1. Introduction
The classic procedure for designing a mine starts by determining the mining method(s) and probable optimum mining rate (discussed in other chapters). This chapter is principally devoted to the next step – determining initial mine layout or “conceptual mine design.” The procedure is also considered initial mine planning.
If the mining method is open pit, the layout starts with the basic design of the open pit itself. This includes pit layouts in intervals up to the final design (ultimate pit). With the pit established, the infrastructure is planned, including surface haul roads, stockpiles, dumps, tailings impoundment, utility corridors, and surface plant layout. The mine layout for an open pit mine might have to be modified if underground mining is contemplated when the pit is exhausted.
If the plan includes underground mining, planning starts with locating and sizing pre-production and on-going development requirements. The initial planning includes determining level intervals, haulage ways, primary access (shaft, ramp or adit), and other major entries. The design of major entries requires considering the requirements for ore handling, waste rock handling, primary ventilation circuit, backfill, transfer, materials handling, access for personnel, refuge stations, and escape route(s). Once the underground mine concept is established, the surface infrastructure is designed, including access roads, dumps, tailings impoundment, utility corridors, maintenance facilities, explosives storage, and surface plant layout.
While the procedures outlined above may appear to be sequential, they are actually iterative to the extent that the process can become tedious. The practical solution for this dilemma is to conduct the exercise employing short-cut methods based on the following activities.
• Comparisons
• Intuitive reasoning
• Rules of thumb
• Tricks of the trade
Comparisons
Comparisons refer to the study of comparable well-engineered projects. In some cases, the layout of another mine may be accepted as a starting model.
Intuitive Reasoning
Intuitive reasoning by the team participants is knowledge-based and relies on rational perception, first-hand mining experience, and good judgment.
Rules of Thumb
Rules of thumb may be applied to break circular references by providing benchmarks and starting points. Rules are also useful in identifying significant planning problems at an early stage.
Tricks of the Trade
Tricks of the trade are particular concepts and efficient procedures employed to save time and effort.
2. Rules of Thumb
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. Source: 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). Source: 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. Source: USBM
• The maximum safe grade over a short distance is generally accepted to be 15%. It may be 12% at larger operations. Source: Kaufman and Ault
• The maximum safe operating speed on a downhill grade is decreased by 2 km/h for each 1% increase in gradient. Source: 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 (twoway), the width of roadway required is 3½ times the width of the widest vehicle. Source: Association of American State 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. Source: Kaufman and Ault
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. Source: Ron Haflidson and others
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). Source: 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). Source: 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. Source: 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. Source: 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). Source: Heinz Altoff
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. Source: 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. Source: Ron Haflidson
• The depth of shaft should allow access to 1,800 days mining of ore reserves. Source: 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. Source: D.F.H. Graves
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). Source: 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. Source: 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. Source: 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. Source: Alan O’Hara
3. Tricks of the Trade
• Job one for mine layout of an open pit (and important for an underground mine) is to obtain aerial photographs and a resulting contour map of the mine area. In the long run, one is better off to get good topographical and survey controls right in the beginning. Source: Richard Call
• Outcrop ore bodies are traditionally open cut to the economic limit, after which mining may take place underground. For a steeply dipping ore body of uniform width, the optimal depth of the open cut is a function of the stripping ratio, which in turn approximates the ratio of underground to surface mining costs. It may be economical to increase the stripping ratio where
waste rock is to be later employed underground as fill. Various Sources
• For an outcrop ore body (ore extends from surface of the bedrock), it is said to be best to mine by open pit down to the point where the cost of mining the last ton is equal to the cost of mining that ton from underground. This concept is not simple to apply. The last cut in the pit is highly profitable and while the first production from underground is the most costly because it
will take months to develop the sequence of stoping required to meet full production capacity. Moreover, the economical depth of an open pit is likely deeper if no underground mining is contemplated. Source: Tim Koniaris
• Haulage costs for open pit are at least 40% of the total mining costs; therefore, proximity of the waste dumps to the rim of the pit is of great importance. Source: Frank Kaeschager
• As a pit deepens, grade control and blending may become more difficult. This is one reason that proposed underground operations should be phased in at an early date. Source: Unknown
• The old rule that says a vertical shaft should be located 200 feet from the crest of an open pit has been proven invalid by sorry experience. The set back distance should be determined by rock mechanics. Source: Jack de la Vergne
