Seismic and Probably Maximum Flood Safety Evaluations for a Concrete Dam
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Project Objective

Stability under seismic and probable maximum flood loads was performed for a 345-foot-high thick arch concrete dam located in the Northern California. The construction of this dam was completed in the 1920’s and its hydropower structures are regulated by the Federal Energy Regulatory Commission (FERC).

Geosyntec’s Scope of Services

A seismic stability evaluation and uplift analysis under probable maximum flood (PMF) was performed as part of a FERC safety evaluation. The 3D analysis was performed using a specialized computer program for three-dimensional analysis of concrete, which modeled the dam as an assemblage of solid elements. For this analysis, the reservoir was idealized with three-dimensional fluid elements to include hydrodynamic effect of water. The model also included absorptiveness of sedimentary materials at the bottom or sides of the reservoir through a wave absorption coefficient for the reflected hydrodynamic pressure waves normal to the reservoir boundary.

In addition to the seismic stability analysis, a three-dimensional sliding stability evaluation of the dam was also performed using an iterative finite element analysis assuming progressive uplift of the base under probable maximum flood load condition. To incorporate the uncertainty regarding the efficiency of drains, a full uplift condition was considered that assumed a linear variation of uplift pressures from the effective reservoir head at the heel to the effective tailwater head at the toe of the dam.

The analysis showed that the unacceptably high stresses in the dam under seismic loads using conventional analysis were unrealistic and that the results from the 3D analysis that considered a more realistic response of the dam including the effects of absorptive reservoir sides and bottom were within acceptable limits.

Notable Accomplishments

Geosyntec’s thorough seismic stability evaluation and uplift analysis under PMF allowed for the successful presentation of the results and acceptance by the FERC regulators.