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January 15, 2008 Page 21 <br />Rock properties were adjusted using the Hoek-Brown method 13 to account for natural <br />fracturing of the rock mass and disturbance induced by the landslide movement. The method <br />makes use of the Geologic Strength Index (GSI), which is essentially the same as RMR89. The <br />generalized strength criterion relates principal stresses to each other at failure according to <br />Equation 2. <br />r a <br />071 = Q3 + as I mb 63 + sJ (Eqn. 2) <br />l 6a <br />where Mb, S, and a are a functions of the material constant, mi, and GSI, as given by Equation 3. <br />mb=mixexp GSI - 1001 <br />/I <br />28-14D <br />C <br />s = exp?GSI -1001 (Eqn. 3) <br />9-3D <br />a= I + I (e -GSI 115 -e-2013) <br />2 6 <br />D is the disturbance factor which ranges between 0-1 where 1 is very disturbed rock. For this <br />work, D = 0 was assumed for all rock below the slide plane and D varied between 0-0.5 for <br />rocks within the landslide depending on loss of core and geologist comments in the logs. The <br />instantaneous uniaxial compressive strength, a, is obtained by setting o3 = 0 in Equation 2 and <br />the tensile strength is obtained by setting 61 = a3 = a, in Equation 2, which result in Equation 4. <br />cc =aaxsa <br />s x Qa (Eqn. 4) <br />Trm = at = - <br />mb <br />The rock mass modulus of deformation is given by Equation 5. <br />E = (1- D 0 ci x 0(GIS-10 / 40) (Eqn. 5) <br />2) 100 <br />Equation 2 is a non-linear relation between o1 and a3. Equivalent linear Mohr-Coulomb <br />strength parameters cohesion, c,,n, and friction, ?jm, for the rock mass were used as input for the <br />stability analyses. The equivalent linear parameters were computed by fitting an average linear <br />relationship to the curve generated by solving Equation 2 for a range of minor principal stress <br />values defined by ut < 03 < 63max. The relation for c„n and Orm are given by Equation 6. <br />13 Hoek E., C. Carranza-Torres and B. Corkum (2002), "Hoek-Brown Failure Criterion-2002 Edition," 51h North <br />American Rock Mechanics Symposium and 17th Tunneling Association of Canada Conference: NARMS-TAC, <br />p. 267-271, http://www.rocscience.com/library/pdf/RL-I.pdf. <br />Agapito Associates, Inc.