The City of Newport, Oregon owns and operates two earthen dams to store water and regulate flows to the city’s water treatment plant. Based on geotechnical explorations, the foundation of the two dams were found to be comprised of low density, high plasticity silt (MH) that would experience large deformations leading to failure of the dams during large earthquake events. In 2021, the upper dam was classified as unsafe by the Oregon dam safety office due to ongoing seepage and internal erosion along the exterior of a spillway conduit penetrating the dam, as well as the concerns related to earthquake response. As a result, both dams have moved to the top of Oregon’s list of unsafe high hazard dams. Through a series of subsequent geotechnical investigations, alternative analyses, feasibility design development, and value engineering, the decision was made to proceed with final design of a new roller-compacted concrete (RCC) dam to replace both the existing upper and lower dams. The new dam will be constructed at a site between the two existing dams. The new dam will be capable of safely performing under the extremely high seismic hazard loading conditions associated with the site’s close proximity to the Cascadia Subduction Zone (CSZ).
This paper will describe the risk-informed design of the new 100-foot-high RCC dam completed through a series of progressively complex 2D stability and both 2D and 3D nonlinear finite element analysis (FEA). Sensitivity analyses have helped to identify the optimum cross-section of the dam combined with an optimum curvature to take advantage of arch action given the site constraints. Curvature of the dam is providing an important balance between deterministic (Maximum Credible Earthquake, MCE) design criteria required by the state dam safety office and federal requirements for risk-informed design that accounts for estimated life loss consequences if federal funding is provided for construction.
