About 2.5 to 3.5 inches (63 mm to 89 mm) of rain fell in the area over about three hours on June 18, swamping I-40 around Exit 451 in Tennessee, just to the west of the state line with North Carolina, officials said.
The Tennessee Department of Transportation originally thought I-40 would be closed until July 3.
The damaged section is part of 12 miles (19 kilometers) of I-40 in North Carolina and Tennessee that were washed away or heavily damaged by flooding that roared through the Pigeon River gorge during Hurricane Helene in late September.
Crews repaired and shored up enough of the old highway to open one narrow lane in each direction in March. The lanes are separated by a curb several inches high.
The permanent fix to stabilize what is left of the road will involve driving long steel rods into bedrock below the highway, filling them with grout and spraying concrete on the cliff face to hold them in place. It will take years.
I-40 runs from Wilmington. North Carolina, to Barstow, California.