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Shrinkage Stope Mining

Shrinkage stope is a generic term used in mining to portray the process of mining upwards from a lower to an elevated horizon, leaving broken rock in the excavation created. The broken rock acts as a working platform and assists to stabilize the excavation by supporting the walls.

The method may be used for ore mining in shrinkage stopes, for rising, and for underground construction projects where excavations of considerable vertical height may be needed, like ore and waste bins, crusher rooms, penstocks and tailrace tunnels.

As blasted rock takes up more volume than rock in situe, some of the broken material must be removed on a periodic manner, to maintain the requisite relationship among the back (or roof) of the excavation, and the level of the broken material in the excavation. This is attained by drawing the blasted material through draw points on the lower level, which is constructed prior to shrinkage starts. Access to the space among the broken material and the back of the excavation ought to be maintained for access of men and materials, and for ventilation. This type of access is usually provided by previously installed raises, normally equipped with ladder ways.

When correctly planned and executed, shrinkage mining is a very effectual technique for ore mining and underground construction. It is used where the hanging and footwalls of the excavation are strong sufficient to be self supporting, although artificial support like rock and cable bolts might be installed as shrinkage progresses. Where the technique is used for ore mining, careful planning and scheduling are requisite to ensure consistency of ore grade and manufacture tonnage.

Shrinkage stoping is used in precipitously dipping, relatively narrow ore bodies with normal boundaries. Ore and waste should be sturdy, and the ore must not be affected by storage in the stope.

From 30 to 40% of the broken ore is taken from the bottom of the stope, and the ore in the slice is blasted down, restoring the volume withdrawn. The miners then reenter the stope and work off the recently blasted ore.

Shrinkage stoping is rather hard to mechanize; in addition, an important period may elapse amid the commencements of mining in the stope and the last withdrawal of all the broken ore.

Shrinkage stoping is a bendy mining method for tapered ore bodies that require no backfill during stoping. Consecutive horizontal slices of ore, normally roughly about 3 meters (10 feet) high, are taken along the length of a stope, in a manner similar to cut-and-fill. The ore is taken out from the stope throughout draw points at the underneath horizon spaced about every 7.5 meters (25 feet) along strike. Just adequate ore is left in the place to give a floor from which to work when taking the next cut. This needs considerable planning and coordination.

When the ore is blazed, it fills a space about 1.5 times the size of the space it filled as a hard mass. This is known as swell and is a vital 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 underneath, therefore "shrinking" the volume of broken ore in the stope.

The procedure is continued upward until the stope either reaches the next level or is stopped at some predetermined elevation. Horizontal crown pillars are left at the back at the top of the stope.

Shrinkage stoping relies on gravity to keep the broken ore moving to the draw points, so it works only in precipitously-dipping ore bodies. There is no provision for hold up, so the wall rocks must be strong and capable. The ore body ought to also be wide adequate to permit a working width all the way up the stope, normally no fewer than two meters.