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Memo to Kate Pickford 2 January 13, 2006 Gravel Pit Access Road Stabilitv File No M-2005-045 • The revised Exhibit C to the permit application "Pre-Mining and Mining Plan Map," revised on October 11, 2005 and received by the DMG Durango Office on November 17, 2005. • The preliminary "Access Road Overall Site Plan," prepared by Michael C. Whitney, P.E., dated August 31, 2005 and received by DMG on December 19, 2005. In order to evaluate the stability of the slope to be traversed by the proposed access road, the DMG modeled and analyzed the slope using Galena version 4.01 slope stability analysis software created by Clover Technology. The Trautner Report provides a description of the stratigraphy of the hillside where the access road is to be constructed. Field investigation by Trautner included drilling of three boreholes into the hillside. lrr each borehole clay soil regolith was shown to overly hard claystone bedrock of the Animas formation. For the DMG model, the soil mantle thickness over the slope cross section under analysis was conservatively set at 16 feet, which is the thickest section encountered in the three test holes. The DMG model includes a water table at 14 feet below ground surface, which is conservative since only one of the three test holes encountered water. Trautner conducted laboratory testing of soil samples collected from the three test borings. Selected samples were sieved and tested for Atterberg Limits in order to classify the soil under the Unified Soil Classification System. In addition, the soil strata were examined in the field, and classified using field techniques. The soil encountered in the test holes ranged from clay of ]ow to medium plasticity (lean clay, classification CL), to clayey gravels (classification GC), to silty clayey sands (classification SC-SM}. The Trautner Report states that selected soil samples were tested for shear strength using a shear box apparatus to test for direct shear strength. A disadvantage to the direct shear test is that the stress state in the sample is inhomogeneous and is utilmown; it is therefore not possible to obtain astress-strain relationship from the test results. For this reason, the utility of the direct shear testis limited when testing cohesive soil, such as that present on the hillside under investigation. The Trautner Report states that, based at least partly on direct shear testing, they estimated the shear strength of the hillside soil at an angle of internal friction of 25 degrees and cohesion of 150 pounds per square foot (psf). In order to assure that the slope stability analyses are conservative, the DMG compared the soil cohesion estimated in the Trautner Report with those published in the geotechnical literature for soil with the same classification. The publication Desiarr of Small Dams' lists an average cohesion value for CL type soil of 1483 psf. The publication Soil Mechanics, Foundations, and Earth StructuresZ lists an average cohesion value for CL type soil of 1358 psf and a value for saturated CL type sail of 459 psf Based on these literature citations, DMG has determined that the cohesion value from the Trautner Report of 150 psf is conservative and appropriate for use in the slope stability analyses. The Trautner Report characterizes the bedrock at the site to be hard claystone and fine-grained sandstone of the Animas formation. It is most likely that the bedrock will have a much higher shearing resistance than the overlying soil mantle, which would limit the depth of a potential soil failure to the depth of the soil bedrock interface. However, since no information was provided in the Trautner Report on the shear strength of the bedrock, the DMG has conservatively assigned to the model shear strength for the bedrock l U.S. Bureau of Reclamation, 1987, Design of Small Dams, 3'd Edition, Water Resources Technical Publication s Il.S. Naval Facilities Engineering Command, 1982, Soil Mechanics, Foundations, and Earth Structures, Design Manual DM-7, Alexandria, Va