SME Chap. 6.1

341

CHAPTER 6.1

Evaluation of Mining Methods and Systems

Michael G. Nelson

INTRODUCTIONThe relative merits of surface and underground mining are widely discussed and frequently debated. Some deposits can be mined entirely with surface methods, while others can only be worked underground. With all other conditions equal, sur-face mining is normally regarded as preferable, because of lower development costs, quicker start-up time, and lower accident rates generally associated with surface mining.

When choosing between surface and underground meth-ods, some of the factors that must be considered include

• Size, shape, and depth of the deposit;• Geologic structure and geomechanical conditions;• Productivities and machinery capacities;• Availability of experienced work force;• Capital requirements and operating costs;• Ore recoveries and revenues;• Safety and injuries;• Environmental impacts, during and after mining;• Reclamation and restoration requirements and costs; and• Societal and cultural expectations.

Some deposits may reasonably be mined entirely by sur-face methods. In general, such deposits are close to the surface and have a relatively uniform geology. Similarly, some depos-its can only be mined economically by underground methods. These deposits are usually deeper, with geological and miner-alogical characteristics that require more selective ore extrac-tion. Finally, other deposits are best mined initially as open pits, with production shifting to an underground method as deeper portions of the ore body are extracted. An example of each type of deposit follows.

In suitable deposits, surface mining is more productive, more economic, and safer for workers. However, changes in environmental regulations and societal expectations may lead to fewer large open-pit mines, particularly if opera-tors are required to backfill open pits and recontour waste dumps. These conditions may result in the development of small, high-grade deposits by very shallow open pits or in the development of high-grade underground mines in place of large open-pit mines. Where applicable, large, low-grade

deposits may be mined by in-situ methods (Hitzman 2005). In some cases, especially in built-up areas, it has become almost impossible to obtain permits for new surface mines. This is the case for producers of crushed stone and dimension stone in large metropolitan areas in many developed countries. For this reason, several underground quarries have recently begun operating in the United States, and many more are in the plan-ning stages.

SURFACE AND UNDERGROUND MINE EXAMPLESIn some cases, the choice of surface or underground is obvi-ous. One such example is the North Antelope Rochelle mine in Wyoming, United States, owned by Peabody Energy.

The North Antelope Rochelle mine shipped 88.7 Mt of compliance coal in 2008 and has produced more than 1,000 Mt since the mine began in 1983. It is the largest coal mining oper-ation in the United States. Remaining coal reserves dedicated to the mine cover nearly 8,800 hectares with about 1,200 Mt of recoverable coal. The coal seam ranges from 18 to 25 m thick and lies from 15 to 105 m below the surface.

The complex employs a large fleet of big equipment, listed in Table 6.1-1. The three draglines have bucket capaci-ties of 84, 76.5, and 65 m3, respectively. The key to success is high volume and low unit costs.

Similarly, in the case of the Henderson mine, underground mining by panel caving was the most logical choice. The Henderson mine, located in Colorado, United States, is owned by Freeport-McMoran Copper and Gold, Inc. A cross section of the Henderson ore body is shown in Figure 6.1-1. Although

Michael G. Nelson, Department Chair, Mining Engineering, College of Mines & Earth Sciences, University of Utah, Salt Lake City, Utah, USA

Table 6.1-1 Major equipment in use at the North Antelope Rochelle coal mine

DescriptionNo.

in Use DescriptionNo.

in Use DescriptionNo.

in Use

Heavy haulers 2 Scrapers 7 Graders 13

Draglines 3 Water trucks 9 Mining shovels 14

Loaders 3 Drills 9 Dozers 27

Backhoes 6 Wheel dozers 13 Haul trucks 73

342 SME Mining Engineering Handbook

the ore body is relatively large, it is also quite deep―about 1,040 m below the top of Red Mountain. Development of an open-pit mine would have required removal of a large amount of overburden before the ore body was exposed. This would have required construction of roads, power lines, and other infrastructure to the top of Red Mountain, at 3,751 m above sea level. Thus development would have been extremely expen-sive, with no initial production to support development costs. The Henderson mine was developed as shown in Figure 6.1-2.

