Page "Coal mining" Paragraph 22

The limitations on contour strip mining are both economic and technical.

When the operation reaches a predetermined stripping ratio ( tons of overburden / tons of coal ), it is not profitable to continue.

Depending on the equipment available, it may not be technically feasible to exceed a certain height of highwall.

At this point, it is possible to produce more coal with the augering method in which spiral drills bore tunnels into a highwall laterally from the bench to extract coal without removing the overburden.