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December 1986 8 863-2060 <br />• <br />Ground-water elevations measured in piezometers G-8, G-9, and <br />G-10 are summarized in Table 1. Additional piezometric data are <br />available for seven piezometers {Appendix A) which were installed <br />in 1985 near the southern boundary of Area 51 (Table 2). Ground- <br />water levels in the vicinity of the 1986 piezometers are cur- <br />rently very near or below the contact between the mine spoil and <br />underlying bedrock units. However, historic piezometric data <br />collected in the vicinity of the Area 51 highwall (Table 2) <br />iddicate that ground-water levels fluctuate seasonally. As ex- <br />pected, ground-water levels in the vicinity of the study area are <br />highest during or shortly after peak snowmelt periods. <br />4.4 Historic Slope Instabilit <br />• Slope stability conditions in Area 9/10 should be evaluated in <br />the context of geotechnical conditions associated with unstable <br />mine areas in the general site vicinity. Based on conversations <br />with YVCC personnel, unstable final slopes at the Colorado Yampa <br />mine have only been observed in Area 1, which is located along <br />Middle Creek east of Area 9J10 near the strip mine's eastern <br />boundary. In addition, a significant mass movement of mine spoil <br />occurred during May of 1984 at the Edna coal strip mine, which is <br />located approximately 4 miles southeast of the study area. Geo- <br />techniCal aspects of these slope instabilities are discussed in <br />the following paragraphs. <br />• <br />Golder Associates <br />