Finally, the Northparkes mine in Queensland, Australia, provides an example of a mine that began as an open pit and is now an underground operation. As shown in Figure 6.1-3, the Northparkes ore body is a narrow porphyry, 200–300 m across and about 900 m in height.

Beginning in the late 1970s, Northparkes was mined with two open pits, each about 150 m deep. Because the ore body has such a small cross section, the stripping ratio increased

rapidly, and in 1993 development was begun for underground mining by block caving, as shown in Figure 6.1-4.

With these examples in mind, it is worthwhile to consider some specific differences between surface and underground mining.

PRODUCTIONMuch more material is produced by surface than by under-ground mining. This is shown in Table 6.1-2, which gives recent data for the United States. It is apparent from Table 6.1-3 that sand, gravel, and stone products represent more than 90% of the material produced in surface mines each year.

The fraction of mined material produced by underground methods in the United States has decreased in recent years, as shown in Table 6.1-3. This results from the decrease in the fraction of coal produced underground; the fractions for metal and nonmetal minerals vary over the period.

Figure 6.1-1 Cross section of the Henderson ore body

Continental Divide

Mine Site

56.3 kmto Kremmling

Rail HeadConcentrator

Mill Site

(SC1)Mill Yard

Drive Houseand Transfer

Station

(PC2/PC3)Transfer

Station andDriveHouse

OverlandProduction Conveyor 3 (PC3)

(6.4 km) Production Conveyor 2 (PC2)(16.1 km)

Ventilation Shaft(Not in Use)

West East

Workers andMaterials Shaft

Red Mountain

Ventilation Shafts

Denver80.5 km

PC1/PC2TransferStation

Inter-LevelRampsProduction

Conveyor (PC1)(1.6 km)

Underground Crusherand Reclaim Gallery

Undeveloped Reserves

7700 Production Level

Future HighliftCave Area

7500 Former Train Level

7225 Production Level7065 Crusher Dump

Exhausted8100 Level

Source: Rech 2001.Figure 6.1-2 Cross section of the Henderson mine

Evaluation of Mining Methods and Systems 343

MINE SIZEIn terms of daily production tonnage, surface mines are almost always larger than underground mines producing the same commodity. This is partially true because open-pit mines must mine much more waste rock (and therefore have much more dilution of the in-situ mineral), whereas many of the under-ground methods can mine the same mineral much more selec-tively, with less dilution and therefore fewer metric tons.

Table 6.1-4 shows approximate daily production rates for selected large surface mines; Table 6.1-5 shows similar data for large underground mines. These tables show the predominance of surface methods for large, high-tonnage operations world-wide. (Tables 6.1-4 and 6.1-5 are not intended to be complete but are included to provide an indication of the respective num-bers and sizes of larger surface and underground operations.)

PRODUCTIVITYWhen productivity is measured in metric tons mined per worker-hour, surface mines are almost always more produc-tive. Table 6.1-6 shows data for coal mining in the United States. During 2006 and 2007, productivity in surface mines was more than three times that in underground mines. However, when choosing a mining method, it is important to go beyond a simple consideration of metric tons per worker-hour. For example, in a gold deposit, it may be more mean-ingful to examine grams or ounces of gold produced per worker-hour. In many gold mining districts, comparing the productivity of the surface mines and underground mines in this way shows much more comparable results.

SAFETYThe mining industry throughout the world continues to reduce the incidence of accidents and fatalities. The underground min-ing environment is recognized as being more hazardous than the surface. Table 6.1-7 shows the incidence rates per 200,000

GEOLOGYLEGENDSulfide Cu-Au OreEquivalent Copper Grade

0.6–0.80.8–1.21.2–2.5>2.5

Andesite Marker Horizon

Quartz Monzonite Porphyry

Silica Flooding

Biotite Monzonite

Zero Porphyry

SCALE

1000 200 m

10200RL 10200RL

10000RL

9800RL

9600RL

9450RL

Surface

Base of Weathering

Base of Oxidation

Gypsum Line

Carbonate Impregnated Zone

10000RL

9800RL

9600RL

10600E

10800E

11000E

Source: House et al. 2001.Figure 6.1-3 Northparkes ore body

Source: House et al. 2001.Figure 6.1-4 Northparkes mine layout


hours for all accidents in the United States during the years 2003–2007; Table 6.1-8 shows the incidence rates for fatal injuries. These rates are higher in all cases for underground mining, and notably higher for underground coal mining.

