2008-04-15_REVISION - M1980244 (193)

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z' Z ~. i -. MainCressonSio eEvaluation AdrianBrown The equation for the failure envelope for this rockmass is given by: s where: ti =shear stress 6 =normal stress 6~ =unconfined compressive strength A, B, and T, s, and m = Hoek and Bray parameters As noted above, the unconfined compressive strength of the intact rock at the Main Cresson Mine has been found to be: 6~ = 17,400 psi = 120 MPa Based on the above values, the' equation for the rockmass failure envelope at the Main Cresson Mine is: 'C = 63 [6/120 + 0.002)0.698 for ti and 6 in MPa. This relationship is plotted in Figure 17. The effective stress friction and cohesion at any depth can be computed from this curve by using the tangent to the curve for the normal stress a, and setting the effective stress friction angle equal to the • angle of the tangent to the horizontal axis, and the intercept of the tangent with the ti axis as the effective stress cohesion for that normal stress. The relationship has been computed for the entire range of normal stresses expected on a possible failure plane, noting that the rock density is approximately 143 lb./cu.ft. The resulting plot of friction angle and cohesion as a function of slope height is presented in Figure 18. The present slope design is fora 700 foot high wall on the east side of the south mine. The Hoek and Bray parameters for this height can be read from Figure 17 using the normal stress, plotting the tangent to the failure envelope at that point, and reading offthe parameters, or alternatively using Figure 18 using the slope height, and reading off the parameters (which have been computed using Figure 17). The resulting parameters are: Walt Hei ht ft 700 Normal stress a 4.9 Effective Stress Cohesion a 2.32 Effective Stress Cohesion si 336 Effective Stress Friction An le ° 44 C7 The total stress which occurs at a depth of 700 feet is approximately 4.9 MPa, or 700 psi. 1386D.980612 41