Chapter 18.3SHRINKAGE STOPING

JACK HAPTONSTALL

18.3.1 INTRODUCTION

Shrinkage stoping is a vertical, overhand mining methodwhereby most of the broken ore remains in the stope to form aworking floor for the miners. Another reason for leaving thebroken ore in the stope is to provide additional wall supportuntil 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) canbe drawn off (“shrunk”) as mining progresses. As a consequence,no revenues can be obtained from the ore remaining in the stopeuntil it is finally extracted and processed for its mineral values.

The method is labor intensive and cannot be readily mecha-nized. It is usually applied to ore bodies on narrow veins orore bodies where other methods cannot be used or might beimpractical or uneconomical. The method can be easily appliedto ore zones as narrow as 4 ft (1.2 m), but can also be successfullyused in ore widths up to 100 ft (30 m).

Logically, the broken ore should be free flowing and notpack in the stope. Neither the ore nor adjacent country rockshould contain undue amounts of clay or other sticky materialto cause the ore to hang together in the stope and either bedifficult or impossible to draw. Additionally, the ore should notreadily oxidize, which may cause the broken pile to re-cementitself, and consequently “hang up.” Oxidation can also have anadverse effect on ore dressing. Ore should be fairly continuousalong the strike of the vein or ore body in order to avoid miningextensive amounts of waste as dilution from the stope back.However, small waste areas may be mined around and left asrandom pillars.

developed extraction system (Fig. 18.3.2). Additionally, stopeswhere ore abruptly extends for great distances beyond stope endlines are also difficult to mine because they often require muchadditional 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 drift-ing in the vein or ore zone on two levels, spaced vertically 100to 600 ft (30 to 180 m) apart. After a viable ore body has beenestablished, the next phase consists of driving one or more raisesto establish vertical ore continuity and also to provide ventilationand access to the stope (Fig. 18.3.1)

Raises may be driven conventionally, with Alimak-type raiseclimbers, or by raise boring machines. Drifting for shrinkagedevelopment is normally done by conventional drill-and-blast,track or trackless methods.

Stopes may be prepared with extraction raises on 25- to 30-ft (7.5- to 9-m) centers over the length of the ore shoot; eachraise is fitted with a chute, normally of timber construction.Extraction raises are belled out and “hogged over” as the under-cut for the start of the first stope cut. This type of preparationis still used but on a very limited basis.

Another method of preparing a stope is to blast down atleast two backs of the ore zone, clean up the broken ore, andinstall stull timbers or timber sets in the drift below the stope.Timber chutes, or even “chinaman” chutes, are installed at ap-proximately 25-ft (7.5-m) intervals as part of the timbering.

Consideration must also be given to the plunge or rake of A more common method of preparing stopes in modernthe ore body, especially where the entire ore body may be mined operations is to drive an extraction drift parallel to the ore bodyas a single stope (Fig. 18.3.1 rather than as pre-established stope development drift, about 25 to 50 ft (7.5 to 15 m) in the footwallpanels with defined vertical end lines. A stope with a shallow of the ore body. Subsequently, drawhole extraction crosscuts areplunge or rake ( < 50”) may be very difficult to mine by shrinkage driven from the footwall drift into the ore drift on 25- to 50-ftmethods because the ore moves away too quickly from the pre- (7.5- to 15-m) centers. The back of the ore body is then blasted

Fig. 18.3.1. Longitudinal section–typicalshrinkage stope.

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SHRINKAGE STOPING 1713

Fig. 18.3.2. Longitudinal section—shrinkage on shallow-raking ore body.

Fig. 18.3.3. Longitudinal section—shrinkageon irregular ore body.

down, and the swell is extracted through the drawholes, eitherwith 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 areusually installed in the raise from the next level. A manwayand service way is normally constructed on one or both end-panels of the stope. Often a timber slide is installed in one of themanways for hoisting and lowering materials into and out of thestope; hoisting is often accomplished with a single-drum air hoistinstalled in the level below the manway. Once the manways,ventilation raises, and service ways have been established for astope, mining can commence.