DEVELOPMENTDevelopment for surface mining of coal and other bedded minerals involves the removing of cover layers of soil and rock to expose the coal. Surface mining is used when the coal seam is relatively close to the surface, usually within 60 m. The time between overburden removal and the min-ing of the product mineral should be as short as possible to optimize overall cash flow. However, for larger depos-its covered by large amounts of overburden and waste, the amount of pre-stripping will also be large, leading to high preproduction development costs. The time required for pre-stripping can range from 2 to 6 years. Thus, interest costs during development will be high and will represent a sig-nificant portion of the pre-mining capital requirement before mining can start. When an ore body is steeply dipping and at or near the surface, open-pit mining can start with a small

amount of stripping. However, as mining of such a deposit progresses, increasing amounts of waste rock must be removed. This must often be done many years before mining of the corresponding amount of ore at deeper levels can take place. Thus, the ultimate pit limits must be projected early in the mine planning process, and the investment cost for waste rock removal in advance of mining must be included in the economic evaluation. Waste rock stripping should be delayed as long as possible to avoid high interest cost for all the money spent in waste stripping activities. The increasing cost of stripping at greater depths is one of the major factors in deciding when to transition from surface to underground mining of a given deposit.

Table 6.1-5 Daily production tonnages for selected large underground mines

Mine t/d Product Country

El Teniente 100,000 Cu ChileGrasberg Underground 50,000 Cu IndonesiaOlympic Dam 25,000 Cu, U AustraliaPalabora 20,000 Cu South AfricaKiruna 40,000 Fe SwedenHenderson 32,000 Mo United StatesNorilsk 30,000 Ni Russia

Table 6.1-2 Ore and coal produced in the United States (in megatons)*Material 2003 2004 2005 2006 2007

Metals

Surface† 1,090 1,190 1,270 1,310 1,330

Underground‡ 14 14 20 16 19

All mines 1,100 1,200 1,290 1,330 1,350

Nonmetals

Surface† 2,850 3,000 3,090 3,130 3,040

Underground‡ 107 139 137 129 129

All mines 2,960 3,140 3,230 3,260 3,170

Coal

Surface† 719 745 763 804 795

Underground‡ 353 368 369 359 352

All mines 1,072 1,112 1,131 1,163 1,147

Total

Surface† 4,659 4,935 5,123 5,244 5,165

Underground‡ 474 521 526 504 500

All mines 5,132 5,452 5,651 5,753 5,667

Source: U.S. Geological Survey 2009; U.S. Energy Information Administration 2009b.*Data are rounded and may not add to totals shown.†Includes materials from wells, ponds, and pumping operations.‡Includes solution mining.

Table 6.1-3 U.S. underground ore and coal production (% of total mine production)

Year

Underground Production, % of total

Metals Nonmetals Coal Total

2003 1.27 3.61 32.92 9.232004 1.17 4.43 33.05 9.552005 1.55 4.24 32.58 9.302006 1.20 3.96 30.88 8.762007 1.41 4.07 30.68 8.82

Courtesy of the Mine Safety and Health Administration.

Table 6.1-4 Daily production tonnages for selected large surface mines

Mine t/d Product Country

CBG Bauxite 32,000 Al GuineaBata Hijau 182,000 Au IndonesiaZarafshan Newmont 38,000 Au UzbekistanGoldstrike 32,000 Au United StatesCripple Creek and Victor 30,000 Au United StatesRhineland Lignite* 274,000 Coal GermanyNorth Antelope Rochelle 251,000 Coal United StatesBlack Thunder 250,000 Coal United StatesCordero Rojo 180,000 Coal United StatesKaltim Prime 100,000 Coal IndonesiaChuquicamata 375,000 Cu ChileEscondida 240,000 Cu ChileGrasberg 240,000 Cu IndonesiaCollahuasi 170,000 Cu ChileBingham 150,000 Cu United StatesEl Abra 120,000 Cu ChileHamersley Yandacoogina 143,000 Fe AustraliaCarajas 100,000 Fe BrazilAlegria 65,000 Fe BrazilSamarco 65,000 Fe BrazilMount Wright 62,000 Fe CanadaIron Ore Company of Canada 60,000 Fe CanadaMt. Keith 32,000 Ni AustraliaSyncrude Oil Sands† 500,000 Oil Canada

Source: Data from InfoMine USA 2009 and Mining-technology.com 2009. *Operation includes three pits.†Operation includes five pits.


