2008-04-15_REVISION - M1980244 (192)

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• GHMXSIopeEvaluation 3. SLOPE DESIGN 3.1 Hoek and Bray Circular Failure Analysis 3.1.1 Method The design of the overall slope angles for the GHMX is performed using the Hoek and Bray circular failure method (Hoek and Bray, 1981). This method applies Hoek-Brown failure criteria for the rockmass to derive the Mohr-Coulomb failure parameters (effective stress cohesion and friction angle), and then applies these parameters to a dry slope of the appropriate height. The analysis is valid when the rockmass does not have persistent discontinuities dipping into the mined excavation, which is the case at GHMX. 3.1.2 Hoek-Brown Failure Criterion The Hoek-Brown failure criterion is defined by (Hoek, 2001): a 61 ~ = 63' + aci [mb 63'~6C1 + S~ where: 6,' =major principal effective stress at failure 63' =minor principal effective stress at failure • 6~; =unconfined compressive strength of intact rock mb =value of the Hoek-Brown constant "m" for the rock mass s = Hoek-Brown fitting parameter a = Hoek-Brown fitting parameter The parameters needed to define the failure criterion are computed generally in reference to the Geological Strength Index (GSI). This is defined by Hoek and Brown (1997) using the Rock Mass Rating (RMR) (Bieniawski, 1976). The RMR is computed for the main rockmass with reference to the evaluation set out in Plate 7. The GSI is computed from the RMR with the following equation (Hoek and Brown, 1997): GSI=RMR- 5 where: GSI = Geological Strength Index RMR =Bieniawski Rock Mass Rating The Hoek-Brown parameters are defined with reference to the GSI as follows: Parameter Formula Comment GSI RMR-5 Geolo ical Stren th Index S ex GSI-100 /9 Hoek-Brown arameter A 0.5 Hoek-Brown arameter mb m; ex GSI-100 /28 Hoek "m" The Mohr-Coulomb parameters can be estimated from the Hoek-Brown failure criterion by • consideration of the tangent to the 61'-63'. The equation to the tangent is: Report 1385E.20071126