Upper Blue Lake Dam Seismic Retrofit Project

High in the mountains of eastern California, Upper Blue Lake provides seasonal storage and the regulation of water for downstream power generation, as well as a host of recreational opportunities. The lake has an available capacity of approximately 7,300 acre-feet at normal maximum water level. It was created by a 786-foot-long earthen embankment dam built in the 1870s and modified extensively over the years.

To improve the stability of the dam in the event of an earthquake, the owner initiated a seismic retrofit project to construct a 175-foot-long rock-fill buttress on the upstream side. The work serves to restrict lateral movement of the embankment slope if it were to lose substantial shear strength during strong ground shaking. It also enhances dam stability by resisting deformations resulting from identified liquefaction hazards in the embankment fill soils. Gannett Fleming designed the retrofit project and provided engineering support during construction.

What We Did

Our firm first investigated the existing condition of the dam to determine seismic vulnerabilities and anticipated deformations. We then designed a stability buttress to mitigate against potential hazards. Regulators were also concerned with the stability of a stacked mortared rock wall that makes up the downstream shell of the dam at the tallest section; stability analyses allayed those concerns, however. Excavation for the buttress foundation was made down to bedrock to provide solid support after groundwater infiltration was found to affect the integrity of on-site soil.

Building the buttress required the demolition of the existing intake structure and construction of a new one approximately 50 feet further upstream, which included extending the two low-level outlet (LLO) pipes. The team used poured concrete to create the new structure’s foundation slab and walls, and the encasement of the LLO pipes. As part of the design, a drawdown analysis was performed to accurately account for the outflow capabilities of the new LLO system, ensuring compliance with California Division of Safety of Dams requirements.

Key Features

Rock-fill buttress strengthens existing dam and mitigates against soil liquefaction hazards.
LLO system extension accommodates new buttress and maintains regulatory water flow.
Lake drawdown accomplished with minimal environmental disturbance.
Drawdown analysis reflects outflow capabilities of extended LLO system.