In an underground mine, a significant amount of infra-structure must be installed before mining begins. This will include shafts, hoists, ventilation fans, underground shops, travel ways for workers and machinery, ore storage bins, underground crushers, and so forth. This requires detailed long-range planning from the very beginning so that the requirements of future workings at deeper levels can be accommodated. A large capital investment is often necessary before production can start.

Underground mining methods require a more careful design and planning process, because it is difficult to make changes in a design after the infrastructure has been installed and the equipment purchased. This condition is often exac-erbated when variables such as ore grade, mine water make, and ground control conditions change or are different than

expected. It is very important that the underground mine design and the machinery capacities are properly chosen from the beginning. For all of these reasons, it is often prudent to develop a small test mine to accurately determine many of the unknown mine characteristics. A test mine and a properly conducted feasibility study will minimize these risks.

The development of a large underground mine can take as many as 5 to 10 years. Interest costs during development will therefore be high and may comprise 30% to 40% of the pre-mining capital requirement before mining can start.

COST COMPARISONSEstimates of capital and operating costs for surface and under-ground mines of various sizes and configurations are compiled regularly and in considerable detail by InfoMine, Inc. Those estimates are provided to customers as the Mining Cost Service and can be purchased in printed or electronic form, or accessed on-line. The cost estimates do not include permitting, environ-mental analysis, reclamation, or closure costs. Figures 6.1-5 and 6.1-6 summarize the cost estimates for surface mines.

The Mining Cost Service also provides estimates of capi-tal and operating costs for underground mining. The data are more extensive, with estimates for eight mining methods, and shaft and adit access for each. Figures 6.1-7 and 6.1-8 sum-marize selected data.

While surface mining methods are relatively simple and uniformly applied, there are many underground mining meth-ods, and application of any given method will vary from mine to mine. Thus it is much more difficult to accurately sum-marize costs for underground mining methods. Nonetheless, Figures 6.1-5 through 6.1-8 show the following trends:

• For small mines, capital and operating costs per metric ton of ore produced are lower for surface methods. Of course, dilution and ore grade must also be considered in a full economic analysis. For large tonnage produc-tion, capital and operating costs may be higher for surface

Table 6.1-6 Coal mine productivity in the United States

Productivity Data 2006 2007

Underground

Number of mines 666 610

Production, kt 430,374 423,296

Number of employees 47,475 46,723

Productivity, t/worker-hour 3.07 3.04

Surface

Number of mines 875 855

Production, kt 1,199,194 1,196,915



Total

Number of mines 1,541 1,465

Production, kt 1,629,568 1,620,210



Source: U.S. Energy Information Administration 2009a.

Table 6.1-8 Mining fatal injury incidence rates per 200,000 hours in the United States

Year

Metal/Nonmetal Coal Total

Underground Surface Underground Surface Underground Surface

2003 0.02 0.02 0.04 0.02 0.03 0.012004 0.03 0.02 0.04 0.02 0.03 0.012005 0.01 0.02 0.06 — 0.03 0.012006 0.01 0.02 0.08 0.01 0.06 0.012007 0.05 0.02 0.05 0.02 0.04 0.01


Table 6.1-7 Mining accident incidence rates per 200,000 hours in the United States

Year

Metal/Nonmetal Coal Total

Underground Surface Underground Surface Underground Surface

2003 5.41 3.49 8.62 2.88 7.98 3.312004 5.30 3.50 8.16 2.54 7.58 3.222005 5.54 3.44 7.43 2.48 7.05 3.152006 4.64 3.18 7.13 2.38 6.62 2.942007 4.84 2.94 6.78 2.20 6.34 2.72



mines, depending on stripping ratio. In these cases, a dual feasibility study must be performed comparing the open-pit option to the best underground mining option.