Drilling of a shrinkage stope back is accomplished withhand-held stopers or jacklegs, although mechanized drill wagonsor stope jumbos may be used in wider stopes. Back stoping isthe normal mode of operation, but breasting down is also com-mon. Up-holes are generally 1.8 to 2.4 m (6 to 8 ft) in length. Inmost cases, all holes are loaded and a complete back is blastedat 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 andeven with slurry blasting agents. Initiation is commonly withnon-electric methods, but electric blasting is also practiced.

After a cut has been blasted in a stope, drawdown of the35% swell is necessary, after which the muck pile must be leveledto facilitate drilling of the next cut. Leveling of the pile can bedone by hand shovels in the case of small stope, with 2- or 3-drum slushers in larger or longer stopes, and even with LHDsin large stopes. After leveling, drilling of the next stope cut,raising of the manways, and so forth are done to continue themining cycle.

Variations for the establishment of openings for manways,ventilation raises, or service ways may include installation ofstrategically placed timber cribbed openings, steel culverts orrings, or timber sets within the broken ore area. These installa-tions may be very desirable during the mining phase, but maycreate safety problems and nuisances with the collapse of thematerials used to construct these openings. Pinning, stulling, orwedging these installations to the stope walls may prevent theirdestruction during drawdown; materials from a destroyed man-way may be drawn down with the broken ore into the chutes ordrawholes and cause hangups.

A stope should have strong, self-supporting walls to permitthe application of shrinkage stoping. Dilution through scaling of

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Fig. 18.3.4. Typical shrinkage stope with LHD extraction.

walls can preclude use of the method. Good mining practicecoupled with state and federal regulations may dictate a least aminimum ground support program.

Wall and back support may be accomplished by leavingrandom or even systematic pillars. Pillars left in ore zones maybe drilled off and blasted upon drawdown of the stope. Tradition-ally, timber stulls fitted with plank headboards have been in-stalled to support suspicious slabs or areas of bad ground ofstope walls. Horizontal stulls and cribbing are also used to sup-port loose areas of stope backs; however, the timbers may besubsequently buried in the muck pile upon shooting the nextstope back, and may become a hazard or, at the very least, anuisance, upon drawdown of the stope. Rock bolting has evolvedinto the preferred mode of wall and back support. Mechanicalas well as grouted types of bolts are used. Correct installation ofbolts in the walls of narrow shrinkage stopes may be difficultbecause of the lack of room to drill the bolt hole perpendicularto the stope wall as well as to install the required length of rockbolt.

Sampling of narrow shrinkage stope backs is usually doneby either taking a channel or chip sample by hand or throughmechanical means. Sampling is usually done at a systematicinterval (say, 5 ft or 1.5 m) along the entire back, ends, and insome cases, the ribs of the stope after every stope cut. In widerstopes, drill sampling of the back and ribs can be done. Thedrill sample may criss-cross the stope back on a predeterminedpattern. Drill cutting samples are collected in a sample bagthrough a hose and funnel or other device.

18.3.4 STOPE DRAWDOWN

One of the most dangerous jobs in a mine is the drawdownof shrinkage stopes, especially where the ore contains stickymaterial to hang up between the stope walls. Hung-up stopes

must either be washed down with water, bombed down withexplosives, picked down by miners (a practice not recom-mended), abandoned, or re-mined. In any case, a hung-up stopeis a costly and dangerous problem, and shrinkage stoping shouldnot generally be used where the ore has a tendency to hang up.

Stopes should usually be drawn down systematically, draw-ing the pile evenly so if the stope walls do peel or slough, thewaste remains atop the pile and does not trap broken ore rilledabove the pile. Once a stope drawdown is started, the operator’scontrol over the walls, pillar recovery, etc., is minimal and inmost cases, the re-entry of miners into a stope under activedrawdown would be considered too great a safety hazard to risk.

