Chief Dam Safety Engineer Placer County Water Agency Foresthill, California
Some say the devil lies in the details, but often, the lack of detailed information causes the most problems. Hell Hole Dam, a remote dam owned and operated by PCWA in California’s El Dorado National Forest, lived up to its name during construction when large storms breached the incomplete dam in December 1964, leading to an urgent reconstruction. Nearly 50 years later, PCWA began evaluating the dam for increased storage potential, designing new spillway crest gates. The name Hell Hole became a pun again when PCWA began construction to raise the insufficient core. In summer 2018, following an initial design, construction successfully exposed the existing core below the crest, but filter materials were not found where anticipated. The coarse filter was lower and further downstream than as-builts indicated, and the fine filter was not encountered. As winter neared, more detailed exploration, an efficient redesign, numerous contingency plans, and rapid approval processes by FERC and DSOD would be required to complete the project.
This presentation describes how PCWA, with GEI’s support and technical expertise, used their 5-month window to facilitate a redesign which included seepage, stability, filter compatibility analyses; hydrologic analyses; material quantities takeoffs and source material identification, completion of final drawings and specifications, and FERC-required submittals. PCWA received regulatory approvals quickly and the Contractor was remobilized in summer 2019.
The 2019 construction was slowed due to high reservoir elevations created by near-record snowpack, yet the first half of the project was successfully completed by October. The 2020 construction season was challenging for other reasons with the Contractor, Engineer, and other personnel living in a remote community setting while adhering to strict COVID protocols, wildfires leading to poor air quality and a site evacuation, and an urgency to top-off the dam before the all-stop deadline, with another winter of hell looming.
