Photo courtesy of Harbor Pacific Contractors
The powerhouse and fish-passage system at Cushman Dam were completed without a single environmental incident.
Cushman Dam No. 2 North Fork powerhouse and fish passage
Location: Hoodsport, Mason County
General contractor: Harbor Pacific Contractors
Owner/developer: Tacoma Power
Primary designer: City of Tacoma
This project consisted of a new powerhouse and an innovative fish-collection and transportation system at Cushman Dam.
Harbor Pacific Contractors engineered and built the 500,000-pound concrete fish-collection structure in the dry area above the dam using a temporary frame and platform. This frame was then utilized to lower the structure into the water where it was grouted to the riverbed.
A 25-ton excavator was used, disassembled at the site and lowered down to the dam’s base for reassembly.
Other project challenges included access logistics, precision rock blasting, geotechnical construction, engineered heavy crane picks and mechanical commissioning, poor rock conditions, and installation of micropile anchors through rubble.
HPC used a negative-pressure cofferdam to contain silt from excavation and high-pH slurry from underwater concrete pours. HPC’s high-capacity pumping system pulled fresh river water into the cofferdam, preventing exfiltration. Silty and high-pH water was then pumped nearly half a mile away to a settling pond.
There was not a single environmental incident through the life of the project.