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: Shaft Design

Area: Depth of Shaft

• The depth of shaft should be such as is able to develop 1,800 days mining of ore reserves. 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 is blind, the measurement is taken from its apex. Ron Haflidson
• In the Canadian Shield, a rectangular timber shaft is satisfactory to a depth of 2,000 feet. From 2,000 to 4,000 feet, it's "iffy." At greater depths, rectangular timber shafts should not be employed at all. Bob Brown

Area: Shaft Clearances

• For a fixed guide system employing steel guides, the minimum clearance between a conveyance and a fixed obstruction (i.e. shaft dividers or shaft walling) is 1½ inches for small, square compartments; otherwise it is 2 inches. Jack de la Vergne
• For a fixed guide system employing wood guides, the minimum clearance between a conveyance and a fixed obstruction (i.e. shaft dividers or shaft walling) is 2½ inches for small, square compartments; otherwise, it is 3 inches. Jack de la Vergne
• For a rope guide system in a production shaft, the minimum clearance between a conveyance and a fixed obstruction is 12 inches and to another conveyance is 20 inches. These clearances may be reduced with the use of rub ropes. George Delorme
• The side-to-side clearance between the skip shoes and guides should be designed ¼ inch and should not exceed 3/8 inch in operation. The total clearance face to face of guides should be ½ to 5/8 inches and not exceed ¾ inch. Largo Albert

Area: Shaft Guides

• The single most important requirement of a guide string is to have near-perfect joints. Straightness is the second most important, and verticality probably the third. Jim Redpath
• The force exerted on a fixed guide from a moving conveyance due to imperfections in the guide string varies (1) in direct proportion to the mass of the conveyance, (2) in direct proportion to the square of the speed of the conveyance, and (3) in inverse proportion to the square of the distance over which the deflection takes place. Lawrence O. Cooper
• For purposes of design, the equivalent static lateral force from a shaft conveyance to the guide string may be taken as 10% of the rope end load (conveyance + payload), provided the hoisting speed does not exceed 2,000 fpm (10m/s). Steve Boyd
• For purposes of design, the calculated deflection of wood guides should not exceed 1/400 and that of steel guides 1/700 of the span between the sets supporting them. German Technical Standards (TAS) 1977
• Acceleration values of 8% -10% obtained from a decelerometer test are reasonable rates to expect from a new shaft in good alignment. Keith Jones
• In an inclined shaft, guides are required for the conveyance cars (to prevent derailing) when the inclination exceeds 700 from the horizontal. Unknown

Area: Shaft Inclination

• In hard rock mines, shafts sunk today are nearly always vertical. Inclined shafts are still employed in some developing countries when the ore body dips or plunges at less than 60 degrees. Jack de la Vergne

Area: Shaft Lining

• The concrete lining in a circular shaft may be put into tension and shear by external forces where the horizontal ground stress in one direction is more than twice the horizontal stress in the other. But only if the lining is "stiffer" than the wall rock and/or is subjected to high pressure grouting that may subject the lining to non-uniform compression. Jack de la Vergne
• The stiffness of concrete (Young's Modulus of Elasticity, E) in a shaft lining is approximately 1,000 times the compressive strength of the concrete (i.e. for 3,600 psi concrete, E is approximately 3,600,000 psi, and for 25 MPa concrete, E is approximately 25 GPa). Troxell and Davis
• The concrete lining in a circular shaft develops greater strength than is indicted by standard concrete cylinder tests, because it is laterally constrained. Tri-axial tests indicate this increase to be in the order of 20%. Witold Ostrowski
• The pressure at which grouting takes place through a concrete lining should not exceed 50 psi (345 kPa) in the shaft collar near surface and at depth should not increase beyond the hydrostatic head by more than 25%. Peter Grant
• Non-reinforced (no reinforcing steel) concrete linings in a circular shaft may be subjected to sufficient tension to result in crack propagation if the temperature environment is varied widely. This is especially relevant to design life if the temperature change routinely falls below the freezing point and moisture is present. It is known that concrete subjected to a tensile stress greater than 30 kg/cm2 (425 psi) will crack. The lining of a circular concrete shaft will crack if it is subject to a fluctuation in temperature greater than 200C (36 0F). This is because the coefficient of linear expansion of concrete is 1 x 10-5/0C (0.56 x 10-5/0F) and the maximum allowable elongation of concrete is 2 x 10-4. This explains why shafts in temperate climates will eventually sustain damage to the concrete walls if the ventilation air inside it is not heated during the winter months. Prof. Yu Gonchum, China Institute of Mining and Technology
• A concrete lining may not be satisfactory in the long run for external pressures exceeding 500 psi (3.5 MPa). Concrete is not absolutely impermeable. When subjected to very high hydrostatic pressure, minute particles of water will eventually traverse the lining and as they approach the interior face (under high differential pressure) they will initiate spalling of small particles of the concrete wall. Eventually, over a period of years, repetitive spalling will destroy the integrity of the lining. Grouting through the lining may temporarily arrest this action, but it will eventually resume. Fred Edwards
• A University of Texas study found that substituting 25 to 35% fly ash for Portland cement in high strength concrete could cut permeability by more than half, extending the life of the concrete. Engineering-News Record, Jan/98

