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Clastic dikes are common in coal beds of the Appalachian basin. Their presence often causes delays in mine production and poses safety hazards to mine personnel. The U.S. Bureau of Mines is conducting research on the occurrence and origin of clastic dikes and associated roof-rock instability, to gain a better understanding of the effect clastic dikes have on ground pressure around mine openings. Geologic mapping and pressure monitoring of clastic dikes at the Greenwich Collieries North Mine, Indiana County, Pennsylvania, has provided insights to this problem.
The North Mine (producing from the lower Freeport coal bed) is experiencing an increase in the number of clastic dikes and associated roof failure as mining advances toward the axis of the Brush Valley syncline. Over 200 individual clastic dikes of claystone matrix with fragments of shale and coal have been mapped at the mine. The dikes range in thickness from a maximum of 2 ft (60 cm) to as small as 0.25 in. (6 mm) and commonly extend vertically from the roof rock 20 ft (6 m) above the coal bed into the coal bed, but rarely through the coal bed to the underclay. Observations of the dikes indicate that they were formed by tensional and compressional forces and may be related to paleostress fields. Characteristically the dikes penetrate the coal bed at high angles to bedding with as mu h as 8 in. (20 cm3) of normal-fault type displacement at the contact of coal bed and roof rock. The measured strike of the clastic dikes shows a strong bimodal distribution of N40°W and N50°E, which is offset 30° to the east of the bimodal distribution of the coal cleat (face N70°W and butt N20°E). Joints in the shale roof rock show a dominant peak of N 40°W, which is coincident with one of the major peaks of the dike distribution.
The depositional environment may have been an important factor in clastic dike formation, as the abundance of preserved casts of tree stumps in the shale roof is inversely proportional to the number of clastic dikes.
Hydraulic pressure-monitoring devices were installed near the clastic dikes to monitor the development of associated roof failure. The results of this monitoring show that the roof behaved as two cantilever beams where a clastic dike was present and that the roof must be supported immediately after mining. Analysis of the anisotropic qualities of the roof strata and rock-pressure conditions suggests that changes in the in-situ stress field near clastic dikes have aided in the propagation of roof failure.
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