correlate fragmentation with dig rate and to catch oversize before it reaches the primary crusher); at the crusher discharge/mill feed (grinding-circuit throughput optimisation, mine-to-mill studies); and in secondary breakage/oversize piles (tracking how much material required costly rework, feeding back into powder-factor decisions for the next round).
Directly: VOD probes (continuous fibre-optic or resistance-wire VOD recorders logging detonation velocity along the charge column in real time) confirm the explosive actually reached (near-)ideal, steady-state detonation rather than a low-order or failed reaction. Indirectly: vibration/airblast monitoring (seismographs) gives energy actually transmitted to the ground/air; high-speed video of face movement/timing confirms designed delay sequence and relief; and fragmentation/muckpile profile analysis (image analysis, dig-rate/productivity data) is the ultimate downstream measure of whether the explosive did the intended work.
Inter-hole delay (within a row) controls how much relief each successive hole has when it fires, sets the effective burden/spacing each hole "sees," and governs vibration/airblast summation (holes closer together in time than ≈8 ms sum as one larger simultaneous charge). Inter-row delay controls the time available for the row ahead to move and create a new free face before the next row fires – too short and the following row fires into an unrelieved face (poor fragmentation, high vibration, back break); too long risks the previous row's muck settling/choking the new free face or excessive throw/looseness. Both together set the blast's overall progression direction, muckpile shape and throw, and are the primary levers for controlling fragmentation/vibration/flyrock simultaneously.
(1) Power/rotation source (rotary head, hydraulic motor, or top-hammer/DTH percussion mechanism) that drives penetration; (2) Drill string (rods/pipe or a single integral shaft) that transmits torque, thrust and (for percussion drills) impact energy from the machine to the bit; (3) Bit (button/tricone/drag bit) that does the actual rock-breaking at the hole bottom; and (4) Flushing (bailing) system (compressed air, water, or air-mist) that removes cuttings from the hole and cools the bit.
Hole diameter sets the achievable burden/spacing and charge per unit length (fragmentation coarsens roughly with the geometric scale of the pattern). Drilling accuracy/deviation directly changes the true burden a charge sees – a deviated hole locally over- or under-bulks the burden, coarsening or over-breaking fragmentation unevenly across the round. Hole depth and subdrill control whether the charge reaches grade (inadequate subdrill leaves toe humps/boulders). Drilling pattern (burden/spacing ratio and layout) sets the powder factor and relief geometry for a given charge. Collaring accuracy and verticality/angle affect both burden uniformity and face profile. Good drilling practice is therefore a prerequisite for the blast design's assumed powder factor and timing to actually be delivered in the ground.
