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TABLE 5.5 <br />MINIMUM FACTORS OF SAFETY FOR SLOPE STABILITY <br />OF EMBANKMENT DAMS <br />Loading Condition Minimum Factor of Safe <br />Steady seepage with phreatic surface fully 1.5 <br />developed for reservoir at normal pool <br />elevation <br />End of Construction 1.3 <br />Rapid draw-down (upstream sloe 1.2 <br />Not applicable for embankment dams on clay shale foundations; residual shear strength may be <br />appropriate and required factors of safety shall be determined on a case-by-case basis by the State <br />Engineer. <br />5.9.4.1.3 For Low hazard or NPH dams, the State Engineer may waive the <br />requirements for stability analysis if it can be demonstrated that conservative slopes <br />and competent materials are used in the dam design. Dams classified as Low hazard <br />or NPH shall have upstream slopes no steeper than 3:1 (horizontal:vertical), and <br />downstream slopes no steeper than 2:1 (horizontal:vertical). <br />5.9.4.2 Seepage and Internal Drainage Design - The evaluation of the steady state <br />seepage and internal drainage conditions shall be performed for all High and Significant <br />Hazard Dams. The seepage and internal drainage analyses shall include, but not be <br />limited to, the following: <br />5.9.4.2.1 Flow nets or numerical analysis computer programs shall be used in the <br />analyses. Data sheets and output files from these analyses shall be provided or made <br />available where requested. The hydraulic conductivity parameters used in these <br />analyses shall be obtained from field permeability tests, laboratory permeability tests, <br />or empirical/correlative permeability determinations, and the sources of the estimated <br />hydraulic conductivities shall be clearly documented. <br />5.9.4.2.2 The analyses shall quantify the anticipated seepage beneath, around and. <br />through the dam. Seepage exiting on the downstream face of the dam shall not be <br />permitted. Internal drains and filters shall be constructed of granular soil materials <br />(sands and gravels), and the filter and drain zones shall be of sufficient thickness to be <br />constructed without significant contamination or loss of continuity that would <br />adversely impact the performance of these features. <br />5.9.4.2.3 The filter compatibility of the drain and embankment material shall be <br />evaluated utilizing current state of the practice methodologies, such as those <br />published by the Natural Resources Conservation Service, the U.S. Bureau of <br />Reclamation or the U.S. Army Corps of Engineers. <br />27