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• When water and/or earthquake conditions are considered the difference <br />between the 2:1 and 2.5:1 configurations is more apparent. For the worst case water <br />condition the factor of safety for the 2:1 configuration is 1.2 (compared to 1.3 for the <br />2.5:1 configuration). As discussed in the previous section the worst case water <br />conditions do not exist, are not considered likely, and were only evaluated as a <br />worst case scenario. With a 20-foot layer of water at the base and an earthquake <br />coefficient of 0.1 g, the factor of safety for the 2:1 model is about 1.15 compared to <br />1.3 for the 2.5:1 model. The 1.15 factor of safety is below the regulatory minimum <br />of 1.2 for earthquake conditions (Reference 14). This is a very conservative model <br />with water included in the earthquake model. <br />Based upon the sensitivity of the proposed configurations to water and <br />earthquake conditions, we recommend the proposed RDA be constructed with <br />• minimum 2.2:1 slopes between the benches. For this configuration we believe the <br />factors of safety are adequate and exceed regulatory minimums for all but the worst <br />case water condition. We do not believe the worst case water condition is likely <br />based upon the results of the current investigation. <br />To summarize, the existing RDA configuration is considered stable. A <br />proposed 2:1 configuration is generally stable but conditions become marginal when <br />water is included in the earthquake model. A configuration with 2.2:1 slopes <br />between benches is recommended. The analysis reveals the significance of water <br />levels on stability, compared to the strength or density of the coal refuse. Although <br />continued compaction and density testing of the refuse is recommended, the <br />recommendations concerning drain construction, surface grading and ground water <br />n <br />21 <br />