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3.5 Rock Mass Rating for the Overburden 0 <br />Bieniawski's RMR system combined five rock mass characteristics of both intact <br />properties and conditions of discontinuities; <br />1. Strength of intact rock <br />2. Rock Quality Designation <br />3. Spacing of discontinuities <br />4. Condition of discontinuities <br />5. Ground water conditions <br />Table 4 presents the geomechanics classification and rock mass conditions determined by <br />adding ratings for these five parameters. As illustrated in plate 1, most overburden rocks <br />are classified as fair. The rating for the Upper Marine Sandstone and few other massive <br />units in the overburden exceed 60, falling into good rock category. <br />46 <br />Plate 1 provides side-by-side comparisons of the variability of RMR in the study area. <br />RMR values have been adjusted to account for orientation of rooms in compare to <br />jointing. Core logs were examined to obtain values for joint spacings; 8 ft was <br />conservative for intervals where no joints were logged. This spacing is based on <br />extensive regional measurements by Tifft (1995). The condition of discontinuities <br />included an evaluation of joint roughness, aperture, and degree of weathering. <br />Table 4-Bieniawaski's Rock Mass Rating. <br />Rating 81-100 61-80 41-60 21-40 <20 <br />Class No. 1 2 3 4 5 <br />Description Very good Good rock Fair rock Poor rock Very poor <br />rock rock <br />0 <br />Maleki Technologies, Inc. Page 25