2001-12-11_PERMIT FILE - C1980007A (2)

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April 1989 Considerable geologic work has been conducted by ARCO geologists at Mt. Gunnison daring the last 21 years. 1`Iumerous drill holes were placed in the property to evaluate the coal reserve and lithology, but very little geotechttical work was done to assess rock quality. As part of this study, available core at West Elk was inspected and RQD"s (Rock Quality Designation) made of the immediate roof and floor of the three seaius to help evaluate ground conditions. The objective was to compare t}te ground conditions experienced from mining the F Seam to those expected itt the E and B Seams by using t}te available drill hole cores. Grottttd conditions in the F seam have varied greatly from good to very poor. The reasons for poor conditions ]rave been due to a combination of structural and lithological features. Coal riders in the immediate roof, together with sandstone channels, kettlebottoms and associated smaller features (such as joints, slickensides, and small faults) have impacted stability. Surface drainages also have influenced stability. Ground control problems have occurred directly beneath or in the promixity of stream valleys dtte to a combination of water and stress- related effects. Water percolating slow 7.y tltrouglt fractures weakens mostly the roof shale beds by destroying the integrity of the clays within the shale strata. Moisture has been found present in many of the roof falls. Abrupt stress differentials due to steep overburden gradients induce anomalous local stress/strain fields that also can contribute to the failures. A recent feasibility study by ARCO"s Mountain Coal Engineering (1987) on the F Seam reviews in more detail the geologic features impacting stability. Fiqure 3.4.A.2 is a map showing the poor roof conditions in the F Seam and possible causes presented itt the ARCO report. The effect of the above geologic features on coal mine stability have been recognized for a Long time and have been often described in the literature. Tltey are relatively common in western coal mines. Unfortunately, medium to small geologic featrres, which are very difficult to identify from widely spaced drill holes ahead of mining, are often responsible for many of the roof falls. Because of this reason, the West Elic core hole information is inadequate for site specific ground condition evaluations in the E and B Seams. Howlever, some genera]. conclusions can be drawn. Roof and floor RQD's are shown in Tal>le 3.4.A.5 for the F, E and R Seams. Appendix A presents further RQD attcl lithological information from the roof and floor of the t}tree seams, as well as t~niaxial compressive strength test results. •