What began as a slight shift in the downstream guidewall of Lock & Dam 15 in Davenport in the 1990s has accelerated into a structural failing that will need a major, multi-million dollar repair.
Employees of the Rock Island District of U.S. Army Corps of Engineers first noticed the slight tipping — about an inch — during routine inspections.
The wall that towboats touch up against to ease their loads into and out of the lock is anchored below water level by timbers that sit on bedrock, Josh Hendricks, a civil engineer with the Corps, explained. Where the bedrock slopes away downstream, the timbers sit on clay, and this is the area of the failure, he said.
This shift wasn't considered a big problem at first, but in the past three years, "it really started to accelerate on us," said Hendricks, who will present a talk on the lock guidewall problem Friday at an engineering conference in Davenport.
Despite underwater inspections by dive teams, engineers weren't sure what was causing the structural failure, Hendricks said.
A possibility, though, could have been the weight of the earthen embankment alongside the wall on the slough side, Hendricks said. To fix this, a maintenance crew with excavators removed the embankment, roughly 5,000 cubic yards of soil, he said.
Still, shifting continued, and came to a head in late May when conditions took a dramatic shift for the worse. A sidewalk above the wall buckled and cracked.
Worried that the wall could fall into the channel, blocking navigation, crews came in with excavators with attachments for breaking up concrete to remove a 120-foot length section of the wall itself, removing the hazard as well as the weight on the timbers below, Hendricks said.
This hole, termed an "interim risk reduction measure" is plainly visible from the greenspace next to the Fort Armstrong replica on Arsenal Island.
The hole makes navigation more difficult, Hendricks said. "If we were to demolish any more (of the wall), the tows would need helper boats (such as tugboats) to continue navigation," Hendricks said.
"They (towboat operators) are used to bumping up against the wall. They are more careful now of how they steer," he said.
For a more permanent fix, the Corps is seeking bids to replace the wall as well as the structure under water that consists of piles, or piers, inside crib boxes. Specifications call for the original wood piles to be replaced with concrete and steel, and they will be drilled into bedrock, Hendricks said.
The work also will involve more wall — 360 feet, or about one-third of the total, compared with the 120 feet that is affected now, he said.
Hendricks said he is not sure when bids will come in and declined to assign a cost estimate except to say that it will be in the multiple millions. This money likely would come out of the Corps operations and maintenance budget, he said.
Work has to be done in the winter, after the navigation season. The very earliest that could happen would be the winter of 2018-19, Hendricks said.
Why the wall is shifting now, after more than 80 years of use, is unknown. The composition of clay might weaken over time, creating an inadequate foundation, but that is speculation, Hendricks said.
The lock wall in Hannibal, Missouri, had a related problem, he said.
Lock and Dam 15 was the first lock completed on the Upper Mississippi River as part of the 9-foot navigation system. Construction was finished in 1934, and the intended design life was about 50 years.
The recent problem is indicative of infrastructure that is well past its prime, Mike Cox, chief of the Rock Island District's Operations Division, said in May when demolition of the failing concrete was announced.
"Almost all of the locks and dams on the Upper Mississippi River system are experiencing varying levels of problems due to the age of the infrastructure," he said in a statement.