2012-12-28_REVISION - M1982131 (3)

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ID Material Moist Density Saturated Density Cohesion Phi Angle 1 Sand and Gravel 121 135 55 40 2 Undisturbed Bedrock 112 132.9 660 64 3 Clay Backfill 112 132.9 274 64 4 Overburden 112 132.9 274 64 Date: November 29, 2012 To: File r ot Applegate Group, inc. 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 used in the past for slope stability calculations and guidelines in the USBR publication Design of Small Dams. 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. The critical section was selected as shown on the following figure. The eastern edge of N 21st Avenue is located approximately 20 -ft from the top of slope of the mined pit. A highway loading was applied to the models at the roadway location. The XSTABL profile plots are also included in this document along with the model output files. The profiles include the sand /gravel layer which is being mined, on top of a bedrock foundation. Weathered bedrock will be removed prior to backfilling the embankment to ensure that competent bedrock is used as the foundation material for the reclaimed condition. A 2 -ft layer of lean clay was assumed over the sandy gravel layer, as indicated by the USDA Soils Report for the vicinity. In the mining condition, the groundwater profile was assumed to be parabolic in nature, 2 -ft below the ground surface away from the cut slope, and daylighting at the toe of slope. This condition would normally be found in a dewatered gravel pit that has reached steady state conditions for the groundwater table. In the reclaimed condition, the worst case situation was assumed in which the groundwater profile was at 2 -ft below the ground surface up to the top of bank, daylighting at the toe of slope. This would result in a saturated slope which should produce the lowest factor of safety. The mined slopes, which have already been completed, have a factor of safety of 1.17. The future reclaimed slopes will have a factor of safety of 5.24. It is noted that the generally accepted factor of safety for mined slopes is 1.2. In this case, the slopes have already been mined, and the failure termination point is within a couple feet of the top of bank. If the bank should begin to fail, the equipment company would notify the City Page 1 of 15