Structural Conditions and Violent Stress Relief in Coal Mines of the Southern Canadian Cordillera

Citation

Norris, Donald Kring (1953) Structural Conditions and Violent Stress Relief in Coal Mines of the Southern Canadian Cordillera. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/K08S-Y213. https://resolver.caltech.edu/CaltechETD:etd-05082003-101319

Abstract

The problem was investigated under the auspices of the Geological Survey of Canada in an attempt to evaluate the relation of geologic structure to the mining practices which gives rise to occurrences of violent relief of stress. An understanding of this problem is imperative to the winning of coal at depth. The faulted and drag-folded condition of the seams in the Crowsnest and Cascade coal areas may be interpreted by the mechanics of shear-thrusting. Gentle flexing of the overthrust masses was effected through similar folding for the combined series of competent and incompetent units. Alternate stretching and compression of the Kootenay formation at Coleman and Canmore resulted in the development of an abundance of extension and contraction faults transecting the seams, and a sheared condition within the coal. Frictional resistance tended to retard those beds in the overriding masses closest to the thrust planes so that in every instance the upper beds moved slightly further to the east relative to those below them. Consistent dragfold patterns within the coal seams were the result. The fractured and sheared condition of the coal is all important in governing the stress relief characteristics of the seams. Bump phenomena occur only in structurally strong coals, whereas blowouts have been experienced where the seams are both sheared and unsheared. In contrast to violent bumps which show a definite increase in frequency with depth, beginning at a cover interval of about 500 feet, blowouts first occur at about the same depth, but after reaching a maximum frequency between 700 and 1100 feet, show a marked decrease at progressively greater cover intervals to the point where no blowouts are recorded in the 1 East mine, Coal Creek, at 2000 feet. Whereas a bump condition is created by faulty mining practices which result in the failure of overstressed coal in abutment zones, a blowout condition is believed to be due in part to redistribution of stresses associated with the advance of the working face, and in part to the physical-chemical relation of the coal to the associated gases. Secular strain is not believed to be a primary factor in the genesis of violent stress relief phenomena.

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