Mining Rule of Thumb

The primary usage of Rules of Thumb should be in the development of conceptual designs and feasibility studies or, when a quick decision is required in the solution of an operating problem. Although an approximated answer, derived from a Rule of Thumb may solve an immediate problem, Rules of Thumb are not a substitute for the application of sound engineering and design methodologies. Although we firmly believe that the presented Rules of Thumb provide great continuing value to our industry, McIntosh Engineering does not guarantee their validity, nor do we (or the referenced individual sources) accept responsibility for application of the Rules of Thumb by others. Where possible, direct quotes have been provided from individual references; however, it is possible that referenced sources may not have directly stated the Rule of Thumb for which they are assigned credit. Although we have endeavored to accurately quote all individual references contained in the Rules of Thumb compilation, we apologize in advance for any misquotes that may be attributed to individual sources. We will provide updates to the Rules of Thumb compilation, as we become aware of corrections that may be necessary.

Field: Explosives and Drilling

Area: Blastholes

• The cost of drilling blastholes underground is about four times the cost of loading and blasting them with ANFO. Present practice is usually based on the historical use of high explosives where the costs were about equal. An opportunity exists for savings in cost and time for lateral headings greater than 12 feet by 12 feet in cross-section by drilling the blastholes to a slightly larger diameter than is customary. Jack de la Vergne
• The "subdrill" (over-drill) for blastholes in open pits is 0.3 times the burden in hard rock and 0.2 times the burden in medium/soft rock. John Bolger
• "Sub-grade" (over-drill) is in the order of 8 to 12 blasthole diameters. Dr. Nenad Djordjevic

Area: Blasting Strength

• Blasting strength is a direct function of density, other things being equal. Typical explosives for dry ground (ANFO) may have a blasthole density (specific gravity) of 0.8 to 1.3, while for wet ground (slurry or emulsion) it varies from 1.1 to 1.3. Developments in explosive technology make it possible to choose any density desired, within the given ranges. Dr. Nenad Djordjevic

Area: Collar Stemming

• The depth of collar for a blasthole in an open pit or quarry is 0.7 times the burden. John Bolger
• The depth of collar stemming is 20-30 times the borehole diameter. Dr. Nenad Djordjevic
• For open pits or back-fill rock quarries, pea gravel of a size equal to 1/17 the diameter of the blasthole should be employed for collar stemming (i.e. ½ inch pea gravel for an 8½-inch diameter hole). Dr. Gary Hemphill

Area: Crater Blasting

• Crater blasting will be initiated if the charge acts as a sphere, which in turn requires the length of a decked charge in the blasthole to be no more than six times its diameter. Mining Congress Journal

Area: Drilling

• Percussion drilling is required for drilling blastholes in rocks with a hardness of 4 or greater on the Mohs' scale (Refer to Chapter 1). These are mainly the volcanic rocks. Rotary drilling is satisfactory for softer rocks, mainly sedimentary. Dr. Gary Hemphill

Area: Explosive Choice

• The strength of pure ammonium nitrate (AN) is only about one-third as great as that of an oxygen balanced mixture with fuel oil (ANFO). Dr. Melvin Cook

Area: Ground Vibration

• The ground vibration produced by the first delay in a burn cut round is up to five times higher than that generated by subsequent delays well away from the cut. Tim Hagan

Area: Labor Cost

• The labor cost for secondary blasting can be expressed as a percentage of the labor cost for primary mucking. For Sub-Level Cave and Crater Blasthole stoping, it is around 30%; for Sub-Level Retreat it is closer to 10%. Geoff Fong

Area: Powder Consumption

• Powder consumption for shaft sinking is 2.5 lb./short ton broken.
  Listed below is typical powder consumption in hard rock.
  Shaft Sinking – 2.5 Lb./short ton broken
  Drifting – 1.8 Lb./short ton broken
  Raising – 1.5 Lb./short ton broken
  Slashing – 0.8 Lb./short ton broken
  Shrink Stope – 0.5 Lb./short ton broken
  O/H Cut and Fill – 0.5 Lb./short ton broken
  Bulk Mining – 0.4 Lb./short ton broken
  Block Cave u/c – 0.1 Lb./short ton to be caved
  Open Pit Cut – 0.9 Lb./short ton broken
  Open Pit Bench – 0.6 Lb./short ton broken
  Various

Area: Relief Holes

• Using a single relief hole in the burn cut, the length of round that can be pulled in a lateral heading is 3 feet for each inch diameter of the relief hole. For example, a 24-foot round can be pulled with an 8-inch diameter relief hole. Karl-Fredrik Lautman
• It has been found that a relief hole of 250 mm (10 inches) will provide excellent results for drift rounds up to about 9.1m (30 feet) in length. Bob Dengler

Area: Spacing and Burden

• For hard rock open pits or backfill rock quarries, the burden between rows can vary from 25 to 40 blasthole diameters. Spacing between holes in a row can vary between 25 and 80 blasthole diameters. Dr. Nenad Djordjevic
• To obtain optimum fragmentation and minimum overbreak for hard rock open pits or backfill rock quarries, the burden should be about one-third the depth of holes drilled in the bench. Dr. Gary Hemphill
• To obtain optimum fragmentation and minimum overbreak for stripping hard rock open pits or quarrying rock fill, the burden should be about 25 times the bench blasthole diameter for ANFO and about 30 times the blasthole diameter for high explosives. Dr. Gary Hemphill
• The burden required in an open pit operation is 25 times the hole diameter for hard rock, and the ratio is 30:1 and 35:1 for medium and soft rock, respectively. The spacing is 1 to 1.5 times the burden and the timing is a minimum of 5 ms (millisecond) per foot of burden. John Bolger
• The burden and spacing required in the permafrost zones of the Arctic is 10-15% less than normal. Dr. Ken Watson
• When "smooth wall" blasting techniques are employed underground, the accepted standard spacing between the trim (perimeter) holes is 15-16 times the hole diameter and the charge in perimeter holes is 1/3 that of the regular blastholes. The burden between breast holes and trim holes is 1.25 times the spacing between trim holes. M. Sutherland