Underground mining

There are numerous methods of underground mining, but mechanised blast hole open stoping (BHOS) is the most often used.

One of the main advantages of BHOS is that it allows a mine to be converted from open-pit to underground because it:

• is able to build tonnage and can easily interface with open-pit layout
• does not require major upfront capital, but does require high long-term development cost
• can selectively mine waste
• is less dependent on the predictability of the rockmass

In BHOS mining, long hole drilling rigs are used to drill rings of holes from drill drives situated perpendicular to the pit faces. These holes are then charged and blasted and the broken ore is loaded by load-haul-dump units (LHDs).

BHOS cannot be used too deeply when kimberlite pipes are situated in unstable country rock such as the Karoo System as it vulnerable to waste dilution. Where this occurs the mining method is changed to modified sub-level caving.

The underground infrastructure (tunnels, level spacing, etc) remains largely unchanged, but the amount of material extracted is reduced to facilitate semi-choke blasting. The broken ore is then loaded via the tunnels by LHDs. This creates a layer of ore above the retreating tunnels that protects against waste ingress with the loaded ore.

This method reduces the amount of ore that can be extracted as some is left behind in the protective layer. It is, therefore, an interim method which is used while the infrastructure for a block cave is being established.

Block caving is the most cost-effective and productive method of mining kimberlite underground. It does depend, though, on the pipe having sufficient cross-sectional area to allow the ore to cave, once it has been undercut.

De Beers underground mines are:

• Finsch (South Africa)
• Snap Lake (Canada)
• Cullinan (South Africa)

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