Chapter 18.3 SHRINKAGE STOPING JACK HAPTONSTALL
18.3.1 INTRODUCTION Shrinkage stoping is a vertical, overhand mining method whereby most of the broken ore remains in the stope to form a working floor for the miners. Another reason for leaving the broken ore in the stope is to provide additional wall support until the stope is completed and ready for drawdown. Stopes are mined upward in horizontal slices. Normally, about 35% of the ore derived from the stope cuts (the swell) can be drawn off (“shrunk”) as mining progresses. As a consequence, no revenues can be obtained from the ore remaining in the stope until it is finally extracted and processed for its mineral values. The method is labor intensive and cannot be readily mechanized. It is usually applied to ore bodies on narrow veins or ore bodies where other methods cannot be used or might be impractical or uneconomical. The method can be easily applied to ore zones as narrow as 4 ft (1.2 m), but can also be successfully used in ore widths up to 100 ft (30 m). Logically, the broken ore should be free flowing and not pack in the stope. Neither the ore nor adjacent country rock should contain undue amounts of clay or other sticky material to cause the ore to hang together in the stope and either be difficult or impossible to draw. Additionally, the ore should not readily oxidize, which may cause the broken pile to re-cement itself, and consequently “hang up.” Oxidation can also have an adverse effect on ore dressing. Ore should be fairly continuous along the strike of the vein or ore body in order to avoid mining extensive amounts of waste as dilution from the stope back. However, small waste areas may be mined around and left as random pillars. Consideration must also be given to the plunge or rake of the ore body, especially where the entire ore body may be mined as a single stope (Fig. 18.3.1 rather than as pre-established stope panels with defined vertical end lines. A stope with a shallow plunge or rake ( < 50”) may be very difficult to mine by shrinkage methods because the ore moves away too quickly from the pre-
developed extraction system (Fig. 18.3.2). Additionally, stopes where ore abruptly extends for great distances beyond stope end lines are also difficult to mine because they often require much additional development work to the stope extraction system (Fig. 18.3.3), especially raising.
18.3.2 DEVELOPMENT AND PREPARATION Sites for shrinkage stoping are generally developed by drifting in the vein or ore zone on two levels, spaced vertically 100 to 600 ft (30 to 180 m) apart. After a viable ore body has been established, the next phase consists of driving one or more raises to establish vertical ore continuity and also to provide ventilation and access to the stope (Fig. 18.3.1) Raises may be driven conventionally, with Alimak-type raise climbers, or by raise boring machines. Drifting for shrinkage development is normally done by conventional drill-and-blast, track or trackless methods. Stopes may be prepared with extraction raises on 25- to 30ft (7.5- to 9-m) centers over the length of the ore shoot; each raise is fitted with a chute, normally of timber construction. Extraction raises are belled out and “hogged over” as the undercut for the start of the first stope cut. This type of preparation is still used but on a very limited basis. Another method of preparing a stope is to blast down at least two backs of the ore zone, clean up the broken ore, and install stull timbers or timber sets in the drift below the stope. Timber chutes, or even “chinaman” chutes, are installed at approximately 25-ft (7.5-m) intervals as part of the timbering. A more common method of preparing stopes in modern operations is to drive an extraction drift parallel to the ore body development drift, about 25 to 50 ft (7.5 to 15 m) in the footwall of the ore body. Subsequently, drawhole extraction crosscuts are driven from the footwall drift into the ore drift on 25- to 50-ft (7.5- to 15-m) centers. The back of the ore body is then blasted
Fig. 18.3.1. Longitudinal section–typical shrinkage stope.
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SHRINKAGE STOPING
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Fig. 18.3.2. Longitudinal section— shrinkage on shallow-raking ore body.
Fig. 18.3.3. Longitudinal section—shrinkage on irregular ore body.
down, and the swell is extracted through the drawholes, either with rail-mounted mucking machines or load-haul-dumps (LHDs) (Fig. 18.3.4).
18.3.3 STOPING OPERATIONS Once a shrinkage stope has been established, manways are usually installed in the raise from the next level. A manway and service way is normally constructed on one or both endpanels of the stope. Often a timber slide is installed in one of the manways for hoisting and lowering materials into and out of the stope; hoisting is often accomplished with a single-drum air hoist installed in the level below the manway. Once the manways, ventilation raises, and service ways have been established for a stope, mining can commence. Drilling of a shrinkage stope back is accomplished with hand-held stopers or jacklegs, although mechanized drill wagons or stope jumbos may be used in wider stopes. Back stoping is the normal mode of operation, but breasting down is also common. Up-holes are generally 1.8 to 2.4 m (6 to 8 ft) in length. In most cases, all holes are loaded and a complete back is blasted at a time. Breasts are drilled with a 8- to 10-ft (2.4- to 3-m) horizontal holes and normally blasted once per shift.
