Contracted workers from Advanced American Construction work to replace the sill at Bonneville Navigation Lock on Wednesday morning. The U.S. Army Corps of Engineers hosted a tour at the lock to show off the emergency repair project. (Amanda Cowan/The Columbian)Photo Gallery
BONNEVILLE DAM — It’s been two weeks since an emergency repair project forced an unplanned closure and drainage of the Bonneville Navigation Lock, and contractors for the U.S. Army Corps of Engineers are working around the clock to meet a goal of reopening the passage by Sept. 30.
The lock is the only way for large boats to pass the adjacent Bonneville Dam, and the closure has brought river traffic to a standstill. To make matters worse, it’s the middle of the wheat harvest season, when barges make frequent trips down the river carrying millions of tons of grain to the Port of Vancouver and other export terminals.
The Corps’ Portland office hosted a media briefing at the visitors center on Wednesday morning to provide an update on the status of construction.
It was a noisy job site — constant sounds of drills and jackhammers echoed from the floor of the 100-foot-deep lock, which opened in 1993. Crews have been working 24 hours a day, and the priority is to restore river traffic as soon as possible, said Alan Davis, with the Corps’ Portland division.
“Right now, all of our efforts are just focused on making this repair,” he said.
Excavation
Problems with the lock became apparent on Sept. 5 when crews reported that something was preventing the downstream lock gates from closing properly. The lock was walled off at both ends using stacks of steel stoplogs, and then drained (or “dewatered,” in Corps parlance) on Sept. 7.
Crews inspected the base of the gate and found cracks in the sill, a concrete slab on the floor that runs across the 50-foot width of the lock. The sill is designed to line up with rubber flaps at the bottom of the gates, creating a watertight seal when the gates close.
According to Patrick Duyck, deputy chief of engineering and construction, the inspections revealed that the sealant between the sill and the lock floor appeared to have failed, allowing water from the lock to penetrate beneath the sill.
The water was pushed into the gap by pressure from the rest of the water in the lock, Duyck said, acting kind of like a wedge that pushed the sill upward, cracking it and dislodging it. The only way to fix it was to remove the entire sill and rebuild it.
The first step was to lower an excavator into the lock to break up the concrete. The process took several days because the concrete chunks all had to be lifted back out of the lock using a crane, said Col. Aaron Dorf, commander of the Portland Corps district. Crews had to switch to hand tools near the end of the demolition to avoid any risk of the excavator damaging the lock floor.
The weather was also a hassle, he said, because rain during the demolition created slurry, which had to be pumped into storage tanks and lifted out by crane to avoid contaminating the river.
“It’s not a massive project,” Dorf said. “It’s just a lot of getting it right.”
Drilling
The second step began Sept. 12 and is still in progress. Crews are drilling holes into the lock floor to accommodate fresh rebar that will anchor the new sill. The floor is 25 feet thick and goes all the way down to bedrock, Dorf said.
The rebar holes only need to be drilled to a depth of about three feet, he said, but that’s still tricky because there are layers of rebar lattices reinforcing the concrete in the floor, and the new shafts must avoid hitting any of them.
“You’re trying to take four tic-tac-toe boards and drill a hole through all of them without hitting a line,” Dorf said.
And just to make it more fun, the gaps in one lattice may not perfectly line up with the gaps in the next. The crews have to make their best guess for the placement of each drill shaft using the original lock blueprints and the “as-built” schematics.
“Is it perfectly aligned? You won’t know until you drill,” Dorf said.
The new rebar will be sealed into the shafts with epoxy, and then crews will be able to attach rebar cages on top that will form the skeleton of the new sill. The final step will be to pour about 40 truckloads of fresh concrete into the molds and then wait several days for the new sill to cure, Doft said.
The new sill will have the exact same shape as the old one, Dorf said, in order to work with the existing lock gates. But the new anchoring rebar is going to be thicker, Duyck said, and the anchors will be spaced closer together. When complete, the new sill will contain about three times as much internal steel as its predecessor, he said.
Crews will take extra steps to seal the gap between the sill and the floor this time, Duyck said, but the project is taking what he referred to as a “belt and suspenders” approach. The extra steel is intended to make the new sill strong enough to resist being dislodged even if water somehow did manage to slip beneath it again.
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