R655-11-6A. Factors of Safety

  A. All dams should meet the following criteria for factors of safety under normal loading conditions.

TABLE

                                                 Minimum Factor

   Condition of Safety

   End of Construction Case--upstream and

     downstream slopes 1.3

   Steady State Seepage--upstream and downstream

     slopes (full pool) 1.5

   Instantaneous Drawdown--upstream slope 1.2

    OR

   Actual Drawdown--upstream slope 1.5

  B. All factors of safety should be generated by methodology acceptable to the State Engineer. In undertaking the analysis, the effects of anisotropy should be considered and a ratio of horizontal to vertical permeability of at least nine should be used in the seepage analysis, unless otherwise justified to the satisfaction of the State Engineer. Ratios of up to 100 should be considered if the material types and construction techniques will cause excessive stratification.

  C. The strengths used in the stability analysis should be obtained from tests which best model the situation being analyzed.

  D. The analysis of the upstream slope stability for actual drawdown should consider drawdown rates which the low level outlets are capable of generating. Actual residual pore pressures should be used.

  E. For low hazard dams the State Engineer may waive the requirements of a stability analysis, including a seismic analysis, if it can be demonstrated that conservative slopes and competent materials are used in the dam, and seismic problems (i.e., liquefiable materials, active faults close to the dam) are not present.