2012-08-14_REVISION - M2002020 (5)

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1,1104 Applegaite Group, Inc. Consultants for Land, Minerals and Water Memorandum Date: 02 -14-02 File No. 01 -101 To: City of Greeley From: Craig Ullman Subject: Slope Stability for 2? Avenue Site As requested by the City of Greeley we have performed a slope stability analysis for the 25 Ave. gravel pits. The purpose of this analysis was to determine a stable slope configuration for additional mining proposed at the site and to determine a proper location for the proposed slurry wall. Two cross sections were selected for the analysis. The first cross section was located along the western boundary of the West Pond and the second section was along the north edge of the east pond. The first cross section poses several problems. The 35 Ave stormwater ditch is in close proximity to the existing pond. The top width of the berm separating the stormwater ditch from the pond is currently 15 ft. A buried gas line is located within this berm. A 40ft wide working platform is proposed for the construction of the slurry wall. Assuming a side slope of 3:1 the platform slope will tie into the existing surface somewhere near the existing toe of slope. Soils in this area consist primarily of approximately 5.5 11 of silts lying atop 50 ft of coarse alluvium. The second cross section was along the border of the Cache La Poudre River. Existing pit slopes appear to vary from 211:1 V to 411:1 V. The stability analysis assumed all pit slopes would be brought to 311:1 V. Topographic data was not available for the water surface or exact location of the river with respect to the ponds. Therefore, the water surface elevation in the river was assumed to be 5 feet below the working platform for the slurry wall and the edge of water was assumed to be 10 ft from the outer edge of the working platform. Borelogs in this vicinity indicate that there is a thin layer (2.5 ft) of coarse alluvium lying atop of approximately 5 feet of fine alluvium or silt. The remaining depth to bedrock consists of coarse alluvium. From this point additional mining could be performed in one of two ways. If mining progresses in such a manner as to expose a high wall of alluvium then the limits of mining will need to be offset a'sufficient•distance from the existing toe of slope. The stability analysis was used to determine an acceptable distance for this setback. After completion of mining the side slopes would need to be reclaimed at a specified slope. The construction condition for this scenario was analyzed due to the presence of the high wall. Mining could also be performed by beginning at the existing toe and mining down to bedrock at a specified slope. Both scenarios were analyzed for stability during a rapid drawdown condition. This scenario assumes that the pore water pressure in the embankment remains unchanged as the reservoir is drained. The minimum allowable factor of safety for all scenarios is 1.20. Several assumptions were made in the stability analysis. It was assumed that the depth of remaining alluvium was 30 feet. Weathered bedrock was assumed to extend 5.5 ft below the bedrock surface. All pit slopes were assumed to be constructed at a slope of 311: IV. Groundwater levels were set at 10 ft below existing ground surface. Soil parameters were unavailable for the site and therefore, were determined based on conservative values for similar materials. Of special note, the materials used to reclaim the slopes of the original pit were placed without controlled compaction and therefore the state of these materials is questionable. Very conservative assumptions were made concerning the soil 1499 West 120 Avenue, Suite 200 • Denver, Colorado 80234 -2728 • (303) 452 -6611 • Fax: (303) 452 -2759 5441 Boeing Drive, Suite 200 • Loveland, Colorado 80538 -8855 • (970) 461 -9884 • Fax: (970) 613 -1177