Stopes can be drawn down from strategically placed chutesor from drawpoint crosscuts. Haulage from the stope extractionpoints may be done with rail equipment or LHDs and/or trucks.Chutes should be robustly designed and constructed to avoiddestroying them through blasting of large slabs in them. Stopesmay also be extracted through slusher trenches developed belowthe stope.

18.3.5 VARIATIONS AND APPLICATIONS

Variations of shrinkage stoping include inclined shrinkageand longhole shrinkage. Recovery of large pillars may be doneby shrinkage methods.

One example of mines that employed shrinkage as a primarystoping method is the Homestake mine at Lead, SD. Fifty-foot(15-m) wide “bull pen” shrinkage stopes were developed trans-versely across the full width of the great Main Ledge ore body.Stopes were mined over a timbered sill where strategic chinachutes were constructed for ore extraction. Stopes were minedover 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 leftbetween stopes, which along with the crown pillars, were subse-quently extracted with square-set stopes. Homestake abandonedthis type of shrinkage stoping just before World War II.

A second example is the Idarado mine located near Ourayand Telluride, CO. Stopes were mined along the veins and thefull width of the veins, which varied from 5 to 25 ft (1.5 to 7.6m). Stope panels were generally 400 ft (122 m) long and wereprepared over a slusher trench developed about 20 ft (6 m) overthe back of the main level drifts. A series of pockets and raiseson 25-ft (7.6-m) centers were developed from the slusher trenchand the pockets “hogged over” to form pillars between the trenchand 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 Morocochaand Casapalca mines of the Cerro de Pasco Corp. located in thecentral Andes mountains of Peru, South America. Stopes inthese mines were prepared over the main development leveldriving 25-ft (7.6-m) raises on 25-ft (7.6-m) centers and “hog-ging” out from the raises to form the first stope cut at about 16ft (5 m) over the level. Each raise was then fitted with a timberedchute for ore extraction.

In all cases, a raise was first developed through each oreblock or stope panel for ventilation and service. Manways wereeither carried as cribbed raises in the stope or, in the case ofIdarado, as boreholes 10 ft (3 m) in the footwall of the vein. Inthe vein mines, drilling was accomplished with either stopers orjacklegs, while at the Homestake, drilling was done with bar-and-column mounted Leyner-type drills.

Some variations of shrinkage stoping include inclined shrink-age, longhole shrinkage, and construction shrinkage.

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Fig. 18.3.5. Shrinkage stope, Rosiclare, IL.

Fig. 18.3.6. Longhole shrinkage stoping.

Inclined shrinkage refers to a rill stoping adaptation wheremultiple faces or benches for drilling are carried along the backof the stope as it is mined upward (Fig. 18.3.5). Stopes aredeveloped conventionally over pillars and chutes or over timbersets fitted with chutes on centers of about 25 ft (7.6 m). Theadvantage of carrying the stope in benches is that multiple facescan be drilled in a given shift where it is desirable to drill thestope with airleg-type drills rather than stopers.

Longhole shrinkage (Fig 18.3.6) is developed conventionallyas described previously. The exception is that drilling of thestope is done from vertical raises driven through the ore zone on50- to 100-ft (15- to 30-m) centers. Raises can be developedwith raise climbers or through cage raising techniques. The raiseclimber or the cage becomes the entry and exit vehicle as wellas the platform for drilling and loading. Parallel longholes aredrilled along the strike of the ore body and loaded from theraises. Initiation normally is done from a safe area on the servicelevel above the stope.

Shafts, winzes, or large break raises for blasthole or sublevelcaving stopes may be developed through shrinkage methods (Fig.

Fig. 18.3.7. Construction shrinkage stoping by con-ventional method.

18.3.7). In many cases, this is done as described in the longholevariation. Given a large enough opening, conventional shrinkagestoping of a shaft or raise may be justified.

