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Y <br />• The slope stability model analyzes the driving forces (i.e., forces causing the slope to move) versus the <br />resisting forces (i.e., the sheaz strength of the soils tending to prevent movement). The ratio of these two <br />forces yields the factor of safety of the slope against movement (failure). A factor of safety of 1.0 <br />indicates failure of the slope and subsequent movement. A minimum factor of safety of 1.5 under static <br />conditions is required for the RDA. Analyses were run to determine the overall factor of safety against <br />failure and the factor of safety against shallower slides. The results of our analyses indicate that <br />constructing the slope to the planned configuration and with the subsurface conditions assumed above, <br />will result in minimum factors of safety ranging from 1.6 [0 1.7. The slope stability analyses and data <br />aze presented in Appendix A. <br />CONSTRUCTION RECOD~AIENDATIONS AND SEQUENCING <br />Based on the stability analyses completed, it appeazs that the waste coal pile should be stable at its <br />proposed configuration. However, due to the moisture content of the processed waste coal materials and <br />the production rate of these materials, it appears that special precautions and construction techniques will <br />have to be followed to ensure that the waste coal materials aze properly drained and dried out prior to <br />placement in the embankment. This section has been prepared to outline the recommended changes in the <br />construction and sequencing of the RDA due to the variable properties of the processed waste coal <br />. materials. <br />In the original report prepared by CTL/Thompson, it was assumed that the processed waste coal materials <br />will be coazse grained materials with very little fines (less than 10 percent passing the No. 200 sieve) and <br />the materials will be cohesionless. It was also assumed that the materials will be relatively free draining <br />and [he possibility of developing saturated zones within the pile are minimal. However, based on the <br />laboratory test results obtained during this investigation, it appeazs that the processed waste coal <br />materials, which are being produced and placed in the RDA, have considerably more fines than were <br />assumed to be in the original design (2 [0 5 times higher). In addition, the materials tested were low to <br />moderately plastic with plasticity indices ranging from ]0 to 17; whereas, the repott prepared by <br />CTL/Thompson indicated that the processed waste coal materials were non plastic. The permeability <br />tests run by our firm on the remolded samples of processed waste coal indicate that these materials are <br />relatively impervious and not free draining. It should also be noted [hat moisture content tests taken <br />previously by our firm indicate that the moisture content of the processed waste coal materials coming <br />from the wash plant and being stockpiled at the RDA Site ranges from LO to 15 percent over the optimum <br />moisture content. <br /> <br />Job Na. 97-J216 Northwest Colorado Consulunts, lnc. Page 7 <br />