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To: Chris Nyikos <br />From: Philip B. Solseng <br />Subject: Stability Analysis Results <br />Date: 16 September 2009 <br />Project: Pond ME Evaluation <br />c: Jed Greenwood, Brad Lindsay <br />STABILITY ANALYSIS METHODS <br />The slope stability of the embankments was analyzed to determine the factor of safety for the following <br />conditions: <br />a. Downstream stability of the embankment when the pond is at maximum pool, 1 foot below crest. <br />b. Upstream stability of the embankment when the pond is at normal pool <br />c. Upstream stability of the embankment when the pond is at normal pool and the river floods to <br />Elevation 6051.5. <br />The assumed typical water level for the North Fork Gunnison River is Elevation 6040. For modeling <br />purposes, it was assumed that the phreatic surface was at least Elevation 6041, which corresponds to the <br />normal permanent pool elevation in the pond. <br />Two types of stability analyses are typically performed for embankments of this type: the Undrained <br />Strength Stability Analysis (USSA) and the Effective Stress Stability Analysis (ESSA). The USSA is <br />performed to analyze the case in which loading or unloading is applied rapidly and excess porewater <br />pressures do not have sufficient time to dissipate during shearing. This scenario typically applies to <br />loading from, for example, embankment construction where the loading takes place quickly relative to the <br />permeability of the soils but loading from floodwaters also meets these qualifications. It is often referred <br />to as the "end-of-construction" case. The ESSA is performed to account for much slower loading or <br />unloading, or no external loading, in which the drained shear strength of the materials is mobilized and no <br />shear-induced pressures are developed. For example, a slowly moving landslide is best analyzed using the <br />ESSA method. For this reason, the ESSA is often referred to as the "long term" case. Both the USSA and <br />ESSA were performed as part of the slope stability analysis for the Pond MBSE project. <br />A seismic analysis was completed for the normal pool condition only because the probability of having <br />the maximum pool and a seismic event occurring simultaneously is extremely low. In accordance with <br />seismic design requirements for impoundments per CDRMS regulations, a minimum seismic factor of <br />safety of 1.20 was used in this analysis. For the pseudo-static stability analysis, a peak ground <br />acceleration (PGA) of 0.166g was used. This PGA is meant to account for an earthquake as specified for <br />buildings in ASCE 7-05. The USSA scenario was modeled for the seismic case.