How do you mine away a pillar if the roof is going to simultaneously collapse in the same location?
How do you mine away a pillar if the roof is going to simultaneously collapse in the same location?
When the pillars are extracted, temporary roof supports are installed to keep the roof in the immediate area above the miners and equipment from falling. The miners and equipment are also positioned to keep the caving roof more ore less in front of them, with supported top over them and behind them. Keep in mind they are retreating, or backing out of the section as the pillars are extracted.
The temporary roof supports are withdrawn as the miners retreat, allowing the roof in front of them to cave.
This is a simplified explanation. The entire process is methodical and accepted practice, but it is the most risky part of the mining process. A miner operator must have nerves of steel when pulling pillars; the folks behind him (helper, haulage operators, etc) have to have big ones as well. As the pillars are removed and before the roof caves, you can hear the weight of the overburden trying to compress the remaining part of the pillar, and the adjoining pillars. The roof makes loud popping and thumping noises, and the coal pillars start to pop and snap. You can see coal popping off of the pillars due to the pressure. It is quite unnerving until you get used to it. When the roof falls, it creates this huge burst of wind and noise. More than one newbie has had to go outside and change his clothes after experiencing pillar extraction for the first time.