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page two <br />28 November 2012 <br />M12058MT <br />- unsaturated, as constructed, <br />- full reservoir water level, saturated embankment steady state <br />from the phreatic surface within the embankment, and <br />- rapid draw down, saturated embankment, reservoir drained. <br />Rapid draw down is the most critical embankment condition. <br />This occurs when the reservoir is full from the impounded water <br />and the reservoir is drained rapidly so that the embankment soil <br />materials remain saturated and there is no upstream water within <br />the reservoir. Because rapid draw down is the most critical <br />embankment condition and the most likely instability this is the <br />scenario that we assessed. The other embankment conditions will <br />be more stable than the rapid draw down scenario. <br />The stability of any slope is dependent on many factors. Typi- <br />cally the theoretical stability of a slope is analyzed by <br />calculating the anticipated gravitational forces that tend to <br />drive the mass of soil downhill and the anticipated internal <br />strength of the soil along the expected plane of failure that <br />will resist the downhill movements. If the driving forces are <br />equal to or greater than the resisting forces then failure is <br />imminent. A theoretical calculated factor of safety of 1.5 is <br />considered by the geotechnical engineering industry as a minimum <br />factor of safety for a slope to be considered as stable. A <br />calculated factor of safety of 1.0 or less indicates that slope <br />movement is imminent or in process. Failure can occur as slow <br />deformation, creep, or as a somewhat spontaneous failure. <br />Factors that have an adverse influence on slope stability can <br />generally be classified as those that increase the stress <br />(driving force) on the system or those that decrease the strength <br />(resisting forces) of the soil. <br />Our stability analyses of the embankment slope was based on the <br />Bishops Method of Slices. This method is based on the assumption <br />that the slope soil mass will fail in a rotation mode on a <br />circular arc plane. In this method of analysis the mass of soil <br />is divided into vertical slices. The forces acting on each slice <br />are evaluated from the equilibrium of the slices; that is, the <br />forces that tend to drive the slice downhill and the forces that <br />tend to resist the movement of the slice. The equilibrium of the <br />CONSULTING GEOTECHNICAL ENGINEERS <br />AND MATERIAL TESTING <br />Appendix 2.05.3(4)-6 Page 3 December 2018 (TR -19) <br />