Holes are loaded with ANFO products or water gels and even with slurry blasting agents. Initiation is commonly with non-electric methods, but electric blasting is also practiced. After a cut has been blasted in a stope, drawdown of the 35% swell is necessary, after which the muck pile must be leveled to facilitate drilling of the next cut. Leveling of the pile can be done by hand shovels in the case of small stope, with 2- or 3drum slushers in larger or longer stopes, and even with LHDs in large stopes. After leveling, drilling of the next stope cut, raising of the manways, and so forth are done to continue the mining cycle. Variations for the establishment of openings for manways, ventilation raises, or service ways may include installation of strategically placed timber cribbed openings, steel culverts or rings, or timber sets within the broken ore area. These installations may be very desirable during the mining phase, but may create safety problems and nuisances with the collapse of the materials used to construct these openings. Pinning, stulling, or wedging these installations to the stope walls may prevent their destruction during drawdown; materials from a destroyed manway may be drawn down with the broken ore into the chutes or drawholes and cause hangups. A stope should have strong, self-supporting walls to permit the application of shrinkage stoping. Dilution through scaling of
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must either be washed down with water, bombed down with explosives, picked down by miners (a practice not recommended), abandoned, or re-mined. In any case, a hung-up stope is a costly and dangerous problem, and shrinkage stoping should not generally be used where the ore has a tendency to hang up. Stopes should usually be drawn down systematically, drawing the pile evenly so if the stope walls do peel or slough, the waste remains atop the pile and does not trap broken ore rilled above the pile. Once a stope drawdown is started, the operator’s control over the walls, pillar recovery, etc., is minimal and in most cases, the re-entry of miners into a stope under active drawdown would be considered too great a safety hazard to risk. Stopes can be drawn down from strategically placed chutes or from drawpoint crosscuts. Haulage from the stope extraction points may be done with rail equipment or LHDs and/or trucks. Chutes should be robustly designed and constructed to avoid destroying them through blasting of large slabs in them. Stopes may also be extracted through slusher trenches developed below the stope.
18.3.5 VARIATIONS AND APPLICATIONS
Fig. 18.3.4. Typical shrinkage stope with LHD extraction.
walls can preclude use of the method. Good mining practice coupled with state and federal regulations may dictate a least a minimum ground support program. Wall and back support may be accomplished by leaving random or even systematic pillars. Pillars left in ore zones may be drilled off and blasted upon drawdown of the stope. Traditionally, timber stulls fitted with plank headboards have been installed to support suspicious slabs or areas of bad ground of stope walls. Horizontal stulls and cribbing are also used to support loose areas of stope backs; however, the timbers may be subsequently buried in the muck pile upon shooting the next stope back, and may become a hazard or, at the very least, a nuisance, upon drawdown of the stope. Rock bolting has evolved into the preferred mode of wall and back support. Mechanical as well as grouted types of bolts are used. Correct installation of bolts in the walls of narrow shrinkage stopes may be difficult because of the lack of room to drill the bolt hole perpendicular to the stope wall as well as to install the required length of rock bolt. Sampling of narrow shrinkage stope backs is usually done by either taking a channel or chip sample by hand or through mechanical means. Sampling is usually done at a systematic interval (say, 5 ft or 1.5 m) along the entire back, ends, and in some cases, the ribs of the stope after every stope cut. In wider stopes, drill sampling of the back and ribs can be done. The drill sample may criss-cross the stope back on a predetermined pattern. Drill cutting samples are collected in a sample bag through a hose and funnel or other device.
