ATLANTA — The full State Transportation Board was expected to approve a contract today for redesign of the interchange of interstates 285 and 20 east of Atlanta after a committee voted Wednesday to approve an agreement with a roadbuilding consortium.

Construction is scheduled to begin in the middle of next year, with completion in late 2026.

The $685.5 million project is aimed at smoothing traffic flow through an interchange rated as the nation's 25th worst bottleneck last year.

The overhaul will include reconstructing ramps to create more direct alignments, adding new collector-distributor and auxiliary lanes, replacing several bridges and erecting new noise barriers.

The work will be done by East Interchange Builders, the consortium the board selected for the project last month. The lead partners are Archer Western Construction and E.R. Snell, two of Georgia’s leading highway contractors.

Representatives of the Georgia Department of Transportation (DOT) and East Interchange Builders have been negotiating the agreement during the last month.

It will be a design-build-finance contract, the same model that was used to build the Northwest Corridor toll lanes along I-75 and I-575 in Cobb and Cherokee counties and in the ongoing overhaul of the I-285/Georgia 400 interchange.

Under the terms of the contract, the consortium will design and construct the project, then turn it over to be managed by the DOT, Meg Pirkle, the agency's chief engineer, told members of the board committee with jurisdiction over projects built through public-private partnerships.

"They are [also] solely responsible for the financing of the project," she said.

Certain contingencies could drive up the cost of the work either for the consortium or the DOT.

Under the contract, the consortium could be assessed penalties for missing project deadlines or for unanticipated lane closures during construction.

The DOT would pick up any additional costs of construction materials.