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~~o~ September 7, 2005 Valco, Inc. P.O. Box 837 Gunnison, CO 81230 Attention: Mr. Ed Stewart Subject: Slope Stability Analysis Valco Gunnison Pit Gunnison, Colorado Project No. GSO4594-105 CTL I THOMPSON This letter provides the results of our slope stability analysis for the Valco Gunnison Pit in Gunnison, Colorado. We determined the theoretical factor of safety (FOS) of a generalized cross-section of the existing pit slopes and then determined what pit slope was required to achieve a FOS of 1.25. To evaluate slope stability it is necessary to approximate the slope geometry, the general profile of the soil, ground water conditions, and strength parameters of the soil. For this study, slope geometry was modeled from a survey by Furey Land Survey, Inc. The generalized existing pit slope consists ofa very steep uppersection, a steep middle section and a flatter lower section. The soil profiles were based on subsurface conditions observed in the pit. Shear strength of soil is defined by friction angle and cohesion when the Mohr/Coulomb model is used. The relatively large diameter ofthe granular soil atthe site make it difficult to perform laboratory strength testing to determine strength parameters. Therefore, strength parameters were based on our experience and published values. The gravels were considered a purely fractional soil with no cohesion. During a slope stability analysis, a "factor of safety" is calculated for a particular modeled slope condition. The factor of safety is the ratio of the strength available to resist sliding compared to forces tending to cause sliding. Therefore, a factor of safety of 1.0 means the strength to resist sliding movement is equal to the driving force trying to cause movement. Ifthe strength to resistsliding is greaterthan the resisting force then the FOS is greater than 1.0. 234 Center Drive ~ Glenwood Springs, Colorado 81601 Telephone:970-945-2809 Fax:970-945-7411