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Hazeltine/Road Runners Rest IllBrinkmann-Woodward Gravel Pits Lining Project -Design Technical Memorandum Denver W ater August 2003 Section 3 -Slope Stability Analyses 3.1 General GEI performed two-dimensional static slope stability analyses to estimate the location of the failure surface for the selected factor of safety (FOS) for the interior (reservoir side) and exterior (Bull Seep and South Platte River sides) slopes along the alignment of the cutoff wall. The purpose of this analysis was to identify the zone where the cutoff wall should be located to provide an acceptable FOS against damage associated with slope movement as shown on Figure 3.1. We used the computer program STABLSM (Purdue, 1998) to perform the iterative task of identifying a potential failure surface. We used the modified Bishop method to perform these limit equilibrium calculations. 3.2 General Assumptions and Slope Stability Design Criteria Reclamation and permanent design of side slopes of the reservoir is not part of the scope of this phase of the project, and analysis of permanent slopes will be conducted by Denver Water during subsequent phases of the project. GEI conducted the slope stability analyses to select the horizontal alignment of the soil-bentonite wall and identify areas where earthwork will be required to support the soil-bentonite cutoff wall until permanent reclamation can be completed. The stability of a slope in engineering practice is typically indexed to a FOS against failure determined using limit equilibrium concepts and the method of slices. In the limit equilibrium analysis, the FOS is the relationship between the resisting forces (shear strength of the soil and/or bedrock) and the driving forces (weight of soil and other loads) satisfying either moment or force equilibrium, or both. A FOS of less than 1.0 indicates instability. The design of the reservoir side slopes should include a FOS larger than 1.0 to account for: 1) variations in subsurface conditions, 2) uncertainties associated with interpreting limited data (i.e., estimating subsurface stratigraphy between widely spaced borings and material properties from limited laboratory tests), and 3) the economic cost and operational impact of slope failure. Our stability analyses were based on the following general assumptions and design criteria: The gravel pits would be mined to the top of bedrock and the bottom of the reservoir will be at the top of bedrock. Top of bedrock is generally around El. 5000, except for the eastern part of the reservoir where the top of bedrock is at about El. 4968. GEI Consultants, Inc. 8 o?osoo?-ov-=i in-uvdn~ro~z~y~r>,