Area: Shaft Location

• The normal location of the shaft hoisting ore (production shaft) is near the center of gravity of the shape of the ore body (in plan view), but offset by 200 feet or more. 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: Shaft Orientation

• The long axis of a rectangular shaft should be oriented perpendicular (normal) to the strike of the ore body. Ron Haflidson
• The long axis of a vertical rectangular shaft should be oriented perpendicular (normal) to the bedding planes or pronounced schistocity, if they are near vertical. RKG Morrison
• The long axis of a rectangular shaft should be oriented normal to regional tectonic stress and/or rock foliation. Jack Morris

Area: Shaft Sets

• Tests initiated at McGill University indicate that a rectangular hollow structural section (HSS) shaft bunton will have 52% of the resistance (to ventilation air) of a standard structural member (I-beam). Bart Thompson

Area: Shaft Spill

• For a well-designed skip hoist installation, the amount of shaft spill will equal approximately ½% of the tonnage hoisted. (This rule of thumb is based on interpretation of field measurements carried out at eight separate mines, where the spill typically measured between ¼% and 1% of the tonnage hoisted.) Jack de la Vergne

Area: Shaft Stations

• At the mining horizon, the nominal interval for shaft stations is between 150 and 200 feet; however, with full ramp access to the ore body this interval can be higher, as much as 400 feet. Jack de la Vergne
• Above the mining horizons, shaft stations are not required for access, but stub stations should be cut at intervals of ±1,000 feet, because this is a good distance for safely supporting steel wire armored or riser teck power cables. Jim Bernas
• Above the mining horizons, full shaft stations are not required for access, but intermediate pumping stations are required at intervals not exceeding 2,500 feet (typically 2,000 feet) when shaft dewatering is carried out with centrifugal pumps. They may still be required for shaft sinking and initial development, even though the mine plans for using piston diaphragm pumps for permanent mine dewatering. Andy Pitz
• The minimum station depth at a development level to be cut during shaft sinking is at least 50 feet (15m). Tom Goodell

Area: Timber Shaft

• The classic three-compartment timber shaft employing one hoist for skip and cage service is normally satisfactory for production up to 1,000 tpd, although there are a few case histories with up to twice this rate of production. Jack de la Vergne
• For a timber shaft, the minimum dimension of the space between the shaft timber and the wall rock should be 6 inches. Alan Provost
• For a timber shaft, set spacing should not exceed 8 feet. J.C. McIsaac
• For a timber shaft, catch pits are typically installed every six sets (intervals of approximately 50 feet). Jim Redpath

Area: Ventilation Capacity

• The maximum practical velocity for ventilation air in a circular concrete production shaft equipped with fixed (rigid) guides is 2,500 fpm (12.7m/s). Richard Masuda
• The economic velocity for ventilation air in a circular concrete production shaft equipped with fixed (rigid) guides is 2,400 fpm (12m/s). If the shaft incorporates a man-way compartment (ladder way), the economic velocity is reduced to about 1,400 fpm (7m/s). A.W.T. Barenbrug
• The maximum velocity that should be contemplated for ventilation air in a circular concrete production shaft equipped with rope guides is 2,000 fpm and the recommended maximum relative velocity between skips and airflow is 6,000 fpm. Malcom MacPherson
• The " not-to-exceed" velocity for ventilation air in a bald circular concrete ventilation shaft is 4,000 fpm. Malcom McPherson
• The typical velocity for ventilation air in a bald circular concrete ventilation shaft is in the order of 3,000 fpm to be economical. Jack de la Vergne