The Upper Baker Dam Spillway Stabilization project addressed potential failure modes in the dam’s spillway chute’s foundation, identified during a 2019 Federal Energy Regulatory Commission (FERC) Part 12D dam safety workshop. After extensive analysis, design, and consultation with FERC, the project came to fruition.
Brennan constructed a concrete buttress within the old river diversion sluiceway channel used during dam construction in 1956—essentially, a giant “doorstop” at the base of the dam. This involved developing temporary access to the tailrace/sluiceway area, dredging, constructing cofferdams and formwork, preparing foundations, placing concrete, drilling, and installing untensioned/passive dowels.
We routinely perform all these scopes of work, but not in extremely remote and limited-access areas. PSE’s property was a vast wooded area with a hilly road network to move around between different site locations. We could only access the work area from above (down into the sluiceway via a complex scaffold and stair tower system) or from downstream via boat/barge. The river downstream was narrow (65 FT at its narrowest) and shallow, with steep rock walls rising on either side of the canyon. Moreover, the river quickly rose over 5 FT when the hydros turned on, becoming a very fast-moving waterway.
Scaffolding access only allows materials/equipment that can be carried in your hands, so Brennan opted to create a temporary downstream access pad in an undeveloped area of the right-descending bank. Basically, we built a small dock wall in the middle of nowhere by strategically burying Flexifloat® modular barges, a Brennan Innovation submitted in Fall 2024.
While we constructed the access pad, a subcontractor rappelled up and down the steep canyon walls to dislodge any loose and precarious rocks, trees, or other worrisome items, allowing us to work safely in the river.
Our next phase of work was dredging. Because the river through the canyon was too narrow to position the material barge alongside the dig barge, we outfitted the dig barge with the John Deere® 470 on the head and a smaller machine on the stern. Remaining mindful of its position, the 470 dug, spun around, and dumped material in the middle of the barge. From there, the smaller machine scooped it and spun around to deposit it on the material barge. The jet boat transported the filled material barge back to the access pad, where a third machine unloaded the material into an off-road dump truck for transport uphill to a rock processing area.
Another subcontractor then processed the material for the owner’s on-site use. In 2 months, Brennan’s team dredged and processed 7,400 CY of rock, gravel, and sand—nearly twice as much as expected, which caused our first major schedule delay. Some of this rock was the size of a small car.
Dredging provided barge/crane access to the downstream side of the dam and sluiceway, giving the river much-needed relief. With no dredging since the dam’s construction, the river amassed almost 70 years of buildup, spreading debris across the entire river channel in some locations. Rockfall debris piles accumulated above the waterline, spanning the whole channel width in some areas. In essence, we had to dredge our way up the tailrace channel so the marine plant could access the main work area.
With access to the sluiceway via barge, we could really get to work. The sluiceway, approximately 140 FT from downstream to upstream, presented our next challenge: the largest crane that fit on the barge could only reach about halfway up. We could not use a bigger barge to hold a larger crane with more reach because it would not fit through the canyon.
Next, we erected a cofferdam/formwork panel to close off the end of the sluiceway, keeping water out during hydro operations and in the event of a high-water event. The panel also served as the downstream-most end of our concrete face. The delay from additional dredging caused this work to start when PSE raised the elevation of Lake Shannon downstream, backing up water; Brennan Divers had to install the cofferdam in 5 FT of water (we planned to do this work in the dry). The large but fairly straightforward cofferdam utilized EFCO® plate girder forms along with large pipe braces that kicked down into the sluiceway. An uneven, sloped rock face complicated the installation as the formwork had to match that surface.
Foundation preparation required the removal of 70 years of muck, rock, debris, and even some old tools from the original dam construction, all on the sluiceway’s bedrock bottom. PSE had numerous fish biologists, geotechnical engineers, and geologists observing the work to ensure fish removal as we dewatered and the foundation was sufficiently clean and ready for concrete. We removed and disposed of approximately 15 CY of various material from the sluiceway.
On to concrete, and a lot of it. In total, we placed 10 mass concrete lifts. The reinforced cap was placed in 11 smaller sections, with construction joints, to allow for final grading and concrete finishing. Lift heights and quantities varied due to the sluiceway’s maximum dimensions; our best day was 585 YD during mass lift #6. The final 2 FT cap was also complicated by its elevations; the top pitched in toward the center (from left to right) with 2 different pitches along the downstream run of the sluiceway for proper water drainage.
Concrete supply offered yet another challenge. We used 2 main mix designs, including 1 self-consolidating concrete (SCC) mixture. Most placements also needed ice to keep the concrete from exceeding certain temperatures. We spent a significant amount of time assisting the plant and our concrete consultant with the mix designs, yet we still encountered quality challenges during most placements.
The point of placement down in the old sluiceway was over 400 FT from the closest a ready-mix truck could get—too far for a pump truck’s boom to reach or for a crane to swing concrete buckets. Our team mounted steel slick line to the dam’s downstream face, transitioning to rubber line as it got down into the sluiceway, using 5 IN line because of the distance we needed to pump. A traditional pump truck conveyed the concrete through the line to point of placement. Placements typically started on the downstream-most end against the cofferdam face and progressed toward the back/upstream end of the sluiceway. The crane on the barge supported the discharge end of the hose for as long as it could; when it could no longer reach, the crew had to manhandle it from there—not easy with a 5 IN line full of concrete.
In the end, Brennan placed 5,202 CY of concrete over 23 different placements, equating to just over 21 million LB (10,500 TON). The schedule delays coincided with the rainy season, so many of the final top cap placements had to incorporate a rainwater collection gutter system and roof panels to ensure proper concrete finishing. The new concrete buttress also required us to cast an 8 FT x 8 FT shaft into it; we had to jump those forms between lifts.
Drilling took place between the mass concrete lifts and the final 2 FT concrete cap, providing the buttress with some shear resistance. With Brennan’s support, a subcontractor drilled and installed 48 un-tensioned/passive bedrock dowels, drilling 5.5 IN holes all the way through the previously placed mass concrete and extending 10–15 FT into the underlying bedrock. Epoxy-coated Williams rods (2.5 IN diameter, 150 KSI, and 40–48 FT long) were inserted at the bottom and terminated 6 IN from the final top of the concrete. Grout filled the drilled holes, locking everything together.
The available physical space on top of the buttress limited our options and required us to perform specific tasks concurrently. At times, we placed the first top cap sections of concrete at one end of the dam while the drillers finished the last holes on the other end. We also had to work around PSE’s requirements to manage water elevations and meet hydropower license requirements. Because we were not allowed on the water during spill or hydropower unit operations, we constantly coordinated with PSE.
Working together through all these challenges, we completed the project after an intense demobilization and site restoration during the cooler winter months.
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.