Reconstruction of the upper portion of a 400-foot-high rockfill dam.

Hell Hole Dam was constructed in 1966 and is a 400-feet-high rockfill dam with a central clay core that stores a reservoir with a capacity of 200,000 ac-ft. After the Placer County Water Agency (PCWA) determined its dam was not constructed to the original requirements, the agency required a quick turnaround repair of the upper portion of the dam.

GEI completed a design for a new section within the first few months. This required various evaluations and analyses, including hydrologic, hydraulic, and geotechnical to develop plans and specifications for approval by the Federal Energy Regulatory Commission (FERC) and the Division of Safety of Dams (DSOD). Once the plans were approved, GEI performed construction management and inspection services for another portion of the project.

The larger project goal was to increase the Hell Hole Dam’s capacity to provide more storage for water supply and hydroelectric power generation, while addressing modern safety standards.

The original design of the core raise was completed (by others), with construction beginning in 2018. The contractor exposed the existing central clay core below the crest fill and found the core material was located lower and further downstream than thought and the fine filter was not encountered. After a series of geotechnical investigations and attempted redesign, construction efforts in 2018 were abandoned, and the excavation was backfilled.

GEI was contracted to complete the core raise project construction documents and creatively engineer the previous design to be more efficient with respect to reservoir levels, material reuse, and excavation sequencing. The design was fast-tracked to ensure construction could proceed. The analyses, design, and approval by FERC/DSOD would need to be completed in five months.

During construction, GEI acted as the owner’s representative and performed full-time construction observation. Work included observation of the dam excavation, foundation testing and verification, and backfill. GEI performed construction management services, including review of all request for information documents and drawings, review of contractors’ pay applications, development of as-built drawings, and the final quantities.

FERC/DSOD approval was received on time and – once construction was complete – PCWA was able to store more water and generate more power.

By increasing the dam’s capacity and ensuring seismic safety, PCWA positioned itself to better serve the growing needs of its customers and manage California’s water challenges, including drought and flood risks and solidify its role as a water resource management leader in the region.