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• ~ 0 balance the forces driving failure, As the FS increases above 1, the forces resisting failure are relatively higher than driving forces and the slope is relatively more stable. Just as importantly, as the FS dips below 1, failure is indicated. For slopes where good confidence exists regarding strength values, internal geometry and ground water conditions, an FS of 1.3 or above is usually adequate for non-critical situations. Instances where a critical installation such as a penstock must be protected, or if there is less confidence in the input parameters, require a higher FS, usually 1.5 or more. For the "dry" condition, with ground water at the natural ground interface, the relative FS increases to 1.2 (Fig. 12). We also modeled the sensitivity of the various embankments to simulate static (non-moving) heavy truck loading that would be legal for highway travel. We have not analyzed the impact of multiple truck trips, other truck configurations or dynamic (moving) trucks, but these are expected to negatively impact relative stability. The relative FS calculated in each instance decreased about 10 percent with static truck loading, as shown on Fig. 13, that indicates a relative FS of 1, compared to the result for Fig. 11, with a relative FS of about 1.1. Based on the assumption the existing slope configurations, truck traffic and ground water conditions result in a condition of imminent failure (FS=1), we considered various methods to increase the relative factor of safety. Our experience indicates the most effective methods involve: Provide drainaoe to control the level of ground water. An increase in the relative height of the ground water level (phreatic surface) in the embankment decreases stability substantially. Drainage could consist of horizontal drains or other means to control ground water. Our analysis indicates drainage has a strong, positive affect on relative stability. However, drainage alone (Fig. 14) does not increase the relative FS above 1.5, which we believe is necessary for a critical installation. Add a buttress fill of gravel and boulders to increase toe weight and thus increase forces resisting failure. The buttress is relatively easily constructed in a short time. Our analysis indicated a buttress fill at the toe of the embankment had the most positive affect on the relative stability. Addition of a buttress fill with relatively high ground water conditions (Fig. 15) brings the relative FS to about 1.3. Reduce the height of the new embankment. By trimming down the new embankment about 10 feet, the driving forces are decreased, thus increasing stability. Our analysis indicates this has a positive, but relatively small affect on overall stability. Stopping truck traffic has a positive effect on overall stability. PUBLIC SERVICE COMPANY OF COLORADO AMES HYDRO PENSTOCK CTLlT 77,Bd8 tl