24-MMP-B2 Rock Fragmentation · December 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams, 09-Mmp-B2 Rock Fragmentation, December 2015, 3 hours, closed book (one double-sided aid sheet permitted). Question 1 plus four (4) of Questions 2-7 constitute a complete exam 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.
Compressed-air (or air-mist/foam) requirements for a rotary blasthole drill depend on: hole diameter (sets the annular cross-section to be swept), penetration rate and bit type (higher drilling rates generate cuttings faster, demanding more air), formation hardness/abrasiveness, hole depth (affects line losses and the static head of cuttings/air column to be overcome), the required bit face/flushing pressure for adequate bottom-hole cleaning, and the desired margin above the minimum bailing velocity needed to lift cuttings (part b/c).
The actual (available) air velocity in the annulus depends on: compressor output (volumetric flow rate at the given pressure and air density), the annular cross-sectional area (hole diameter minus drill-string/bit diameter), air density (itself a function of altitude, temperature and compressor discharge pressure), and any leakage or pressure losses along the drill string and connections.
The required (minimum) bailing velocity to keep cuttings in suspension depends on: cuttings size distribution and particle density (specific gravity of the rock, which sets each particle's terminal settling velocity), hole depth/angle (a longer or inclined annulus needs more margin above terminal velocity for reliable transport), the desired penetration rate (faster drilling produces cuttings faster, raising the required lift rate), and hole-cleaning efficiency needed to avoid bit balling, reduced penetration rate, or cuttings regrinding.
At higher altitude, ambient air density is lower, so a compressor of a given displacement delivers a lower mass flow of air at a given discharge pressure – directly reducing both the effective bit-cleaning/cooling airflow and the cuttings-lifting capacity of the same nominal compressor. Diesel-engine-driven compressors also lose power output at altitude (reduced combustion-air density), further derating available air/hydraulic power. Practical consequences: either a larger/higher-pressure compressor must be specified, or the achievable penetration rate must be reduced to match the reduced available bailing capacity, and larger-diameter air lines/hoses may be needed to keep line losses acceptable at the lower air density.
Blasthole drilling deviation is controlled by: geological structure (bedding planes, joints, and hardness contrasts deflect the bit toward weaker planes or away from harder zones); drill-string stiffness and stabilisation (stabilisers and rod/rigid-rod stiffness resist lateral deflection); applied thrust/collar weight (excessive thrust for the formation promotes bit walk); bit design and gauge condition (a symmetric, sharp bit tracks straighter than a worn or asymmetric one); hole angle (steeply inclined or near-vertical holes drift differently under gravity than shallow-angle holes); and rig set-up/alignment accuracy at the collar, since any initial mis-alignment compounds with depth.
