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\V YY C~ ^ CL. <br />. These tests indicated a cohesion of 1,100 psf and a 12 degree angle of internal friction. It is <br />important to note that these tests were conducted under saturated conditions. The Mancos <br />Shale, however, is not saturated in the waste bank location, and therefore, the Maxim results <br />are not representative of site conditions. WESTEC envisions the Mancos Shale to remain in its <br />in-situ dry state during the construction and life of the waste bank because of the various <br />hydraulic components, including underdrain and surface runoff control ditches, that take water <br />away from the site. Since the coal mine waste bank will be constructed relatively slowly, <br />undrained conditions will not be induced. <br />Using the strength parameters for the Mancos Shale obtained in the WESTEC geotechnical <br />investigations described above, and the strength properties obtained by Maxim (Maxim, 1997) <br />for the colluvium and fill, the stability analysis presented in the design report (WESTEC 1996) <br />has been revised. The material properties used in the revised stability analysis are summarized <br />in the following table: <br />Material Properties Suggested for Revised Slope Stability Analyses <br />• <br />Material Total Unit <br />Weight, y,,, <br />f Saturated Unit <br />Weight, y,,, <br />f <br />Cohesion, c <br />f Internal Angle of <br />Friction, ~ <br />r <br />Coal Mine Waste 65 78 800 36 <br />colluvium 120 125 900 32 <br />Fill 110 115 200 36 <br />Mancos Shale 130 135 750 46.7 <br />Attached are the revised stability analyses for static and pseudostatic conditions. All of the <br />conditions remain the same as the described in the WESTEC design report, except the revised <br />material properties. Results indicate a static factor of safety of 2.7, which is above the <br />Colorado coal regulation Rule 4.10.4(2) for static stability (factor of safety greater than 1.5). <br />Under pseudostatic conditions the waste bank remains stable with a factor of safety of 2.0. <br />CONCLUSIONS AND RECOMMENDATIONS <br />It is demonstrated by this report the importance of estimating strength properties of materials <br />under conditions that will be produced in the field. WESTEC believes that the strength tests <br />presented in this report for the Mancos Shale closer represent conditions in the field. <br />By comparison of slope stability results presented here (2.7 static and 2.0 pseudostatic) and <br />those presented in the design report (1.5 static and 1.1 pseudostatic), it is apparent that the <br />material properties used in the design report are conservative, thereby resulting in lower factors <br />of safety than those presented in this report. The stability results calculated in this report with <br />revised material properties enhance the conclusion reached in the design report of general slope <br />• stability. <br />