Site Adjustment Factors Applied to Response Spectra for Hard Rock Dam Sites
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Project Objective

A power utility previously conducted seismic hazard evaluations for their portfolio of dams in the Sierra Mountain region of California. Those seismic hazard evaluations utilized a reference rock condition with a VS30 equal to 760 m/s. In 2018, Part12D Safety Inspections were completed for six of the dams and included recommendations that site-specific adjustment factors be evaluated and applied prior to performing seismic analyses of the dams. The objective of this project was to develop site adjustment factors that can be used to adjust the response spectra from the reference rock conditions used in the seismic hazard evaluations to the site-specific foundation conditions for each site.

Geosyntec’s Scope of Services

Geosyntec’s scope of services included 1) evaluating available site adjustment models, 2) characterizing the foundation conditions at each dam, and 3) developing site adjustment factors for the six dams. Available site adjustment models are largely empirically based and suffer from several shortcomings including a lack of ground motion recordings on hard-rock sites and few hard-rock stations with measured VS30 values, leaving a gap in the practice for the development of hard-rock site adjustment factors. At the time of the project, a study by Al Atik et al. (2022) was underway to address this gap by providing analytical site adjustment factors that can be used to adjust response spectra from a reference site condition with VS30 = 760 m/s to a site-specific VS30 between 680 – 2200 m/s.

The foundation conditions at each dam site were characterized based on available documentation and data. Three of the dam sites had shear-wave velocity measurements from geophysical investigations that were used to calculate site-specific VS30 values. Shear-wave velocity measurements were not available for the other three dam sites; thus, a desktop study was initiated to use available regional data and knowledge about the foundation conditions at the site to develop a range of VS30 for the sites without VS data.

Geosyntec worked with Dr. Al Atik to apply the work that was later published as Al Atik et al. (2022) and develop site adjustment factors for the six dam sites. Application of the site adjustment factors included an approach that differentiated between sites with measured and estimated VS30 values and ensured consistency between the site adjustment factors and the treatment of uncertainty in the seismic hazard. The project approach, site characterization, and adjustment factors were documented in a report which was submitted to FERC.

Notable Accomplishments

Geosyntec completed one of the first applications of newly developed site adjustment factors by Al Atik et al. (2022) to six dam sites in the Sierra Mountain region of California. Geosyntec’s knowledge of the ongoing study facilitated the development of site adjustment factors for the six dam sites using the latest available science, while at the same time assisting the power utility’s dam safety program in meeting their federal regulatory requirements.