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Ms. Christina L. Kamnilrar February 22, 2000 Page 2 oj4 In engineering where there is no conclusive test data to base an analysis and design upon, a more practical solution should be used. We have reviewed the Division's stability models that show slope failures with a zero cohesion value used for the bedrock in the stability model. Unfortunately, these models are not supported by the conditions that occur in the field. If the bedrock is modeled with zero cohesive strength, then the mining highwall will not be stable. However, there are currently several hundred miles of gravel highwalls that aze stable in the State of Colorado. There should be a very high incidence of highwall failures if the DMG mode] is con•ect. Practical experience shows that this mode of failwe razely happens. The bedrock for this site was modeled with a "c" value 300 psf a.~d a phi angle of 45degrees. We believe that the values aze a conservative estimate of the strength of the material. In the stability model, the cohesive strength of the underlying bedrock can range from a value of 25 pcf [0 500 pcf with little to no change in the stability of the highwall. The reason for this is simple. A slope failure that occurs in a gravel highwall will occur in the gravel and not in the bedrock underneath the gravel reserve. The gravel is a "weaker" material than the bedrock. Item #3 -The USCS's soil classifications were included on the bore logs in the geotechnical report ~ prepazed by Terracon. The strength parameters were not taken from a published document, but aze ~.~ ~ based on the years of experience of both the engineers and the operators involved with the soils in v' this region. Please clarify exactly what you would like to see from the "lithographic profile of the oils". All of the items that you have requested have already been shown in the bore logs and in~ Table 1 of the Stability report. ~em #4 -Please see our letter to your office dated January 11, 2000 concerning the drain tiles. ' \~ ~~ V ~~ l\ Item #5 -The fish hatchery tanks have been included in the stability analysis (please see Case 5). To simulate the presence of a "tank", a 5000 psf foot surchazge load has been used in the model. This is greater than the bearing capacity of the soil (see also Item 7). The. water surface in the stability model (phreatic surface) was shown to be approximately 5.0 feet below the existing ground surface. This water surface elevation would be consistent with a pond (ground water lake) or a storage tank that has a considerable leakage rate. We believe that the analysis presented depicts an overly conservative estimate of the impact the fish hatcheries would have on any proposed mining highwall. r.1.,:~ ~ ..' Item #6 -There aze three cases where the fences could be impacted by a potential slope failure. Casel and Case 2 deal with the fence along the ROW of the roads that surround the property. The 'a setbacks in this case are already from the ROW line, therefore an additional setback is not warranted. Case 4 deals with the surrounding homes and fences. This case shows the critical slope failure 45' back from the edge of the pit. In addition to the 100 foot setback requirement proposed for the homes, an additiona145 foot setback from any fence line should be adhered to in this azea.