Stoping methods

The following is an excerpt from the ninth edition of Mining Explained, published by The Northern Miner.

Stopes are the production centres of the mine. It is here that the ore is first broken.

The safety of the miner is a big consideration in selecting a suitable mining method. Even then, miners must be constantly on their guard to make their workplace as safe as possible. Loose rock is a constant potential danger. As soon as a miner enters a stope, the ceiling (or “back”) is examined carefully, and any loose material is brought down with a scaling bar.

A stope must also be designed so that miners can reach their working places, remove the broken ore, and bring in supplies, tools, explosives and equipment. It must also be properly and continuously ventilated, so that the air does not fill with dust and machine exhausts. In deep mines, where the wall rocks can be quite hot, ventilation also helps to keep the air temperature low enough for miners to work.

Stopes are started from the main levels in a mine. The first step of the operation is known as “silling” — cutting a drift across the top or bottom of the ore to be mined.

There are several ways to remove ore from a stope. The sill may consist of a timber structure placed in a section of a crosscut that has had a slice taken out of the back. This structure will include chutes for the removal of the broken ore from the stope to be developed above the crosscut. It will also include the beginnings of the manways, which must be maintained for access and for services such as compressed air, water and electricity.

In other instances, a series of short raises or box holes is opened in the rock. These are excavated at short intervals along the drift, or in the footwall parallel to it.

Development work for some stoping methods calls for an elaborate system of box holes. This is required for the removal of ore from scraper drifts, which in turn are fed from drawpoints underlying a stope that extends for some distance along strike. Development work for a stope often includes rib pillars (oriented at right angles to the strike) and sill pillars (oriented parallel to the strike) over the main haulageway. In any case, development work provides access, ore removal, service and supply to a stope.

In trackless mining, box holes and chutes are replaced with a system of drawpoints spaced equally along the footwall of the orebody. These are developed so that they break into the ore at the sill level.

Cut-and-fill

Cut-and-fill stoping is suited to irregular orebodies with wall rocks that cannot support loads over large stoping heights. Since backfilling adds a step to the mining of each slice of ore, the ore has to have a high enough grade to offset this added cost.

The stope is mined upward from below, in horizontal slices or cuts. Each slice is blasted on to the floor of the stope, and the ore is mucked to the stope mill hole, which leads to the chutes in the haulageway below. The mill hole is a vertical opening and usually lined with steel. Mining a slice leaves a space along the entire length and width of the stope.

After the mill hole and manway structures have been extended, the stope is backfilled, leaving enough work space above. Usually the top portion of each new layer of fill is a concrete mix that provides a solid work floor to support heavy equipment. The process is repeated until the level above is reached, or to some predetermined pillar line.

Cut-and-fill is flexible. Ore production can be carried out in one part of the stope while another part is being filled. The method also allows miners to mine ore and waste selectively, with little dilution. Waste is identified and left behind in the stope as fill. With a competent ore rock and proper support, it is also quite safe.

Blasthole stoping

The low-cost bulk method known as “blasthole” or “open” stoping is suitable for large, regularly shaped, steeply dipping orebodies. The wall rock must be competent; the stope has to be able to stand open without support. It is also a convenient way to excavate large underground openings, such as crusher stations and storage bins.

Typically, a block of ore is prepared by driving sub-levels through the orebody at vertical intervals of about 20 metres (66 ft.). Then a raise is made between sub-levels and opened across the width of the stope into a slot shape by successive blasts, providing an opening in which to blast the remainder of the ore in the stope.

Blastholes are drilled in a fan-like pattern into the ore across the entire face of the stope. They are loaded with explosives and detonated to break into the slot raise. Broken ore is either removed by load-haul-dump machines from drawpoints or by rail cars loaded by slushers.