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weakly cemented sandstones and conglomerates, with no clearly defined structure or discontinuities, we considered the deposits a heterogeneous and isomorphic rock mass. In addition, we assumed the exposed rock slopes represented the most weathered and weakest conditions for the rock mass. GEOMECHANICAL ROCK MASS CLASSIFICATIONS Our team classified the rock mass of the slopes using Bieniawski's (1989) Rock Mass Rating (RMR) and Hoek and Brown (1997) Geologic Strength Index (GSI). This approach by Hoek and others has been modified and improved over the last 10 years (Marinos and Hoek, 2001; Hoek and others, 2002; RocLab, 2012). In general, because the rock mass is amorphous with no clear structure, it was difficult to establish an accurate RMR and subsequent GSI. We estimated a GSI of about 40; however, after review of Hoek and Brown's charts for GSI, we downgraded the value to about 35; which seemed more appropriate. The intact strength of the rock matrix ranged between 145 to 725 psi with a mean strength of about 483 psi. In addition, since the matrix of the conglomerate consisted of a weakly cemented sandstone, we selected a Hoek -Brown intact rock materials index of 17 for sandstone (conglomerate is 21 and not appropriate for this material; RocLab, 2012). The stability of the slope was very sensitive to the intact rock strength. To evaluate and select conservative strength data for the design, we used the "Three -Sigma Rule" described by Duncan (2000). This rule is based on the fact that 99.73% of all values of a normally distributed parameter fall within approximately three standard deviations of the mean value. Therefore, if HCV equals the highest conceivable value of the parameter, and LCV equals the lowest conceivable value of the parameter, these are approximately three standard deviations above and below the average value. One may elect to choose a value that is one to three standard deviations below the mean value to be conservative or in situations where there is limited data available. Therefore, for these slopes we estimated the strength as follows: HCV = 725 psi, LCV = 145 psi. Standard Deviation = a = (725 - 145)/6 = 97 psi, 3a = 290 psi Mean = 483 psi Mean - 3a = 483 psi — 290 psi = 193 psi. Therefore, intact rock strength of 193 psi was used to model the global stability of the cut slopes. This rock strength is probably conservative with respect to short term stability because it does not reflect the less weathered condition of the rock on the interior of the slope. However, we assumed that it was representative of the long term strength of the rock mass further into the slope if the rock mass is allowed to continue to degrade over time. MOHR- COULOMB SHEAR STRENGTH BY BACK ANALYSIS We estimated the Mohr- Coulomb shear strength of the rock mass exposed in the cut slopes using back analysis. The geologic conditions of a rock mass at both cut slopes demonstrated there were no apparent distinct discontinuity surfaces on which sliding might take place during failure. The sliding surface in these rock masses will be along both natural discontinuity surfaces (indistinct bedding, joints or faults) together with shear through the intact rock or the matrix that holds the rock mass together. Two empirical methods have been established to establish the Mohr- Coulomb shear strengths; back analysis and general Hoek -Brown criterion. According to Wyllie and Mah (2004), the most reliable method to establish the strength of the rock mass is to back analyze a failed or failing slope. Only one strength parameter can be calculated in a back analysis. Typically, one would select a reasonable friction angle for the rock mass based on testing or the literature. The engineer would then employ a computer program