Shrinkage Stoping Mining Method
Shrinkage stoping is a flexible mining method for narrow orebodies that need no backfill during stoping. Successive horizontal slices of ore, usually about 3 metres (10 feet) high, are taken along the length of a stope, in a manner similar to cut-and-fill. The ore is removed from the stope through drawpoints at the bottom horizon spaced about every 7.5 meters (25 feet) along strike. Just enough ore is left in the place to provide a floor from which to work when taking the next cut. This requires considerable planning and coordination.
When the ore is blasted, it fills a space about 1.5 times the size of the space it filled as a solid mass. This is called swell and is an important factor in determining how much ore to draw from the bottom of the stope in order to maintain adequate working room. The broken ore is drawn down from chutes below, thus “shrinking” the volume of broken ore in the stope.
The process is continued upward until the stope either reaches the next level or is stopped at some predetermined elevation. Horizontal crown pillars are left behind at the top of the stope.
Shrinkage stoping depends on gravity to keep the broken ore moving to the draw points, so it works only in steeply-dipping orebodies. There is no provision for support, so the wall rocks must be strong and competent. The orebody must also be wide enough to allow a working width all the way up the stope, generally no fewer than two meters.
Sublevel Caving
In sublevel caving, ore is developed from a series of sublevels spaced at regular intervals throughout the orebody. Mining begins at he top of the orebody. A series of ring patterns is drilled and blasted from each sublevel. Broken ore is mucked out after each blast and the overlying waste rock caves on top of the broken ore.
This technique is inexpensive, highly mechanized and yields a large amount of muck. It is normally used in massive, steeply-dipping orebodies with considerable strike length. Since dilution and low recoveries are unavoidable, sublevel caving is used to mine low-grade, low-value ore-bodies.
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