UTAH COAL MINE COLLAPSE

By Iona Evans & Jonathan Price

INTRODUCTION

• Crandall Canyon Coal Mine, 140 miles south of Salt Lake City

•At least 1,500ft deep

•3.5million tonnes produced per year

•Room and pillar mining

WHAT HAPPENED?

“This tragedy was caused by an earthquake, not something our employees did.” Robert Murray, owner.

• August 6th 2007

• Seismic activity registered 3.9 on the Richter Scale

• 6 miners trapped behind collapsed rock, never found

• 3 miners killed in rescue attempt

SEISMIC ACTIVITY

Results from seismic stations

• Small open circles represent S waves – shear.

•Filled circles would imply P waves – compression.

•Earthquakes cause P waves

•Seismic activity recorded at the coal mine must have been related to the collapse of an underground cavity

•NOT AN EARTHQUAKE as claimed by mine owner.

“ROCK BUMPS”

As coal is extracted, pressure is redistributed onto the pillars and can increase to the extent that the pillar explodes, shooting coal and rock at lethal speeds.

CAUSES

• Strong roof and floor strata

• Sandstone channels

• Strong coal seams

• Fault and shear zone structures

• Mining deeper 1,200 feet

PREVENTION OF “ROCK BUMPS”• Mine design

softening the ground pillar size

• Destressing intentional fracture load redistribution can cause bumps elsewhere

• Volley firing

explosives cause fracture to advance high stress zones away from working face

• Hydraulic fracturing injection of fluid to cause failure

ENGINEERING ERRORS

• 325 safety warnings, 116 “significant and substantial” by the mine safety authority

• $152,000 fines over 3 years

• Pillars reported to be inadequate in size

• No escape route provided ($60 fine in 2006)

• Engaged in dangerous ‘retreat mining’

RETREAT MINING

• Retreating back towards the mine's entrance, miners pull down the last standing pillars of coal and let the roof fall in.

• Pillar removal has to occur in a very precise order in order to reduce the risks to workers, due to the high stresses placed on the remaining pillars.

CONCLUSION

• Mine Safety authority overstretched and has insufficient power

• No precautions taken to prevent rock bumps

• Need to carry out proper load analysis and mine design

• Retreat mining is unsafe and should not be used