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0 2.0 CONCLUSIONS <br />Based on a review of regional data, examination of existing core, analyses of mechanical <br />properties, and numerical modeling, the following conclusions were made regarding <br />strength and rock mass quality of selected overburden units and stability of the Third <br />North Mains pillars, Hubbard Creek study area: <br />• Tested overburden rocks are of medium-to-high strength (7,600 to 22,000 psi) and <br />have a low-to-medium modulus ratio, with Young's modulus ranging from <br />1,700,000 to 2,600,000 psi. This is considered very typical for sedimentary rocks <br />in the Somerset Coal Field. <br />• RQD values are generally low near the cleated B Seam, but increase rapidly into <br />the roof. The Upper Marine Sandstone (or C- Sandstone), which is approximately <br />50 ft thick, has excellent rock quality. Because of lateral consistency of this unit, <br />this sandstone will contribute significantly to stability of the overburden. <br />• Considering both intact strength and condition of fractures, the selected <br />overburden rocks are classified mostly as "Fair." The rating for the Upper Marine <br />Sandstone and the immediate roof strata exceed 60, falling into "Good" rock <br />category. <br />• The average factor of safety for mains pillars ranges from 2.2 to 4 using the <br />NIOSH method and boundary-element modeling results. MTI considers such <br />high levels of factor of safety very beneficial to long-term stability of the mains. <br />• <br />Maleki Technologies, Inc. Page 5