• In all cases, capital costs increase and operating costs decrease with increasing production tonnage.

ENVIRONMENTAL AND CLOSURE REQUIREMENTSSurface mines create a much larger footprint than underground mines. In the United States, surface coal mines are required to backfill mine excavations, and recontour and revegetate waste piles. This is not the case for metal, but there are strong indica-tions that the situation is changing. Indeed, in most countries, society no longer looks favorably on large, abandoned excava-tions, and the mandated costs of reclamation and closure for large surface mines are likely to increase. A permit for con-struction of a new surface mine or expansion of an existing surface mine cannot be obtained in some areas. In these cases, underground mining should be examined.

Costs of environmental compliance, reclamation, and closure are seldom published. However in 1999, Mudder and Harvey reported that closure costs for U.S. surface mines ranged from US$1,236 to US$3,707 per disturbed hect-are, with coal mine costs on the higher end of the range at US$2,471 to US$3,707 per hectare. Costs for metal mine sites were lower, yet they were much higher in cases where exten-sive water management and acid rock encapsulation were required. In 1996, Homestake Mining Company reported aver-age company-wide reclamation costs of US$3,361 per hect-are. Between 1980 and 1992, 136 abandoned coal mine sites

in Pennsylvania were reclaimed, at a cost of about US$2,348 per hectare (Bogovich 1992).

In 2004, Wilson and Dyhr estimated environmental and closure costs as a percentage of total operating costs for medium-sized mines with on-site processing and tailings dis-posal. Those estimates are summarized in Figure 6.1-9, where the higher costs associated with surface mining are clearly shown.

SELECTION OF A MINING METHODBased on this brief introduction, it may appear that surface mining is preferable to underground methods, particularly in regard to productivity and worker safety. However, as has been pointed out, selection of the best mining method for any deposit requires analysis of many factors besides the simple productivity in metric tons of ore per worker-hour.

The following subsections discuss in detail the proce-dures for selecting a mining method and include factors that influence the choice between surface and underground mining.

Location of the DepositIn some cases, a mineral deposit may be located in a place where a large surface mine is simply unacceptable. The case of stone and other construction materials, already mentioned briefly, is an excellent example. These materials have a rela-tively low value and are used in large quantities, so it is impor-tant that they be mined as close as possible to the locations where they will be used. Those places are almost always heav-ily built-up areas. For example, the amount of concrete used in

1

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1,000

100 1,000 10,000 100,000

Cap

ital C

osts,

Mill

ion

US$

Daily Ore Production, metric tons

Cut-and-FillEnd SliceSublevel Long HoleRoom-and-PillarSublevel CavingBlock Caving

Figure 6.1-7 Estimated capital costs for six types of underground mines, all with shaft access

1

100

10

100 1,000 10,000 100,000

Ope

ratin

g C

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Mill

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Cut-and-FillEnd SliceSublevel Long HoleRoom-and-PillarSublevel CavingBlock Caving

Figure 6.1-8 Estimated operating costs for six types of underground mines, all with shaft access

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10

100

1,000

10,000

100 1,000 10,000 100,000

Cap

ital C

osts,

Mill

ion

US$


1:12:14:18:1

Figure 6.1-5 Estimated capital costs for surface mines at four stripping ratios

1

10

100

100 1,000 10,000 100,000

Ope

ratin

g C

osts,

US$

/t O

re


1:12:14:18:1

Figure 6.1-6 Estimated operating costs for surface mines at four stripping ratios


Manhattan every 18 months—3.33 million m3—is about the same amount as was used in the construction of the Hoover Dam on the border between Arizona and Nevada in the United States (Owen 2003). In such areas it is difficult, if not impos-sible, to expand an existing stone quarry, let alone open a new one, and the production of stone increasingly comes from underground quarries.

