Waterproofing a Multiple Arch Dam in a Cold Mountainous Environment with PVC Geocomposite Membrane

Florence Lake Dam, a 92-year-old concrete structure, is located high in the central Sierra Nevada Mountains, where it is subject to freezing temperatures each winter and has experienced freeze-thaw degradation. A thick PVC geocomposite membrane system was designed and installed on the upstream dam face of select arches to dewater the concrete and mitigate freeze thaw damage. This paper will describe the Florence Lake Dam geocomposite installation projects.
The dam is situated at an elevation of 7,330 feet and is part of the Big Creek Hydroelectric System. Below Florence Reservoir is the Ward Tunnel leading to the Portal Powerhouse, which discharges into Huntington Lake downstream and into six subsequent powerhouses before the water reaches PG&E’s Kerckhoff Lake and subsequently water flows into Millerton Lake, operated by the Bureau of Reclamation. Florence Lake Dam consists of 58 concrete arches, with a crest length of about 3,156 feet and a height of 149 feet, all founded on granitic bedrock. Each arch is an inclined cylinder that sweeps out approximately 75 feet of circumference before meeting the adjacent arch.
Carpi began work at Florence in 2014 and has completed membrane installation on the upstream face of 22 arches over 2014, 2015, 2016, and 2017. A multi-year phasing of the PVC geocomposite membrane installation was necessary as no upstream reservoirs are available to be used to restrict inflow and the short construction season runs late summer through October, weather permitting.
Several campaigns of shotcrete placement on the upstream face of Florence Lake Dam over its history provided a rough uneven surface on which to install the membrane system. A grout curtain was required around the submersible seal, near the upstream dam toe, to reduce seepage that could travel around the submersible perimeter seal between original concrete and the shotcrete overlay. Not previously executed on any Carpi projects, a unique strategy of utilizing boats and pontoons to deliver the swingstages to the upstream face of the dam, via the water surface, was utilized at Florence Lake Dam.