Chelan County Public Utility District (PUD) operates the Rock Island Hydroelectric Project on the Columbia River approximately 14 miles downstream of Wenatchee, WA. The project has two powerhouses, a gated spillway, and three fish passages.
Brennan was contracted to perform concrete overlays in the draft tube of Unit B-8 in Powerhouse #1 to increase the efficiency of the hydro turbine and reduce future cavitation. At the same time, the client took advantage of this outage to perform rehabilitation and upgrades to the hydro unit and generator.
Accessing A Draft Tube 100 Feet Below the Surface
To access our work area, the Chelan County PUD installed upstream and downstream bulkheads to dewater the draft tube, removing the entire hydro turbine to assist in other planned rehabilitation. Dewatering pumps lowered the water level to approximately 3 feet, and the rest was up to us.
Brennan hired a scaffolding subcontractor to assemble a stair tower going down into the draft tube. All said and done, it was approximately 100 feet tall. Then, we installed aluminum dewatering panels around the pier of the draft tube outlet to further dewater the draft tube walls and adjacent work areas. With a little bit of cleanup and some lighting, our crew was ready to get to work.
Surveying The Structure and Preparing the Concrete Surface
The original draft tube walls had a unique geometry of angles, radii, and swooping curves. A FARO® laser survey was performed to lay out the formwork extents and dowel locations. Then Brennan saw-cut the perimeter and started drilling holes, using a tracked Brokk® machine to speed up the process. Although there were some difficulties with the small work area, the Brokk machine drastically reduced our physical labor for this portion of the work.
With all the holes drilled, Brennan crews installed rebar hook dowels while a subcontractor tied mats. In total, there were around 1,200 hook dowels to install along with four layers of rebar mats.
Installing Custom Formwork for Complex Geometry
Following rebar installation, Brennan installed custom wooden formwork panels and associated steel walers. The formwork was designed for full fluid-head pressure because there could not be construction joints in the concrete, and we wanted to use self-consolidating concrete (SCC) to eliminate the need to vibrate the concrete amid the steel jungle of rebar and formwork ties.
Therefore, the form panels required two-by-sixes 7 inches on center, with structural ¾‑inch plyform facing. We anchored ¾ inch coil rod into the wall every 3 feet to provide support out to the walers.
Placing Concrete Deep Inside the Powerhouse
With forms in place, it was finally time to place concrete and finish the project. Due to the exceptionally unique location of the work area down in the draft tube, our only feasible option was to place the concrete using 5‑cubic-yard concrete buckets. Concrete trucks backed down the dam to the gantry crane and loaded one bucket at a time, after which the crane carried the bucket inside the powerhouse, where a series of gantry cranes took them across the powerhouse to Unit B-8.
From there, another overhead gantry crane lowered the bucket down into the hole and offloaded the concrete into an electric concrete pump. Although each 60‑cubic-yard wall was poured separately, the pours took the entire day due to the lengthy process of getting the concrete to the placement area.
Delivering A High-Quality Finished Product
A few days later, Brennan stripped the forms and found the newly placed concrete to be in great condition with no signs of honeycombing or voids. The SCC was very flowable and managed to fill the forms nicely. The Chelan County PUD was very impressed with our work and final product and gave great feedback on the on-site crew that made it happen.
Thank you to our client, Chelan County PUD, and every Brennan team member who helped plan and execute this project.
For more details on our dam construction and repair services, visit our Dam Construction service page or connect with our team.
About J.F. Brennan Company
J.F. Brennan Company, Inc. (Brennan) is a family-owned marine construction firm specializing in environmental remediation, dam construction, commercial diving, harbor management, and submarine cable services. Working closely with public and private owners of water-based infrastructure since 1919, Brennan operates throughout waterways across the United States and Canada.
