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Frog Hollow Dam work shows exposed bedrock and HDPE outlet pipe; city told project should finish by July
Summary
City staff presented photos and a construction update on the Frog Hollow Dam replacement, describing excavation to bedrock, placement of dental concrete and a high‑density polyethylene outlet pipe that expands thermally; staff said the contractor expects project completion by July.
City staff provided a construction update and photographs showing progress on the Frog Hollow Dam replacement, including excavation to exposed bedrock, placement of dental concrete and installation details for the outlet pipe.
The presentation showed the outlet structure that existed since 1978 and pictures of substantial material removed around the structure. Staff described exposing bedrock and noted a pipe visible in photos that is approximately 2.5 feet in diameter for scale. Staff explained the use of “dental concrete” — a fill material to stabilize rough bedrock cavities — and showed poured dental concrete and backfill work.
Staff described a high‑density polyethylene (HDPE) outlet pipe that was supported on stirrups and strapped down to prevent thermal movement. “Throughout the course of the day, that pipe would expand and contract by over a foot,” a staff presenter said, and crews managed that movement by securing the pipe. Rebar cages and poured concrete slabs that will support the outlet pipe were discussed and shown.
Staff said crews have poured a roughly 400‑foot concrete slab to support the outlet conduit and that backfill and compaction work are underway. A coffer dam was noted as providing protection for near‑term storm events. The presenter said the contractor expects the project to be completed by July unless weather or other conditions cause delays.
Council members asked technical questions about pipe‑to‑concrete interactions; staff replied that HDPE is inert and will not react chemically with the concrete and that the pipe walls are about two inches thick. The update also included other local construction projects: a 1,100 widening project, natural gas pipeline replacements and irrigation tie‑ins that are proceeding in adjacent neighborhoods.
No council action was required at the pre‑meeting; the presentation was informational and aimed at keeping the council and public apprised of construction progress.
