2009-12-03_REVISION - C1996083 (5)

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6.0 CONCLUSIONS AND RECOMMENDATIONS 6.1 Conclusions Based on geotechnical drilling, examination of core, analyses of mechanical properties, and numerical modeling, the following conclusions are made regarding strength and stability of the overburden rocks at the project site: • In average, overburden rocks are of medium strength (8,500 psi) and have a medium modulus ratio, with Young's modulus averaging 2,000,000 psi. This is considered very typical for sedimentary rocks in the Somerset Coal Field. • RQD values are low near the cleated B Seam, but rapidly increase into the roof. The C- Sandstone, which is approximately 50-ft thick, has excellent rock quality. Because of lateral consistency of this unit, this sandstone will contribute significantly to the stability of the overburden. • RQD values improve generally from TC-03-01 toward the other holes within the corridor. The highest average RQD is attained at TC-03-03 (68 %). • Considering both intact strength and condition of fractures, the overburden rocks are classified mostly as "Fair". The rating for the Upper Marine Sandstone and few other massive units in the overburden exceed 60, falling into good rock category. • The average factor-of-safety for mains pillars ranges from 1.6 to 2.5 within the corridor using Mark-Bieniawski method and boundary-element modeling results. By limiting the mining height to 9-ft (as planned), number of entries to 5 or less, and by following careful excavation and support practice, long-term pillar stability can be enhanced for these set of mains. • FLAC modeling results clearly show insignificant changes in mining-induced stress or the factor-of-safety levels for many overburden stratigraphic units including the C-Sandstone and siltstone intebeds. Yielding is expected within the Upper B Seam and in the roof and floor particularly in the crosscuts. Yielding should be controlled through careful design of excavation and support practice. • Maleki Technologies, Inc. Page 35