Work on the Standedge Road/Acker Hall Farm project in Diggle, near to the A62 Huddersfield Road junction, is set to start on August 13.
The work is expected to last until January 2019.
The existing wall, supporting the A670, will be fully demolished and reconstructed.
Diversion routes are in operation.
Any vehicles travelling along the A62 from Marsden will be unable to turn left towards Uppermill, except during Yanks weekend (August 11-12), and Rushcart celebrations on August 25.
Instead, they will be directed along the A62 to Cross Street in Oldham Town centre before being turned back towards Saddleworth along the A669.