The Washington State Department of Transportation says Highway 97A will remain closed at least partially between Entiat and Chelan until June 15.
Work began Wednesday on removing loose and unstable rocks on the hillsides above the highway.
The $660,000 dollar project at three different locations, including Knapps Hill Tunnel, which has been under an emergency closure since May 13.
WSDOT says rockfall on the highway near the tunnel had increased in intensity and frequency, prompting the closure ahead of the scheduled work.
If that emergency closure is lifted, there still will be closures from 8 a.m. to 5 p.m. during the work.
The state continues to advise motorists to use Highway 97 on the other side of the Columbia River.