24-MMP-B2 Rock Fragmentation · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams, 09-Mmp-B2 Rock Fragmentation, May 2015, 3 hours, closed book (one double-sided aid sheet permitted). Five (5) questions constitute a complete paper; every question (1-7) 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.
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.
Charge diameter relative to the explosive's critical diameter – below critical diameter the reaction cannot sustain steady VOD; density – over-compression ("dead-pressing") of a granular or emulsion explosive above its critical density collapses the void/microballoon structure that normally provides hot-spots, killing sensitivity; temperature – both very low temperatures (product becomes sluggish, VOD drops) and elevated temperature (near/above the freeze or exudation point of an emulsion) degrade sensitivity; degree of confinement – a poorly confined or air-gapped charge loses lateral energy and can fail to propagate; water and hydrostatic pressure – can desensitise gassed/sensitised products by collapsing the sensitising bubbles under pressure; ageing/shelf life – crystal growth in AN-based products and bubble coalescence in emulsions reduce sensitivity over time; and primer strength, size and placement – the initiating shock must exceed the explosive's own critical energy threshold.
The minimum primer is the smallest booster (mass and/or detonation pressure) that will reliably bring the surrounding column explosive up to its own steady-state VOD before that shock front has decayed below the explosive's critical initiation threshold. Undersized ("sub-minimum") primers can achieve a low-order, unstable detonation, or fail outright, leaving unreacted or partially reacted explosive in the hole – a serious misfire/undetonated-explosive hazard.
The optimum primer is larger than the minimum but chosen for reliability and consistency rather than the bare theoretical minimum: it should have a detonation pressure and VOD equal to or greater than the column explosive (so it drives rather than lags the column), be centrally/axially located in the charge (not against the wall) to give symmetric shock loading, and be sized with an engineering margin above minimum to absorb manufacturing variability, temperature effects and any deck or coupling losses – in practice, 2-4× the theoretical minimum primer mass is common design practice for bulk ANFO and emulsion columns.
In tunnel/underground blasting, small-diameter (32-50 mm) holes push the charge close to the emulsion's critical diameter, especially in cold rock/wet holes where the emulsion's own microballoon sensitiser can be compressed by hydrostatic water head, raising the effective critical diameter above the hole diameter and causing erratic propagation or "cut-offs" between adjacent charged holes if rock movement from an earlier hole shears or displaces a not-yet-fired charge. In large-diameter, decked surface holes, the opposite problem occurs: the column can be dead-pressed by the detonation shock of an earlier deck in the same hole (dynamic over-compression through the intervening stemming plug), desensitising or destroying the sympathetic-detonation margin of the next deck unless decks are adequately separated and independently primed; sleep time (time between loading and firing) in hot or deep holes can also allow bubble coalescence and desensitisation before the deck actually fires.
Detonating cord (Primacord): surface-cord noise/airblast and cut-off risk if crossed by a later-firing downline; unsuitable alone for precise millisecond timing. Pyrotechnic (non-electric, Nonel) shock-tube: fixed factory delay times only (cannot be re-programmed on site), delay scatter (tolerance) increases with delay number, and shock tubes are vulnerable to being cut by flyrock from an earlier-firing hole in the same round. Electric detonators: vulnerable to stray currents, static electricity and RF/lightning – a significant premature-initiation hazard, and each circuit must be tested and balanced, slowing hook-up on large rounds. Electronic detonators: highest cost per unit and require a dedicated logger/programming and blasting-machine system, plus more complex QA/QC procedures, though they remove the timing-scatter and stray-current drawbacks of the other three systems.
