R655-11-5. Seismic Design

A. Because each dam site has a unique seismic and geological setting, detailed direction cannot be provided for seismic design which is applicable to all dams. Rather, an order of evaluation is presented beginning with more simplified methods and progressing, as required, to more rigorous procedures. In determining the sophistication of analysis required, the State Engineer may consider factors including consequences of failure, available freeboard, duration of reservoir pool, and site geometry. Regardless of the method of analysis, the final determination of seismic adequacy of a dam will be based on all pertinent factors involved and not strictly on the numerical analysis. The order of progression of the seismic analysis follows:

1. Undertake geological and seismological investigations to determine the potential for earthquakes and associated ground motions at the site, including the source and magnitude of the earthquakes to be considered and the selected motions, including potential fault rupture.

2. Undertake field and laboratory investigations of the dam and foundation materials to determine their properties and liquefaction potential.

3. Undertake an appropriate analysis for seismic events to predict factors of safety against slope failures, structural deformations, and liquefaction resulting from earthquake shaking or fault rupture.

4. Incorporate defensive design measures based on the analysis and proven practices.

B. In many instances, an adequate seismic analysis can be determined from the geological study and determination of the general properties of the dam and foundation. Other projects may require more detailed investigations and analyses. Decisions as to seismic safety and risk should be made as the analysis progresses and the extent of further investigations required after each step should be determined following consultation with the State Engineer as necessary.