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Anderson Consulting Engineers DRMS Title 34 Slope Stability Analyses Page 2 lining Rigden Reservoir. Excess excavated overburden and claystone material was, and is currently being disposed of elsewhere, one location of which is the POE pit, located immediately to the east of Rigden Reservoir. The POE pit project is intended to reduce the exposed water surface area and the City's future augmentation requirements, stabilize the site with respect to flood events from the Cache la Poudre River, and ultimately reclaim the site as a future Natural Area for the City. Subsurface Conditions In 2013, Brierley was retained by Anderson to perform a subsurface investigation for the Rigden Storage Reservoir project. Brierley observed the excavation of nine test pits on March 7, 2013 and drilled three exploratory borings on March 14, 2013. Test pits and test borings were located along a pipeline alignment, and within the pump station and Fossil Creek Reservoir Irrigation Ditch diversion structure footprints. A subsurface investigation was not performed in the vicinity of either modeled section. However, observations from a site visit revealed similar subsurface conditions to those in the area of our investigations. More detailed information on the subsurface conditions is presented in Brierley's Geotechnical Investigation Report dated June 12, 2013. In general, the materials encountered during our subsurface explorations consisted of fine alluvium, coarse alluvium and claystone bedrock of the Pierre Shale. The fine alluvium was composed of sandy -lean clay and silt. Coarse alluvium consisted of clayey sand, silty sand with gravel and poorly - graded gravel. Groundwater was encountered as shallow as 5 feet and as deep as 25 feet during the investigation. Brierley utilized existing geotechnical and DRMS published data (Allen Sorenson memo dated March 12, 2003) in assigning soil strength parameters to existing and backfilled materials. Analyses Critical cross sections were selected by Brierley and Anderson for each of the areas of interest based on an evaluation of the completed site. Brierley analyzed two conditions for each area: 1) stability immediately following construction and 2) rapid draw down. These analyses were performed using the computer program SLOPE/W based on limit (moment and force) equilibrium theory to compute a Factor of Safety (FOS). The Spencer method was selected to determine FOS values and considers both shear and normal interslice forces, satisfies both moment and force equilibrium and assumes a constant interslice force function. Model definitions and results including the FOS for these scenarios are included in Appendix A. Groundwater levels were provided by Anderson. The rapid drawdown phreatic surface used in the analyses is conservative. It would take approximately 48 days to drain the reservoir if completely full at the maximum pumping rate. Therefore, a saturated embankment ponded elevation is conservative. Results of the laboratory data were referenced in determining strength values for the subsurface materials. Additionally, strength properties were determined with guidance from the DRMS. The DRMS strength values, in particular the residual strength value for the weathered bedrock, represent the worst -case for potential slope instability. Mohr - Coulomb assumes an inherent cohesion in over - consolidated fine - grained or cemented soils and bedrock. The selected values used in the analyses are summarized in Appendix A. Analysis 1: Steady State Conditions The slope stability for the critical cross sections associated with County Road 40 Right -of -Way and the Interstate 25 Right -of -Way were analyzed under drained long -term steady state flow BRIERLEY ASSOCIATES Creating Space Underground