18.3.4 STOPE DRAWDOWN One of the most dangerous jobs in a mine is the drawdown of shrinkage stopes, especially where the ore contains sticky material to hang up between the stope walls. Hung-up stopes
Variations of shrinkage stoping include inclined shrinkage and longhole shrinkage. Recovery of large pillars may be done by shrinkage methods. One example of mines that employed shrinkage as a primary stoping method is the Homestake mine at Lead, SD. Fifty-foot (15-m) wide “bull pen” shrinkage stopes were developed transversely across the full width of the great Main Ledge ore body. Stopes were mined over a timbered sill where strategic china chutes were constructed for ore extraction. Stopes were mined over the sill for about 70 vertical ft (20 m) to within 30 ft (9 m) of the next level. Twenty-five-ft (7.6-m) wide pillars were left between stopes, which along with the crown pillars, were subsequently extracted with square-set stopes. Homestake abandoned this type of shrinkage stoping just before World War II. A second example is the Idarado mine located near Ouray and Telluride, CO. Stopes were mined along the veins and the full width of the veins, which varied from 5 to 25 ft (1.5 to 7.6 m). Stope panels were generally 400 ft (122 m) long and were prepared over a slusher trench developed about 20 ft (6 m) over the back of the main level drifts. A series of pockets and raises on 25-ft (7.6-m) centers were developed from the slusher trench and the pockets “hogged over” to form pillars between the trench and the first cut of the stope. Ore was extracted from the stope, slushing from the pockets to a chute in the center of the stope. Stopes were normally mined from level to level or about 200 ft (60 m) along the dip. A variation of the above was practiced at the Morococha and Casapalca mines of the Cerro de Pasco Corp. located in the central Andes mountains of Peru, South America. Stopes in these mines were prepared over the main development level driving 25-ft (7.6-m) raises on 25-ft (7.6-m) centers and “hogging” out from the raises to form the first stope cut at about 16 ft (5 m) over the level. Each raise was then fitted with a timbered chute for ore extraction. In all cases, a raise was first developed through each ore block or stope panel for ventilation and service. Manways were either carried as cribbed raises in the stope or, in the case of Idarado, as boreholes 10 ft (3 m) in the footwall of the vein. In the vein mines, drilling was accomplished with either stopers or jacklegs, while at the Homestake, drilling was done with barand-column mounted Leyner-type drills. Some variations of shrinkage stoping include inclined shrinkage, longhole shrinkage, and construction shrinkage.
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Fig. 18.3.5. Shrinkage stope, Rosiclare, IL.
Fig. 18.3.7. Construction shrinkage stoping by conventional method.
Fig. 18.3.6. Longhole shrinkage stoping.
18.3.7). In many cases, this is done as described in the longhole variation. Given a large enough opening, conventional shrinkage stoping of a shaft or raise may be justified. Inclined shrinkage refers to a rill stoping adaptation where multiple faces or benches for drilling are carried along the back of the stope as it is mined upward (Fig. 18.3.5). Stopes are developed conventionally over pillars and chutes or over timber sets fitted with chutes on centers of about 25 ft (7.6 m). The advantage of carrying the stope in benches is that multiple faces can be drilled in a given shift where it is desirable to drill the stope with airleg-type drills rather than stopers. Longhole shrinkage (Fig 18.3.6) is developed conventionally as described previously. The exception is that drilling of the stope is done from vertical raises driven through the ore zone on 50- to 100-ft (15- to 30-m) centers. Raises can be developed with raise climbers or through cage raising techniques. The raise climber or the cage becomes the entry and exit vehicle as well as the platform for drilling and loading. Parallel longholes are drilled along the strike of the ore body and loaded from the raises. Initiation normally is done from a safe area on the service level above the stope. Shafts, winzes, or large break raises for blasthole or sublevel caving stopes may be developed through shrinkage methods (Fig.
18.3.6 CASE STUDY: LA LIBERTAD MINE, PUEBLO NUEVO, DURANGO, MEXICO The small La Libertad mine was brought on-stream in July 1977 by Minas de San Luis, S.A., a 51% Mexican-owned company (Haptonstall, 1980). The mine was developed entirely for shrinkage stoping. GENERAL. La Libertad was essentially a virgin silver-gold deposit in a very rugged location in the Mexican Sierra Madre mountains. The only previous mining done in the area was a small tonnage extracted from the outcrop of the principal Santa Rosa vein in the 1930s. Production ceased in the mine in 1985 due to political strife in the area. The total investment to bring La Libertad on stream was about $3.5 million (in 1975 dollars). GEOLOGY. The ore deposit occurred in quartz veins hosted in tertiary rhyolite intrusives and tuffs. The principal oreshoot on the Santa Rosa vein is 1150 ft (350 m) long, 450 ft (145 m) high, and on average 20 ft (6 m) wide. Dip of the vein is 70°W.
