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.
Collaring accuracy (setup/layout error at the surface); rig alignment and mast rigidity – poor levelling or mast flex lets the hole start off the design angle; rod/steel stiffness and hole depth – long, slender strings deflect and "walk" under their own weight and the bit's side-load, especially in angled holes; ground structure – bedding, joints and alternating hard/soft bands deflect the bit toward the path of least resistance; bit type and condition – a worn or asymmetrically-worn bit drills off-line; operator skill and drill guides/stabilisers; and vibration/torque control during drilling.
Hole deviation changes the actual burden and spacing seen by each charge, which directly distorts the design powder factor at that point in the pattern: excessive local burden gives poor fragmentation, toe problems (unbroken rock at the bench floor) and violent confined "cratering"; insufficient local burden risks flyrock and airblast from an under-confined charge venting toward the free face; converging/diverging holes can cause holes to intersect (sympathetic detonation risk while loading) or leave unbroken pillars between them; and cumulative deviation over a multi-row round degrades overall fragmentation uniformity, muckpile shape and dig rates.
| Fault | Compensation |
|---|---|
| Excessive burden | Increase charge/powder factor locally (heavier bottom charge) or, if severe, add a satellite/relief hole between the intended and actual toe position. |
| Insufficient burden | Reduce the charge in that hole (shorter column, more stemming) and/or increase stemming length to control flyrock/airblast risk. |
| Short holes | Increase subdrilling on adjacent holes to compensate the toe, or accept a higher powder factor bottom charge in the short hole; expect a toe ridge to require secondary breakage if not corrected. |
| Long holes | Reduce charge length (raise the bottom of the charge with extra inert fill) to avoid excessive subdrilling and undesirable sub-grade damage/vibration. |
| Smaller diameter (than design) | Increase charge length (lower stemming top) to recover the design charge mass, since mass per metre falls with the diameter squared; reassess burden/spacing as they were sized for the larger design diameter. |
| Larger diameter (than design) | Reduce charge length / increase stemming to avoid over-charging, or accept the extra mass only if the pattern's burden/spacing is widened to match – otherwise reduce explosive density (aerated product) to hold mass constant. |
Air-bailing (cuttings-removal) velocity up the annulus is controlled primarily by the compressor air volume and pressure delivered down the drill string relative to the annular flow area (hole diameter minus drill-pipe/rod diameter) – a wider annulus at the same air volume gives a lower uphole velocity. It must be kept above the minimum cuttings-lifting (terminal settling) velocity for the largest expected chip size and rock density, which in turn depends on rock hardness/friability (harder rock → larger, denser chips needing higher velocity), hole depth (frictional and hydrostatic air losses increase with depth, reducing effective bottom-hole velocity for a given surface supply), and hole deviation/inclination (an inclined or deviated hole lets cuttings settle back on the low side unless velocity is increased further).