• To design the optimum layout for a new underground mine, it is important to first determine the planned mining methods and stoping sequence. Conceptual engineering should be referenced first to the ore body. Mine layout serves the miners; it is not the other way around. Source: Jack de la Vergne
• Bad ground is traversed at less risk with a vertical shaft than a lateral or inclined heading. Where a choice must be made, a shaft should be located in the bad ground and the lateral access to the ore body in the good ground – not the other way around. Source: Jack de la Vergne
• In the case of a deep ore body, it has already been well proven that a twin shaft layout can be used to bring a new mine into a high rate of production at an early stage, which must be the aim of every new mining venture. Sinking two shafts simultaneously also provides desirable insurance against the possibility of one shaft encountering serious sinking difficulties. Source: L.D. Browne
• For deep mines with a natural rock temperature exceeding 1000 F (380 C), the size and location of shafts should be determined mainly by ventilation considerations. Source: Dr. J.T. McIntyre
• A twin shaft layout (shafts close together) for a deep mine will require twin cross cuts to the ore body for an efficient ventilation circuit. It may be better to set the shafts far apart. Source: Jozef Stachulak
• Normally, the concentrator (mill) should be located close to the mine head. Pumping tailings from the mill is less expensive than truck hauling ore over a similar distance. When pumping water to the mill, hauling concentrate from the mill and use of a portion of the tailings for paste fill is also considered, the argument is even stronger. Source: Edgar Köster
• The mine administration offices should be located as near as possible to the mine head to reduce the area of disturbance, improve communications, and reduce transit time. Source: Brian Calver
• When a mine has a camp incorporated into its infrastructure, the campsite should be as close as practical to the mine to minimize the cost of service and utility lines, as well as to expedite emergency call-outs. Source: George Greer
• It is normally false logic to consider particular items of used plant and equipment at the conceptual design stage. The conceptual engineering should consider all new plant and equipment sized and built to provide optimum extraction and recovery. In this manner, a benchmark reference is provided against which opportunities to provide particular items of used
plant and equipment may be later evaluated. Source: Jack de la Vergne
4. Strategy for Underground Mines
Ramp Haulage
For small ore bodies, ramp haulage is the default selection because it normally provides the most flexible and economical choice. (In a cordillera, the terrain may provide relief adequate for a level entry or “adit.”) A ramp (or adit) drive can typically be oriented to provide an underground diamond-drilling base and provide shorter crosscuts to the ore zone. The crosscuts are provided rapidly and economically because they provide a second heading for the main drive. It is possible to sink and develop from a shaft at the same time; however, this is a difficult and expensive procedure.
Another advantage to the ramp or adit entry is direct access by mobile equipment when trackless mining is to be employed. For a typical shaft, the equipment must be dismantled and reassembled underground. The set-up time required to initiate ramp driving is usually shorter than for a shaft. One to three months may be required to provide access and collar a ramp portal, while the collar, hoist, and headframe required for a shaft may take six months of site work.
For medium sized ore bodies, ramp haulage may still be the best choice where the ore body is relatively flat lying. In this case, the ramp may have to be enlarged to accommodate larger trucks. In some cases, it may be practical to provide twin ramp entries to handle two-way traffic.
Belt Conveyor
For large flat-lying ore bodies, a belt conveyor is typically the most economical method of hoisting ore. The legs of the conveyor are put into a ramp that has been driven straight (i.e. a “decline”) for each leg of the proposed conveyor way. If the soil overburden is very deep, or deep and water bearing, a ramp or decline may not be a practical method due to the extraordinary cost of excavating and constructing a portal. If the ground (rock) beneath the overburden is not competent or is heavily water bearing, a ramp or decline access may be impractical due to the driving time and cost.
Shaft System
For large steeply dipping ore bodies, a shaft system is usually best. In this scenario, it may be advisable to have a ramp entry as well to accelerate the pre-production schedule and later to provide service access to the mine.
Conventional Methods of Ore Transport
At the conceptual stage, it is normally better to consider only conventional methods for the transport of ore and resort to the unusual methods only under unusual circumstances. A good example of “unusual” is the aerial tramway installed across a fjord at the Black Angel Mine in Greenland to access an ore body located high on a cliff face.
Table 4-1 lists methods employed at underground mines for ore transport.
Table 4-1 Ore Transport Methods Employed at Underground Mines
The following flow chart (Figure 4-1) summarizes rules of thumb for bringing ore to surface from an underground mine. The flow chart is only a guideline. For example, one underground mine employed truck haulage (for a very small rich ore body) to a vertical depth of 700m before the operation was abandoned.
Figure 4-1 Moving Ore to Surface
Main Entry
The foregoing strategy determines a main entry to the underground on the basis of ore transport. In many cases, this entry also serves for personnel and materials transfer, particularly at small operations. Consideration should be given to a separate entry for man and materials handling when it can be afforded. For example, some mines use the production shaft for ore/waste hoisting, main exhaust, and alternate escape while a second shaft provides cage service in the main fresh air entry.
If a shaft system is employed at an operation of substantial capacity, it is not uncommon to find a ramp access from surface as a third entry. This is a logical progression when an internal ramp system is required by the mining method to be employed. The access ramp has advantages described in previous text of this chapter.