Una Persson,
I read the info you provided about the Springfield (W. Ky. No. 9) Coal. Not a miner myself (shocking, I know), I am wondering if the description of this particular vein is indicative of a tougher than normal mining situation and if this would lead to the mythical toughness of the “number nine coal” and the miners that mine it.
Specifically the following:
“coal is usually overlain by an immediate roof of hard, black shale (miners’ “slate”), which commonly contains “slips,” a miners’ term for slickensides … probably the single most common geologic obstacle encountered during mining of the Springfield coal”
“In some areas, the coal itself may contain very hard, brown carbonate masses called “coal balls.” … They can be a significant nuisance to mining, since they can stop a continuous miner and cause excessive wear on bits.”
“Rotated and deformed bedding formed by ancient failures of channel margins, called “paleoslumps,” is also common. … Because of the deformed and high angle of bedding, such features are very
difficult to support underground”
“Springfield commonly rests on a well-developed underclay … Floor heave has been documented where the underclay is thick”