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: Mine Layout

Area: Crown Pillar

• A crown pillar of ore beneath the open pit is usually left in place while underground mining proceeds. The height of the crown pillar in good ground is typically made equal to the maximum width of stopes to be mined immediately beneath. When the overburden is too deep, the ore body is not mined by open pit, but a crown pillar is left in place of height the same as if it were. If the outcrop of the ore body is badly weathered ("oxidized") or the ore body is cut by major faults, under a body of water or a muskeg swamp - the height of the crown pillar is increased to account for the increased risk. Ron Haflidson and others

Area: Mine Entries

• Small sized deposits may be most economically served by ramp and truck haulage to a vertical depth of as much as 500m (1,600 feet). Ernie Yuskiw
• A medium-sized deposit, say 4 million (short) tons, may be most economically served by ramp and truck haulage to a vertical depth of 250m (800 feet). Ernie Yuskiw
• In good ground, at production rates less than one million tons per year, truck haulage on a decline (ramp) is a viable alternative to shaft hoisting to depths of at least 300m. G.G. Northcote
• Shallow ore bodies mined at over 5,000 tpd are more economically served by belt conveyor transport in a decline entry than haul trucks in a ramp entry. Al Fernie
• As a rule, a belt conveyor operation is more economical than rail or truck transport when the conveying distance exceeds one kilometer (3,281 feet). Heinz Altoff

Area: Pit Layout

• The overall slope (including berms, access roads, and haul roads) of large open pits in good ground will eventually approach the natural angle of repose of broken wall rock (i.e. 38 degrees), except for the last few cuts, which may be steeper. Jack de la Vergne
• When hard laterites are mined in an open pit, safe pit slopes may be steeper than calculated by conventional practice (as steep as 50 degrees between haul roads). Companhia Vale do Rio Doce
• For haul roads in general, 10% is the maximum safe sustained grade. For particular conditions found at larger operations, the grade has often been determined at 8%. It is usually safe to exceed the maximum sustained grade over a short distance. USBM
• The maximum safe grade over a short distance is generally accepted to be 15%. It may be 12% at larger operations. Kaufman and Ault
• The maximum safe operating speed on a downhill grade is decreased by 2 km/h for each 1% increase in gradient. Jack de la Vergne
• Each lane of travel should be wide enough to provide clearance left and right of the widest haul truck in use equal to half the width of the vehicle. For single lane traffic (one-way), the travel portion of the haul road is twice the width of the design vehicle. For double lane (two-way), the width of roadway required is 3½ times the width of the widest vehicle. American Association of Highway Officials (AASHO)
• A crushed rock safety berm on a haulage road should be at least as high as the rolling radius of the vehicle tire. A boulder-faced berm should be of height approximately equal to the height of the tire of the haulage vehicle. Kaufman and Ault

Area: Shafts

• The normal location of the production shaft is near the center of gravity of the shape (in plan view) of the ore body, but offset by 200 feet or more. Alan O'Hara
• The first lift for a near vertical ore body should be approximately 2,000 feet. If the ore body outcrops, the shaft will then be approximately 2,500 feet deep to allow for gravity feed and crown pillar. If the outcrop has been or is planned to be open cut, the measurement should be made from the top of the crown pillar. If the ore body does not outcrop, the measurement is taken from its apex. Ron Haflidson
• The depth of shaft should allow access to 1,800 days mining of ore reserves. Alan O'Hara
• For a deep ore body, the production and ventilation shafts are sunk simultaneously and positioned within 100m or so of each other. D.F.H. Graves

Area: Underground Layout

• Footwall drifts for blasthole mining should be offset from the ore by at least 15m (50 feet) in good ground. Deeper in the mine, the offset should be increased to 23m (75 feet) and for mining at great depth it should be not less than 30m (100 feet). Jack de la Vergne
• Ore passes should be spaced at intervals not exceeding 500 feet (and waste passes not more than 750 feet) along the footwall drift, when using LHD extraction. Jack de la Vergne
• The maximum economical tramming distance for a 5 cubic yard capacity LHD is 500 feet, for an 8 cubic yard LHD it is 800 feet. Len Kitchener
• The amount of pre-production stope development required to bring a mine into production is equal to that required for 125 days of mining. Alan O'Hara