18.3.6 CASE STUDY: LA LIBERTAD MINE,PUEBLO NUEVO, DURANGO, MEXICO

The small La Libertad mine was brought on-stream in July1977 by Minas de San Luis, S.A., a 51% Mexican-owned com-pany (Haptonstall, 1980). The mine was developed entirely forshrinkage stoping.

GENERAL. La Libertad was essentially a virgin silver-golddeposit in a very rugged location in the Mexican Sierra Madremountains. The only previous mining done in the area was asmall tonnage extracted from the outcrop of the principal SantaRosa vein in the 1930s. Production ceased in the mine in 1985due to political strife in the area. The total investment to bringLa Libertad on stream was about $3.5 million (in 1975 dollars).

GEOLOGY. The ore deposit occurred in quartz veins hostedin tertiary rhyolite intrusives and tuffs. The principal oreshooton 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.

1716 MINING ENGINEERING HANDBOOKORE RESERVE. Mine commenced with 193,800 tons

(176,200 t) averaging 11.5 oz/ton (400 g/t).MINING METHOD. Shrinkage stoping.EQUIPMENT. 1-yd3 (0.8-m3) LHDs, 2-drum air slushers,

stopes and jacklegs, on-highway trucks.PRODUCTIVITY. 7.7 tons (7.0 t)/employee-shift in stope.

18.3.7 SUMMARY

Under most economic evaluations, the labor intensity ofshrinkage stoping precludes its widespread application in mod-ern mining situations. However, it may be the only possiblemethod applicable in the case of a mine in which the ore bodiesoccur in very narrow veins and cannot be stoped by other meth-ods. Shrinkage may also be used in special situations where smallore blocks cannot be extracted economically any other way orin conjunction with other stoping methods.

18.3.7.1 Parameters

The following is based on Boshkov and Wright (1973), Lucasand Haycocks (1973), Morrison and Russell (1973), and Lyman(1982):

Ore characteristics: requires strong ore, non-oxidizingore, ore that does not pack or stick together, and ore thatdoes not spontaneously combust.Host rock characteristics: requires strong to moderatelystrong walls.Deposit shape: almost any shape but should have uniformdip and boundaries.Deposit dip: greater than angle of repose (> 45°), andpreferably steeper than 60°.Deposit size: narrow to moderate width (3 to 100 ft, or 1to 30 m); length minimum of 50 ft (15 m) to unlimitedpanel 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. Small to moderate production rates.2. Gravity drawdown of stope.3. Simple method, especially for small mines.4. Low capital investment, some mechanization possible.

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.1.

2.3.4.

5.6.

7.

Productivity low to moderate, 3 to 10 tons (2.7 to 9 t)/employee-shift in stopes.Mining costs moderate to high.Labor intensive, mechanization limited.Difficult working conditions, especially in narrow and/or short stopes.About 60% of ore tied up in stope until completed.Ore can pack, oxidize, or spontaneously combust instopes.Risk of loss of stope during drawdown if not properlycontrolled.

REFERENCES

Boshkov, S.H., and Wright, F.D., 1973, “Underground Mining Systemsand Equipment ,” SME Mining Engineering Handbook, Sec. 12, A.B.Cummins and I.A. Given, eds., SME-AIME, New York, pp. 12.1to 12.13.

Hamin, H., 1982, “Choosing an Underground Mining Method,” Under-ground Mining Methods Handbook, W.A. Hustrulid, ed., SME-AIME, New York.

Haptonstall, J.C., 1980, “La Libertad, Making a Small Mine Work InMexico,” World Mining, Vol. 33, No. 5, May, pp. 42–47.

Hustrulid, W.A., 1982, “Shrinkage Stoping at the Idarado Mine,” SMEUnderground 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 ofMining, Chap. 9, 3rd ed., Wiley, New York, pp. 249–261.

Lucas, J.R., and Haycocks, C., eds., 1973, “Underground Mining Sys-tems 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 Under-ground Mining Methods Handbook, Sec. 3.1, Chap. 1, W.A. Hus-trulid, 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 Hand-book, 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.