MINING ENGINEERING HANDBOOK
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O RE R ESERVE. Mine commenced with 193,800 tons (176,200 t) averaging 11.5 oz/ton (400 g/t). MINING METHOD . Shrinkage stoping. 3 3 E QUIPMENT. 1-yd (0.8-m ) LHDs, 2-drum air slushers, stopes and jacklegs, on-highway trucks. PRODUCTIVITY. 7.7 tons (7.0 t)/employee-shift in stope.
5. Ground support of ore and walls minimal. 6. Stope development moderate. 7. Good ore recovery (75 to 100%) low dilution (10 to 25%). 8. Reasonable selectivity possible. Disadvantages.
Productivity low to moderate, 3 to 10 tons (2.7 to 9 t)/ employee-shift in stopes. 2. Mining costs moderate to high. 3. Labor intensive, mechanization limited. 4. Difficult working conditions, especially in narrow and/ or short stopes. 5. About 60% of ore tied up in stope until completed. 6. Ore can pack, oxidize, or spontaneously combust in stopes. 7. Risk of loss of stope during drawdown if not properly controlled. 1.
18.3.7 SUMMARY Under most economic evaluations, the labor intensity of shrinkage stoping precludes its widespread application in modern mining situations. However, it may be the only possible method applicable in the case of a mine in which the ore bodies occur in very narrow veins and cannot be stoped by other methods. Shrinkage may also be used in special situations where small ore blocks cannot be extracted economically any other way or in conjunction with other stoping methods.
18.3.7.1 Parameters The following is based on Boshkov and Wright (1973), Lucas and Haycocks (1973), Morrison and Russell (1973), and Lyman (1982): Ore characteristics: requires strong ore, non-oxidizing ore, ore that does not pack or stick together, and ore that does not spontaneously combust. Host rock characteristics: requires strong to moderately strong walls. Deposit shape: almost any shape but should have uniform dip and boundaries. Deposit dip: greater than angle of repose (> 45°), and preferably steeper than 60°. Deposit size: narrow to moderate width (3 to 100 ft, or 1 to 30 m); length minimum of 50 ft (15 m) to unlimited panel stopes on long strike lengths. Ore grade: moderate to high.
18.3.7.2 Features The following is based on Morrison and Russell (1973), Hamrin (1982), and Lyman (1982): Advantages.
1. 2. 3. 4.
Small to moderate production rates. Gravity drawdown of stope. Simple method, especially for small mines. Low capital investment, some mechanization possible.
REFERENCES Boshkov, S.H., and Wright, F.D., 1973, “Underground Mining Systems and Equipment ,” SME Mining Engineering Handbook, Sec. 12, A.B. Cummins and I.A. Given, eds., SME-AIME, New York, pp. 12.1 to 12.13. Hamin, H., 1982, “Choosing an Underground Mining Method,” Underground Mining Methods Handbook, W.A. Hustrulid, ed., SMEAIME, New York. Haptonstall, J.C., 1980, “La Libertad, Making a Small Mine Work In Mexico,” World Mining, Vol. 33, No. 5, May, pp. 42–47. Hustrulid, W.A., 1982, “Shrinkage Stoping at the Idarado Mine,” SME Underground Mining Methods Handbook, Sec. 3.1, Chap. 3, W.A. Hustrulid, ed., SME-AIME, New York, pp. 495–507. Lewis, R.S., and Clark, G.B., 1964, “Mining Methods,” Elements of Mining, Chap. 9, 3rd ed., Wiley, New York, pp. 249–261. Lucas, J.R., and Haycocks, C., eds., 1973, “Underground Mining Systems and Equipment,” SME Mining Engineering Handbook, Chap. 12, A.B. Cummins and I.A. Given, eds., SME-AIME, New York, pp. 12.1 to 12.262. Lyman, W., 1982, “Introduction to Shrinkage Stoping,” SME Underground Mining Methods Handbook, Sec. 3.1, Chap. 1, W.A. Hustrulid, ed., SME-AIME, New York, pp. 485–489. Morrison, R.G., and Russell, P.L., 1973, “Selecting a Mining Method: Rock Mechanics, Other Factors,” SME Mining Engineering Handbook, Sec. 9, A.B. Cummins and I.A. Given, eds., SME-AIME, New York, pp. 9.1 to 9.22. Smith, M., 1988, “Trackless Mining at JCI,” Mining Magazine, Vol. 158, No. 4, Apr., pp. 264–273. Wyllie, R.J.M., 1988, “El Indio,” Engineering and Mining Journal, Vol. 180, No. 3, Mar., pp. 34–38.
