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• WHEXSIopeEvaluation AdrianBrown <br />6. SLOPE DESIGN <br />6.1 Me#hod <br />Design of the WHEX mine slopes has been performed using the Hoek method (Hoek and Bray, 1981, <br />Hoek and Brown, 1980), which comprises the following steps: <br />1. Derive the Hoek-Brown failure criterion using rock mass characteristics deternuned from <br />drilling information. <br />2. Derive Mohr-Coulomb strength parameters using the Hoek-Brown failure criteria, segregated by <br />depth where the material conditions change. <br />3. Compute the factor of safety for the wall using the Hoek stability charts for circulaz failure, or <br />wedge failure analysis methods where through-going adverse weak planes may exist. <br />6.2 Hoek-Brown Failure Criterion <br />6.2.1 Definition <br />The Hoek-Brown failure criterion is defined by (Hoek, 2001): <br />• 61' = 63' + 6C1*[mb 63'~6C1 ~- S]a <br />where: al' =major principal effective stress at failure <br />a3' =minor principal effective stress at failure <br />6~; =unconfined compressive strength of intact rock <br />mb =value of the Hoek-Brown constant "m;" for the rock mass <br />s = Hoek-Brown fitting pazameter <br />a = Hoek-Brown fitting pazameter <br />6.2.2 Geoloaical Strength Index (GSI) <br />The pazameters needed to define the failure criterion are computed generally in reference to the <br />Geological Strength Index. This is defined by Hoek and Brown (1997) using the Rock Mass Rating <br />(RMR) (Bieniawski, 1989). The RMR is computed on a localized or areal basis with reference to the <br />evaluation set out in Table 6. <br />The GSI is computed from the RMR with the following equation (Hoek and Brown, 1997): <br />GSI = RMR(89)' - 5 <br />where: GSI = geological strength index <br />RMR(89)' = Bienawieski (1989) Rock Mass Rating with GW=15 and Zero joint adjustment <br />• <br />Report 1385L.200309017 10 <br />