The A39 in North Devon will be closed for 13 weeks while essential stabilisation work gets underway in the New Year.
The section of the road, north of Barnstaple, showed signs of cracking and subsidence earlier this year, and it was feared the embankment would completely collapse.
A new 60m long concrete retaining wall will be built 3.5m below road level, and socketed into the underlying bedrock.
As well as the concrete retaining wall, the £200,000 scheme also involves drainage improvements and resurfacing.
Devon County Council councillor Stuart Hughes, who is responsible for the county’s highway network, said: “The cracks and deformities in the road are signs that it isn’t stable. While it’s unfortunate that the road will have to be closed, it’s vital that this work is carried out now in order to prevent it from collapsing in the future.”
The work will be carried out by local civil engineering firm MacPlant Construction.