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• WHEXSIopeEvaluation <br />AdrianBrown <br />6.2.3 Hoek-Brown Parameters <br />The Hoek-Brown pazameters aze defined with reference to the GSI for the material at the locations being <br />considered, as follows: <br />GSI < 25 > 25 <br />s 0 ex GSI-100 /9 <br />a 0.65 -GSI/200 0.5 <br />m m• ex GSI-100 /28 <br />mb = value of the Hoek-Brown constant "m" for the rock mass <br />mi =value of the Hoek-Brown wnstant "m" for intact rock <br />GSI =Geological Strength Index <br />s,a = Hoek Brown rock mass parameters <br />6.2.4 Mohr-Coulomb Parameters <br />The Mohr-Coulomb parameters can be estimated from the Hoek-Brown failure criterion by <br />consideration of the tangent to the 6t'-a3'. The equation to the tangent is: <br />61' = 6~~ -}- k 63' <br />where: 61' =major principal effective stress at failure <br />Q3' =minor principal effective stress at failure <br />• aC1= al' axis intercept of the tangent to the failure criterion (unconfined rock mass strength) <br />k = slope of the tangent to the failure criterion at 63' <br />From the k value, the Mohr-Coulomb parameters aze computed: <br />~' = asin[(k-1)/(k+l)] <br />c' = 6~; [1-sin{~')]/[2 cos(')] <br />where: ~' =effective stress friction angle <br />c' =effective stress cohesion <br />6~; = uniaxial compressive strength of the intact rock <br />k =slope of the line relating 61' and 63' <br />6.3 Overall Wall Slope Design <br />6.3.1 Hoek and Bray Method <br />Analysis of the slope stability of the WHEX project walls can be achieved by the Hoek and Bray circular <br />failure analysis (Hoek and Bray, 1981). This analysis has been performed using the Mohr-Coulomb <br />parameters derived by the method described above. These parameters are computed piecewise (typically <br />on a 10 foot vertical increment of the geological section of the wall investigated by each drill hole), and <br />the factor of safety of the slope computed assuming that the entire wall is comprised of that material. <br />This is achieved by computing the dimensionless Hoek and Brown chart pazameter: <br />Report 1385L.200309017 12 <br />