Deep beneath the surface of the earth lie some of the most frightening factories in the world: underground mines. Because surface mines become generally inefficient at depth greater than 60 meters, underground mines usually are used alternatively. Underground mines pose a greater safety risk and limit the size of equipment that can be used. For information, this kind of mine only is saw generally on uranium mining and gold mining.
When building an underground mine, we dig a tunnel to get to the minerals. This can be a straight vertical tunnel called a shaft or a tunnel that spirals gradually downwards, called decline. To access the ore from the shaft or decline, we dig other tunnels.We also mine out tunnels to provide proper ventilation and emergency exits.
We mine the tunnels and the ore bodies by drilling and blasting. The broken up ore is then transported to the surface for processing. Waste rock may be transported to the surface or left in the mine and used to fill empty space.
Once we remove all material inside the tunnels, we support them to make them safe. The type of ground support needed depends on how stable ground is and how long the tunnel is going to be used. These factors are identified in advance so that engineers can design the mine for maximum safety and value.
Ground support may be provided by rock bolts or split sets, which are forced into drilled holes to exert pressure on the surrounding rocks. Chemicals or grouts are sometimes added with rock bolts to give them a greater strength. We also install wire mash to stop smaller rocks from failing down.
High-pressure spaying of shotcrete (a mortar/concrete mix) onto the tunnels' walls and backs provides more support. As we complete mining in each stope, we backfill it with a cement mixture as well.