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Mr. Peter Hays <br />RE: Second Adequacy Review Response <br />January 30, 2013 <br />Page 2 of 3 <br />The Division's Review of Slope Stability Analysis memo is attached for reference. <br />Answer: The assumption of a 20 ft setback was based on an approximation from aerial <br />imagery. Mr. Bucklen measured this setback from the top of his bank to the edge of the <br />asphalt of N. 21s Avenue, and found the distance to be 84 ft. A structure agreement is <br />included from the City of Greeley, and the Slope Stability Memo has been revised <br />accordingly and is included as an attachment to this document. <br />3. The Applegate Report states the soil parameters used for the analysis were based on <br />values used in the past for slope stability calculations and guidelines in the USBR <br />publication "Design of Small Dams ". The soil parameters used by Applegate in the <br />stability analysis differ considerably from the DRMS requirements for temporary pit <br />slopes and the DRMS accepted geotechnical standards. A copy of the DRMS <br />requirements and standard soil properties for stability analyses when no laboratory <br />strength tests were performed on the on -site soils is attaches for reference. Please <br />review the Division's requirements and standard soil properties and either provide <br />justification for the USBR soil parameters or re- analyze the slope stability calculations <br />utilizing the Division's standard soil properties. <br />Answer: Applegate Group does not feel that the assumption of 0 psf effective cohesion is <br />reasonable for undisturbed alluvial sand and gravel deposits. We have attached two <br />documents which calculate cohesion in such formations. <br />The first document, by Mohamad, et. al. finds a relationship between the water content of the <br />alluvial material and its water content. The lowest cohesive strength was found in saturated <br />material at 9.5 kPa, or 199.4 psf. In the mined condition prior to reclamation and filling the <br />reservoir this is a conservative value since the groundwater has been drawn down for <br />dewatering operations of the reservoir. It is more likely that the sand and gravel will be in a <br />dry condition, with a cohesive strength of 21.0 kPa, or 439.5 psf. <br />The second document, by William Gates of Jacobs Associates back calculates the cohesive <br />strength of alluvial deposits based on field observations of stable slopes. Their findings <br />indicated an angle of internal friction between 25° and 35°, and cohesive strengths ranging <br />from 200 to 500 psf. <br />Based on these findings, we submit that a cohesive strength of 55 psf is reasonable and <br />conservative. Also, per the DRMS Slope Stability Analysis, the top 2.5 ft of bedrock was <br />considered to be weathered with no cohesive strength. As indicated in our slope stability <br />memo, prior to backf lling any exposed bedrock will be removed to competent bedrock and <br />the backfill material will be keyed into this competent bedrock material. For the mining <br />condition, the weathered bedrock was considered in the excavated pit bottom. All other soils <br />parameters are as recommended in your Review were utilized in our stability analysis. For <br />the mining condition we found a factor of safety of 1.087, and for the reclaimed condition, a <br />factor of safety of 1.685. <br />