The northbound lanes are expected to reopen within two to four weeks after additional repairs are made, said Geoff Sundstrom, a spokesman for the Delaware Department of Transportation.
Built in 1974, the structure crosses the Christina River just west of the Delaware, carrying 90,000 vehicles a day.
The $35 million repair job is, by any measure, a massive undertaking, aided by emergency shipments of materials and equipment from job sites as far away as Texas and Washington. That includes giant, cylindrical cages made of steel rebar, purchased from an ongoing project at the Tappan Zee Bridge, in New York, said engineer Javier G. Torrijos, DelDOT's assistant director of construction.
"We got parts from all over the country," Torrijos said.
A passing motorist called 911 in April to report that something seemed amiss, according to a chronology provided by state officials. A DelDOT consultant then inspected the site but deemed it OK. The department's bridge division did not learn of the issue until May 29, when an engineering firm reported concerns. After further inspection, the bridge was closed June 2.
Four sets of concrete piers were found to be leaning by as much as 2.3 degrees from their proper vertical alignment, as the bottoms of the support piers had started to shift sideways, underneath the busy lanes of traffic overhead.
Up on top, the bridge decks on both the northbound and soundbound lanes also were tilted as a result. The concrete barriers separating the two sides, normally flush with each other, were separated by as much as 18 inches.
State officials say there is no doubt that the 50,000 tons of dirt is the culprit. Indeed, the bridge realigned itself somewhat once the dirt was removed, said engineer Barry Benton, DelDOT's assistant director for bridges.
The dirt had compressed the underlying soft soil, forcing it sideways, he said.
"It's kind of like you've put peanut butter between two hard surfaces," Benton said.
Engineers considered tearing down and rebuilding the leaning portion of the bridge, but ultimately decided that a repair job was better, as it would get the roadway back in service months sooner.
Though faster than building a new bridge from scratch, working underneath an existing structure presented challenges.
Contractors needed low-clearance rigs to drill the 150-foot shafts underneath the existing bridge deck, which is just 50 feet high. The steel rebar cages were then lowered into the shafts from above, through holes cut into the existing bridge deck.
Each cage weighs 50,000 pounds - equal to the combined weight of 10 Ford F-150 pickup trucks. There are 32 such cages in all, four surrounding each of the piers that were compromised.
The project was designed by AECOM, a global firm based in Los Angeles, and the lead contractor is J.D. Eckman Inc. of Atglen, Chester County.
The construction firm that stored the dirt under the bridge was Keogh Contracting Co. of Wilmington. Company officials did not return a request for comment, though in June, speaking to the News Journal, owner James Thomas expressed a willingness to cooperate. The state has retained counsel to see if any funds for the repair job can be recouped.
Michael J. Chajes, a University of Delaware professor of civil and environmental engineering, agreed that repairing the bridge was preferable to rebuilding.
"Once they discovered what happened, I think the situation has been handled pretty well," said Chajes, who is not involved with the project but has visited the site.
The project involves two different kinds of fixes.
Four concrete piers were found to be leaning enough that engineers decided to replace them. That required the construction of 16 temporary steel towers - four surrounding each concrete pier - to hold that section of the bridge until the piers can be replaced.
The degree of leaning was insignificant in four other four piers, engineer Benton said. With those, the design calls for pouring new concrete footings around the existing piers.
In both cases, underneath the four new piers and under the four that are not being replaced, the structure is supported underground by the new concrete columns, which extend 150 feet down to bedrock.