Of course, underground mining methods can also have adverse effects when operated under built-up areas. Surface subsidence and mine water release must both be monitored and controlled. Other factors also enter into choosing the location for a mineral deposit, including processing require-ments, political and social conditions, and work-force avail-ability, which are discussed in the following sec tions. Another factor, environmental and permitting requirements, is not dis-cussed in this chapter.

Geology of the DepositThree aspects of a deposit’s geology relate to the choice of surface or underground mining: the intrinsic value or grade, the morphology, and the structure.

A material with a higher intrinsic value will support a more expensive mining method. For example, Jim Walter Resources mines high-quality metallurgical coal at its Blue Creek mine in Alabama (United States) under very difficult conditions that include spontaneous combustion, deep cover (450 to 730 m), and high methane levels (Howell et al. 1991). A lower-quality coal would not support the high costs of min-ing in this geological setting. Similarly, an unusual narrow-vein gold deposit, where the gold occurs in very high-grade but sporadic pockets, supports a labor-intensive underground mining method (Original Sixteen to One Mine 2009). In gen-eral, deposits with lower intrinsic value or grade are more amenable to surface mining methods when other conditions permit.

Deposit morphology, including shape, extent, and depth, is also important. The economics of most surface mining methods (and some underground methods) are based on high production volume and low unit costs, and use of equip-ment that has high capital costs. These require large depos-its with relatively uniform grade and few irregularities in shape or extent. Deposits that meet these criteria can often be mined profitably, even when the ore grade or product value

is relatively low. Good examples of such surface mines are large coal mines in northeastern Wyoming, as described pre-viously; the large porphyry copper mines, such as Bingham Canyon in Utah (United States) and Chuquicamata in Chile; and the large, low-grade gold mines such as Round Mountain and Goldstrike in Nevada. Similarly, coal deposits that can be mined by the underground longwall method must have large areas of coal with relatively uniform thickness to allow the development and production of large panels that will support the costs of development and purchase of equipment. The depth of a deposit also influences the surface versus under-ground decision. The depth of the Blue Creek mine requires the use of barrier pillars between longwall panels, at a cost that could probably not be supported by a lower-value coal. In other cases, metal deposits are often mined initially by the open-pit method but switch to an underground method when the costs of removing overburden become too high. This has happened, for example, at Kiruna in Sweden, Northparkes in Australia, and Palabora in South Africa.

Finally, the geologic structure of a deposit must be con-sidered. It is more difficult to generalize about this factor, but a good example is the Homestake deposit in South Dakota (United States). George Hearst, who consolidated the claims and put them into production, is reported to have said to his partners, “Here’s to low-grade ore and plenty of it” (Smith 2003). During its 125 years of operation, the Homestake mine produced almost 1.2 million kg of gold and 0.3 million kg of silver. Of course, open-pit mining was unknown when opera-tions began at Homestake, but lacking other information, one might conclude that this large, low-grade deposit was an ideal candidate for that method. However, the deposit was highly folded and faulted, and required selective mining to extract the ore in a manner that could only have been done by under-ground methods.

Processing RequirementsThe processing required to produce an economic product also influences the choice of mining method. It may be possible to mine a low-grade ore at very low cost using a surface method, but the resulting dilution may make processing so expensive that the overall operation is not profitable. In such a case, more selective mining using an underground method may be used to produce a higher-grade ore, which is less expensive to concen-trate. Such selective mining can also be used to leave in place portions of the deposit containing impurities or contaminants that can increase reclamation or remediation costs if they enter the process stream.

It is also important to consider the locations of available processing facilities and the ease with which new facilities can be permitted and built. The difficulty of obtaining permits for new operations in built-up areas for aggregate pits and stone quarries was described previously. The same challenge has been encountered in permitting new coal preparation plants in the eastern United States, and in some cases this has been the main factor in deciding how to mine new coal resources.

Political and Social ConditionsPolitical and social conditions can determine not only whether or not a mineral deposit can be mined, but also the method by which it is to be mined. There may be significant opposition to the large, highly visible disturbance that occurs in surface mining, making permitting too expensive or impossible. In other cases, the legal rights to the minerals in an area may be

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1955 1965 1975 1985 1995 2005

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enta

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Source: Data from Wilson and Dyhr 2004.Figure 6.1-9 Environmental costs as percentage of total operating costs


separated from the rights to control the surface in the same area, so that accessing and removing the minerals by an under-ground method is preferable.

