Shrinkage

Chapter 18.3 SHRINKAGE STOPING
J ACK H A P T O N S T A L L 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 upwardin 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 onnarrow 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 orother 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 avoidmining 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 orrake ( < 50”) may be very difficult to mine by shrinkage methods because the ore moves away too quickly from the predeveloped 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), especiallyraising.

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 timberconstruction. 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...