Blasthole Stoping

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

As is typical, a block of ore is prepared by driving sublevels through the orebody at vertical intervals of about 66 feet (20 metres). Then a raise is made between sublevels, and opened across the width of the stope into a slot shape by successive blasts, which provide an opening in which to blast the stopes ore remainder.

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

Vertical Crater Retreats
Since the drills capable of drilling large-diameter (15 cm) holes up to 60 metres (200 feet) in length were developed, conventional blasthole stoping in part has been replaced by a more efficient stoping method.

The cost of mining wide, steeply-dipping orebodies is reduced with the vertical cráter retreat (VCR), also known as vertical retreat mining (VRM). The stope has a similar shape, but, large-diameter blastholes are drilled vertically from a top sill to break through into a bottom sill on the sublevel below instead of small-diameter longholes drilled in fans. This makes it possible for the ore to be broken into the bottom sublevel in successive horizontal slices using the same blasthole for each successive blast or deck. One does not need a slot raise in this case.

Just the bottom of every hole is loaded for each successive blast. This breaks a slice of ore off from the lower part of the ore block, which then falls into the drawpoint level which is below, where it can be mucked out. Mucking gives enough room in the stope for the blast that comes next. Dilution is controlled by removing just sufficient ore to create a void enough for the subsequent blast.

This method of mining is especially safe, due to the fact the miner does not have to enter the area where the ore is blasted. Drilling and loading are done from the top part of the sill. The method mentioned also eliminates the necessity of supporting the ground in the stope after every blast.

Mucking can be done by using load-haul-dump machines that are remote-controlled.

Room-And-Pillar
In the cases where the orebody is flat-lying and narrow, as we can find that is the case with many potash, salt, coal and Mississippi Valley-type deposits of zinc and lead, a method of mining which is known as room-and-pillar is used very frequently. As the name mentions, ore is mined from large rooms or voids, and pillars of ore are left between the rooms to support the overlying strata. The pillars of ore remain upon completion of mining, and they are not recovered.