Work ForceIn general, underground mining methods require a more specialized work force than surface methods. Workers with experience in operating heavy equipment in agriculture or construction can often transfer their skills for use in surface mining operations, but underground mining equipment and processes are significantly different. For a deposit in which there is no clear choice between surface and underground min-ing, based on other constraints, the presence or absence of a suitably skilled work force can be a deciding factor.

CONCLUSIONSelection of the best mining method for a given deposit, including the choice between surface and underground min-ing, is a complex process involving the analysis of many inter-related variables. These variables are not just technical; they include consideration of environmental, social, and politi-cal conditions and constraints, and of the time and expense required to obtain the required government permits.

The process is usually iterative in nature, looking at many possible approaches and determining how all the variables interact in each. Mining companies and consultants now use detailed and sophisticated models that incorporate all the tech-nical and financial data, and provide detailed output showing mine and mill production, direct and indirect costs, taxes and royalties, cash flows, internal rate of return, and net present value for each alternative considered. These models often incorporate probabilistic routines for sensitivity analysis so that decision makers can look at how the predicted outcomes for each alternative are affected by changes in the values of key variables such as ore grade, labor and material costs, and commodity prices.

REFERENCESBogovich, W. 1992. Twelve years of abandoned mineland rec-

lamation activities by the U.S. Dept. of Agriculture and Soil Conservation Service in Southwest Pennsylvania. In Land Reclamation: Advances in Research and Technology. Publication 14-92. St. Joseph, MI: American Society of Agricultural Engineers.

Hitzman, M.W. 2005. (R)evolution in mining—Implications for exploration. Min. Eng. 57(1):30–33.

Homestake Mining Company. 1996. Environment, Health, and Safety Report. San Francisco: Homestake Mining Company.

House, M., van As, A., and Dudley, J. 2001. Block caving Lift 1 of the Northparkes E26 mine. In Underground Mining Methods: Engineering Fundamentals and International Case Studies. Edited by W.A. Hustrulid and R.L. Bullock. Littleton, CO: SME.

Howell, R.C., McNider, T.C., and Stevenson, J.W. 1991. Mining with spontaneous combustion problems at Jim Walter Resources, Inc.—No. 5 mine. In Proceedings of the 5th U.S. Mine Ventilation Symposium. Littleton, CO: SME.

InfoMine USA. 2009. Various pages on Web site. www.infomine .com. Accessed July 2009.

Mining-technology.com. 2009. Various pages on Web site. www .mining-technology.com/projects/cbg/. Accessed July 2009.

Mudder, T., and Harvey, K. 1999. The state of mine closure: Concepts, commitments, and cooperation. SME Preprint No. 99-47. Littleton, CO: SME.

Original Sixteen to One Mine, Inc. 2009. Home page. www .origsix.com. Accessed July 2009.

Owen, D. 2003. Concrete Jungle. New Yorker (November 10): 69.Rech, W.D. 2001. Henderson mine. In Underground Mining

Methods: Engineering Fundamentals and International Case Studies. Edited by W.A. Hustrulid and R.L. Bullock. Littleton, CO: SME.

Smith, D. 2003. “Here’s to low-grade ore and plenty of it,” the Hearsts and the Homestake mine. Min. Eng. 55(1):24–29.

U.S. Energy Information Administration. 2009a. Annual Energy Review. Coal. www.eia.doe.gov/emeu/aer/coal .html. Accessed July 2009.

U.S. Energy Information Administration. 2009b. Coal mine safety statistics. www.eia.doe.gov/emeu/aer/coal.html. Accessed July 2009.

U.S. Geological Survey. 2009. Metal and nonmetal mine safety statistics. http://minerals.usgs.gov/minerals/pubs/commodity/m&q/index.html#myb. Accessed July 2009.

Wilson, T.E., and Dyhr, T.M. 2004. Cost trends—Environmental management of mine operations. SME Preprint No. 04-125. Littleton, CO: SME.

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SME Chap. 6.1