Pillar Removal

Provided that the iron ore layer is thick and uniform enough, up to about 90% of the ore can be mined.

From a side tunnel, mining tunnels are driven all the way to the concession boundary.

At the end of the process, the mining tunnels are connected to one another. Only a few remaining pillars are left standing.

The remaining pillars are blown up; the roof collapses.

The surface terrain subsides more or less evenly. After a few years, the underlying ore layer can also be mined if the intervening layer is thick and strong enough.