Page "Coal mining" Paragraph 28

Room and pillar mining consists of coal deposits that are mined by cutting a network of rooms into the coal seam.

Pillars of coal are left behind in order to keep up the roof.

The pillars can make up to forty percent of the total coal in the seam, however where there was space to leave head and floor coal there is evidence from recent open cast excavations that 18th century operators used a variety of room and pillar techniques to remove 92 percent of the in situ coal.

However, this can be extracted at a later stage ( see retreat mining ).