PILLAR STRENGTHS
Mines are erected by pillars, which have factors of safety. The factor of safety is the average strength divided by the average stress ratio. When this ration falls below 1.00, there is pillar instability. If one pillar in a mine collpases, the result is greater force acting on the rest which in turn increases their stress and decreases their factors of safety. If all of the pillars are highly stressed, one pillar failure could lead to a domino effect and the collapse of a mine. 1.60 is considered a good factor of safety for pillars.
http://www.minerals.nsw.gov.au/photos/metals_1.htm
Good factor of safety
http://www.wwnorton.com/college/history/america6_brief/dig_hist/mining/
Bad factor of safety