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6.0 CONCLUSIONS AND RECOMMENDATIONS 0 <br />6.1 Conclusions <br />Based on geotechnical drilling, examination of core, analyses of mechanical properties, <br />and numerical modeling, the following conclusions are made regarding strength and <br />stability of the overburden rocks at the project site: <br />• In average, overburden rocks are of medium strength (8,500 psi) and have a <br />medium modulus ratio, with Young's modulus averaging 2,000,000 psi. This is <br />considered very typical for sedimentary rocks in the Somerset Coal Field. <br />• RQD values are low near the cleated B Seam, but rapidly increase into the roof. <br />The C- Sandstone, which is approximately 50 -ft thick, has excellent rock quality. <br />Because of lateral consistency of this unit, this sandstone will contribute <br />significantly to the stability of the overburden. <br />• RQD values improve generally from TC -03 -01 toward the other holes within the <br />corridor. The highest average RQD is attained at TC -03 -03 (68 %). 0 <br />• Considering both intact strength and condition of fractures, the overburden rocks <br />are classified mostly as "Fair ". The rating for the Upper Marine Sandstone and <br />few other massive units in the overburden exceed 60, falling into good rock <br />category. <br />• The average factor -of- safety for mains pillars ranges from 1.6 to 2.5 within the <br />corridor using Mark - Bieniawski method and boundary- element modeling results. <br />By limiting the mining height to 9 -ft (as planned), number of entries to 5 or less, <br />and by following careful excavation and support practice, long -term pillar <br />stability can be enhanced for these set of mains. <br />• FLAC modeling results clearly show insignificant changes in mining- induced <br />stress or the factor -of -safety levels for many overburden stratigraphic units <br />including the C- Sandstone and siltstone intebeds. Yielding is expected within the <br />Upper B Seam and in the roof and floor particularly in the crosscuts. Yielding <br />should be controlled through careful design of excavation and support practice. • <br />Maleki Technologies, Inc. Page 35 <br />