Underground mining is the process of extracting minerals and ores that are located too far underground to be mined efficiently using surface mining methods. The primary objective of underground mining is to extract ore safely and economically while minimizing handling of waste rock. Typically, underground mine has less environmental impact. However, it is often more costly and may entail greater safety risks for workers than surface mining. There are several different methods of underground mining. The selection of underground mining methods is primarily based on a set of factors associated with geology of the deposit (e.g. Geometry, quality of rock and ore variability) and economics. Moreover, Candidate methods can therefore be chosen and ranked based on estimated operational/capital cost, production rates, availability of labors and materials/equipment, and environmental considerations. The method offering the most reasonable and optimized combination of safety, economics, and mining recovery is then chosen.
Reflecting the importance of ground support, underground mining methods are categorized in three classes on the basis of the extent of support required: unsupported, supported and caving (Adapted from Okubo and Yamatomi, 2005; RitchieWiki Team, 2009).
The unsupported methods of mining are used to extract mineral deposits that are roughly tabular (plus flat or steeply dipping) and are generally associated with strong ore and surrounding rock. These methods are termed unsupported because they do not use any artificial pillars to assist in the support of the openings. However, generous amounts of roof bolting and localized support measures are often used.
- Room-and-pillar mining is the most common unsupported method, used primarily for flat-lying seams or bedded deposits like coal, trona, limestone, and salt. Support of the roof is provided by natural pillars of the mineral that are left standing in a systematic pattern.
- Stope-and-pillar mining (a stope is a production opening in a metal mine) is a similar method used in non-coal mines where thicker, more irregular ore bodies occur; the pillars are spaced randomly and located in low-grade ore so that the high-grade ore can be extracted. These two methods account for almost all of the underground mining in horizontal deposits in the United States and a very high proportion of the underground tonnage as well. Two other methods applied to steeply dipping deposits are also included in the unsupported category.
- Shrinkage stoping is the method characterized by the mining advance being upward, with horizontal slices of ore being blasted along the length of the stope. A portion of the broken ore is allowed to accumulate in the stope to provide a working platform for the miners and is thereafter withdrawn from the stope through chutes Shrinkage stoping is more suitable than sublevel stoping for stronger ore and weaker wall rock.
- Sublevel stoping differs from shrinkage stoping by providing sublevels from which vertical slices are blasted. In this manner, the stope is mined horizontally from one end to the other.
Supported methods of mining are often used in mines with weak rock structure.
- Cut-and-fill is the most common of these methods and is used primarily in steeply dipping metal deposits. The cut-and-fill method is practiced both in the overhand (upward) and in the underhand (downward) directions. As each horizontal slice is taken, the voids are filled with a variety of fill types to support the walls. The fill can be rock waste, tailings, cemented tailings, or other suitable materials. Cut-and-fill mining is one of the more popular methods used for vein deposits and has recently grown in use.
- Square-set stoping also involves backfilling mine voids; however, it relies mainly on timber sets to support the walls during mining. This mining method is rapidly disappearing in North America because of the high cost of labor. However, it still finds occasional use in mining high-grade ores or in countries where labor costs are low.
- Stull stoping is a supported mining method using timber or rock bolts in tabular, pitching ore bodies. It is one of the methods that can be applied to ore bodies that have dips between 10° and 45°. It often utilizes artificial pillars of waste to support the roof.
- Vertical Crater Retreat (VCR) can be either a supported or unsupported method based on the carter blasting technique in which powerful explosive charges are placed in large-diameter holes and fire. Part of the blasted ore remains in the stope over the production cycle, serving as temporary support for the stope walls.
Caving methods of mining are varied and versatile and involve caving the ore and/or the overlying rock. Subsidence of the surface normally occurs afterward.
- Longwall mining is a caving method particularly well adapted to horizontal seams, usually coal, at some depth. In this method, a face of considerable length (a long face or wall) is maintained, and as the mining progresses, the overlying strata are caved, thus promoting the breakage of the coal itself.
- Sublevel caving is employed for a dipping tabular or massive deposit. As mining progresses downward, each new level is caved in into the mine openings, with the ore materials being recovered while the rock remains behind.
- Block caving is a large-scale or bulk mining method that is highly productive, low in cost, and used primarily on massive deposits that must be mined underground. It is most applicable to weak or moderately strong ore bodies that readily break up when caved. Both block caving and longwall mining are widely used because of their high productivity.
There are examples of deposits which can be mined first by surface mining, and then, as the pit deepens, by underground methods. Some deposits are even exploited simultaneously by both methods, typically during the transition period between open pit to underground. Also, every underground mining method requires a point of entrance from the surface (e.g. adit, mine shaft, or vertical or horizontal tunnel). This entry is a key element of the development phase associate with underground mining. (Adapted from Shinobe, 1997; Hartman and Mutmansky – 2002; Dunbar, 2012)
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Shinobe, A.,“Economics of Underground Conversion in an Operating Limestone Mine” M.Sc. Thesis, McGill University, Montreal, Canada. (1997).
Hartman, H. L. and Mutmansky, J. M. “Introductory Mining Engineering”, Published by Wiley, Hoboken, NJ, USA , (August, 2002).
Okubo S. and Yamatomi, “Underground Mining Methods and Equipment, Civil Engineering- Vol. II, Encyclopedia of Life Support System (EOLSS), Tokyo, Japan. 23 pp. (2005).
Dunbar, W. S., “Basics of Mining and Mineral Processing”, Americas School of Mines, University of British Columbia, PWC. (2012). Last accessed on 07/21/2016 at https://www.pwc.com/gx/en/mining/school-of-mines/2012/pwc-basics-of-mining-2-som-mining-methods.pdf
Hustrulid W. A. and Bullock, R. L., “Underground Mining Methods: Engineering Fundamentals and International Case Studies”, Editors, W.A. Hustrulid and R.L.Bullock, Society for Mining, Metallurgy, and Exploration, Inc.(SME) 8307 Shaffer Parkway, Littleton, CO, USA 80127, 718 pages. (2001).
Brady, B.H.G., Brown, E.T., “Rock Mechanics for Underground Mining” 3rd edition reprinted with corrections, Springer, Dordrecht, NL (2006).
Bullock, R. L., “Notes on Underground Mining Methods and Equipments (Mi Eng 324)”, Missouri University of Science and Technology, Rolla, MO, USA, (2010).
RitchieWiki Team, “Underground Mining”, Ritchie Bros. Auctioneers (2012). Last accessed on 07/21/2016 at https://www.pwc.com/gx/en/mining/school-of-mines/2012/pwc-basics-of-mining-2-som-mining-methods.pdf
Feature: Shearer at work in a coal mine. Author: http://www.eickhoff-bochum.de/de/
Figure 1: Underground mining methods. Author: Brady and Brown, 2006
Figure 2: Room-and-pillar. Author: Hustrulid and Bullock, 2001
Figure 3: Shrinkage stoping. Author: Hustrulid and Bullock, 2001
Figure 4: Sublevel stoping. Author: Hustrulid and Bullock, 2001
Figure 5: Cutting and fill. Author: Hustrulid and Bullock, 2001
Figure 6: VCR. Author: Hustrulid and Bullock, 2001
Figure 7: Longwall . Author: Hustrulid and Bullock, 2001
Figure 8: Sub-level caving. Author: Hustrulid and Bullock, 2001
Figure 9: Block caving. Author: Hustrulid and Bullock, 2001