On Sept. 5, 2019, a long unknown and undetected flaw became an impending failure at Bonneville navigation lock.More than 70 feet underwater, sections of rebar, threaded through 40,000 pounds of concrete, had stretched and bent until the strain snapped them. The now deformed steel bars were no match for the force of water that came every time the lock filled with water to pass vessels through the lock.Yearly inspections and five-year “deep” inspections never revealed the nature of the impending failure inside the concrete.Sometime in those first days of September, the concrete near the downstream lock gates began to crumble away in the cold Columbia River water, leaving broken chunks of concrete in the path of the navigation lock gates.***A WATERBORNE ELEVATORThe U.S. Army Corps of Engineers owns Bonneville Lock and Dam, a project that, when built, was the leading edge of President Franklin Roosevelt’s “New Deal” policies to restart the economy in the wake of the worst financial crisis in U.S. history.The navigation lock works as a waterborne elevator – it is a sealable basin placed between the difference in water levels on the river. Lock operators either fill or drain water to match either the downstream or upstream water elevation.Once vessels are inside the lock, lock operators close the gates, which close against a raised concrete lip, or sill, in a water-tight seal. In filling the lock to raise the water, valves open under the base of the lock, allowing upstream water to fill the lock.The original navigation lock – big enough to accommodate ocean-going ships and barges as they shifted from higher water – became outdated in the 1980s. The lock was too small for larger barges being constructed to move commerce up and down the Columbia River.In the early 1990s, Portland District contracted out work for a new navigation lock at Bonneville, and crews began construction, pouring concrete for the massive concrete sill and the lock floor under it.***A GROWING PROBLEMLock operators began to log anomalies in the operation of the downstream lock gate in early September, 2019.Debris can become trapped on the bottom of the navigation lock chamber, inadvertently becoming a door stop for the massive steal doors that must be water-tight when filling the bathtub-like chamber. Debris is a regular occurrence at the lock.The standard procedure for lock operators is a series of opening and closing movements with the gates to clear the debris.But the rebar had yielded its last effort in securing the concrete sill to the floor of the lock, and pieces of concrete were working their way loose from the raised concrete sill.The jam at Gate 4 would not clear. Metal began to drag over the crumbled concrete pieces.Mike Adams, the operation project manager at Bonneville Dam, remembers his phone ringing.“The lock operators were reporting that, with the lock full, a visible plume of water was seen downstream of the miter gate,” Adams said. “The lock operator could not maintain a full lock chamber due to the leakage.”Massive forces of water had started to lift the concrete sill – which was never meant to move – from the floor of the lock.Upstream of the navigation lock, barges with tons of wheat and millions of dollars of goods were making their way down the Columbia River to pass through the Bonneville navigation lock.***CEMENTED FLAWSThe current navigation lock was completed in 1993 to replace the original lock that was built in the 1930’s.In the construction phase, the sill block formation began to differ from what the plans on paper laid out.The concrete sill works in the way a raised threshold in a doorway operates –it’s a raised surface to keep water in or out of the lock by meeting the bottom edge of the closed “miter” gate.The miter gate seals to the concrete sill on the upstream side. The floor of the lock was designed to have a step up of 18 inches at the downstream gate, and it was against this step that that concrete sill was designed to sit. However, instead of creating that step, concrete was poured to form a slope.It was on this slope the massive concrete sill was built.The original design of the “step up” in concrete would have prevented water filling the space between the sill concrete and the floor of the lock. But slanted surface became a weak interface that would be exposed to full lock water pressure every time the lock filled with water – up to 12 times a day.“Over the years, this would have been between 40,000 and 60,000 cycles of water pressure,” said Matt Hanson, chief structural engineer at the Portland District.Engineers estimate the lock was stable in its first years, but at some point the water began to exploit the space between the sill and the lock – the rebar joining the two structures could only stand fast so long.Once the interface was filled with water, it overloaded that section of reinforcement and stretch the rebar so that further areas of the weak bond could be exposed to water pressure. The original reinforcement was not designed for this water infiltration.Through the slow, steady pressure of water and time, the space grew.“Concrete is great at putting up with stress in the form of compression,” said Hanson. “But, it’s terrible at handling the stretching forces of tensile stress.”The tensile-stress problem is solved by the use of rebar throughout the concrete: Rebar can handle the stretch of tensile stress. When it is used in concrete, it removes concrete’s tensile weakness and decreases the speed at which failure occurs, giving engineers crucial time to spot an impending failure.***A MYSTERY OF A PROBLEMOn Sept. 5, lock operators recorded the alarms at the navigation lock – the kind of alarms that indicated trapped debris.A log of the reports from the lock operator that day list a total of four navigation “lockages” for vessels, and a scraping sound when closing the downstream gates. Then, the lock master recorded a 6 inches loss of water in the navigation chamber – indicating a leak.“We keep an underwater remote operated vehicle in its inventory for occasions like this, when something at the dam or navigation lock needed a closer investigation,” said Adams.In the turbid, murky water, engineers and experts were treated to an underwater view of something they’d never thought they’d see: areas of concrete missing from the sill, and exposed and broken rebar.Engineers needed the locks emptied for a closer look.That Friday afternoon, a crane roared to life, and began putting stoplogs in, one-by-one, to isolate the navigation lock from the Columbia River.Concurrently, engineers poured over blueprints of the navigation lock, trying to understand what caused the concrete to spall, loosening chunks of concrete.The engineers had never seen a navigation lock sill fail like this, and when the first group of experts were craned down Saturday morning, the flaw, 30 years in the making, was apparent for all to see.The concrete sill, according to Hanson, was “toast.”Soon afterward, the Portland District leadership announced to local and international stakeholders – that the navigation was shut down, halting all traffic on the Lower Columbia River.***A WAY AHEADThe plan was simple but had a crushing amount of work: demolish the concrete sill. Lift out dozens of tons of concrete. Update key stakeholders. Drill and cut into the concrete floor. Insert new rebar. Update the river users and key stakeholders. Create a notch, or “key,” in which the concrete sill would sit – as the original 1993 design called for. Pour concrete. Stress test. Update the river users and key stakeholders. Wait. Stress test concrete again.Demolition of the concrete were brisk, and demo operations clipped along 24 hours a day to pulverize and extract the concrete sill from the lock chamber. In all, crews removed thousands of pounds of concrete.The crew then began the next step: drilling more than 300 holes into the concrete floor to place rebar that would stabilize the new concrete sill.During drilling, the contractor was only able to drill 240 holes in 48 hours – the drill kept hitting reinforcement before reaching the desired depth. The contractor drilled approximately 1,500 holes in order to achieve 353 successful anchor holes for the new concrete sill.After three days of drilling in rainy conditions, crews installed the rebar and built the form into which they’d pour the concrete for the new sill and they began piping down concrete into the navigation lock.***Late on September 26, the lock operators opened the valves that feed water into the navigation lock, and began to operate the downstream miter gates – successfully.Almost 24 hours later, in the cover of night, but also at the first opportunity, the first barges locked through Bonneville Navigation Lock for the first time in three weeks.Barges that had been stranded upriver of the dam, full of fall’s international wheat exports, began to move again.Adams said he was relieved to see vessels moving through the lock again but was even more relieved knowing that something long overdue was coming:“Good sleep.”