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excluded from these logs for clarity. These data are used in the following sections to <br />calculate RQD. <br />As illustrated in figure 4, the immediate roof above the Upper B Seam has medium to <br />high strength ranging from 14,600 to 19,000 psi. The strength increases into the roof, <br />reaching a maximum of 22,000. With a Young's modulus of approximately 2,600,000 <br />psi, the immediate roof falls into the low modulus category. <br />Based on four uniaxial compressive strength tests, the B Seam has moderate strength <br />(averaging 2,675 psi), which is very similar to the average values found in the B Seam in <br />the West Elk Mine (Maleki and others 1999) and the Orchard Valley Mine (USBM <br />1986). A rather large inherent variability in strength is seen, ranging from 2,000 to 5,800 <br />psi. Such a level of variability is typical for cleated coal seams. Lower strength values are <br />reported to the west near Terror Creek and are influenced by changes in the depositional <br />environment of the B Seam to the west, among other factors (MTI 2004). <br />0 3.4 Rock Quality <br /> <br />The in situ strength and deformation properties of rocks are influenced by intact strength <br />(uniaxial compressive strength) and the condition of geologic discontinuities within the <br />rock mass volume of interest. A number of indices, from simple to sophisticated, have <br />been used by practitioners in rock engineering to characterize rock mass quality. A useful <br />parameter commonly used in geomechanics is rock quality designation, or RQD, which is <br />the sum of all core pieces greater than 4 inches divided by the total interval length. Table <br />3 presents a relationship between RQD and rock quality suggested by Deere (1969). <br />Another, more sophisticated, index (see section 3.5) is Rock Mass Rating, or RMR <br />(Bieniawski 1984). <br />Table 3-Deere's classification based on RQD <br />RQD Description of rock quality <br />0-25 Very poor <br />25-50 Poor <br />50-75 Fair <br />75-90 Good <br />90-100 Excellent <br />Maleki Technologies, Inc. Page 10