2013-02-01_REVISION - M1982131 - Laserfiche WebLink

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Date: November 29, 2012 Revised January 29, 2013 To: File Applegate Groupo.. Water Resource Advisors for the West Memorandum From: Carrie R. Herbolsheimer, PE Subject: Bucklen Equipment Company Pit Area A - Slope Stability Calculations AG Job No.: 07 -148 Slope stability analyses were performed for the Bucklen Equipment Company Pit, Area A. The analyses were on two different conditions, one for the mining condition, during which slopes were excavated at a 1H:1V grade, and one for the reclamation condition, during which the embankment backfill will be placed at no steeper than a 3H:1V grade. The calculations were performed using the XSTABL slope stability software. The following soil parameters were used for the project, and are based on values provided by the DRMS in their review of the 11/29/12 slope stability analysis. Applegate Group does not feel that the assumption of 0 psf effective cohesion is reasonable for undisturbed alluvial sand and gravel deposits. We have attached two documents which calculate cohesion in such formations. The first document, by Mohamad, et. al. finds a relationship between the water content of the alluvial material and its water content. The lowest cohesive strength was found in saturated material at 9.5 kPa, or 199.4 psf. In the mined condition prior to reclamation and filling the reservoir this is a conservative value since the groundwater has been drawn down for dewatering operations of the reservoir. It is more likely that the sand and gravel will be in a dry condition, with a cohesive strength of 21.0 kPa, or 439.5 psf. The second document, by William Gates of Jacobs Associates back calculates the cohesive strength of alluvial deposits based on field observations of stable slopes. Their findings indicated an angle of internal friction between 25 and 35 and cohesive strengths ranging from 200 to 500 psf. Based on these findings, we submit that a cohesive strength of 55 psf is reasonable and conservative. We believe that this is an appropriate assumption also based on the fact that there are many sand and gravel pits in the Platte River Valley with almost vertical highwalls which have stood for many years. If a slope stability model were applied to these existing pits using the numbers that DRMS proposes, the model would show that they have a safety factor less than one and these slopes should have failed. Based on the field evidence that these slopes have remained stable for many years during mining, it is reasonable to assume that there is adequate cohesion to keep these vertical highwalls standing, and that the alluvial sand and gravel in the Platte River Valley are weakly cemented, similar to the studies referenced above. The USDA Web Soil Survey website was used to create a custom soil report for the site. A copy of this survey is included as an attachment to this document. Page 1 of 15