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~~ <br /> <br />• <br />Main CressonSlopeEvaluation AdrianBrown <br />Table 5. Approximate Classification of Rock <br />UCS RANGE CLASS STRENGTH DESCRIPTION <br />150-3,500 psi Rl Very weak rock Crumbles under sharp blows with geological <br /> pick point, can be cut with pocket knife. <br />3,500 - 7,500 psi R2 Moderately weak rock Shallow cuts or scraping with pocket knife <br /> with difficulty, pick point indents deeply with <br /> firm blow. <br />7,500-15,000 psi R3 Moderately strong rock Knife cannot be used to scrape or peel surface, <br /> shallow indentations under firm blow from <br /> pick point. <br />15,000-30,000 psi R4 Strong rock Hand-held sample breaks with one firm blow <br /> from hammer end of geological pick. <br />> 30,000 psi RS Very strong rock Requires many blows from geological sample <br /> to break intact sample. <br />3.3 Frictions! Properties <br />3.3.1 Laboratory tests <br />Source: Hoek and Bray, 1980, Table II, p. 99 <br />Joint fractures observed at the mine site and in core from geotechnical investigation drilling aze rarely <br />coated with clayey materials; most joints observed in the mine materials aze either clean, or are filled <br />with iron-rich cement. To provide a quantification of the shear strength of joints at the Main Cresson <br />Mine, a number of sheaz strength tests were performed for mated fracture surfaces as part of the initial <br />slope design investigation (Call and Nicholas, 1994). The results for best linear fit effective stress angle <br />of friction and cohesion aze presented in Table 6. <br />The joints tested exhibit little cohesion, and the following effective stress friction angles: <br />1. porphryitic phonolite (3 tests} 37.9° <br />2. Breccia (3 tests) 40.6° <br />3. Phonolite dike (2 tests) 40.2° <br />The large strain post-failure strength of the rock samples tested showed very little drop-off from peak <br />strength. The friction angles found aze consistent with the observed absence of clay infilling of fractures. <br />In summary, the behavior of mated joints was essentially the same for the principal materials present in <br />the Main Cresson Mine area, regardless of joint type and orientation. A reasonable set of effective stress <br />Mohr-Coulomb strength parameters is the following: <br />• Effective stress cohesion = 0 psi <br />Effective stress angle of friction = 39° <br />1385D.98o612 g <br />