Dorset County Council said they are now checking the structure of Beaminster Tunnel in Dorset.
It's parapet was destroyed in a landslide caused by heavy rain.
The Council said they will need to tackle the drainage before they address the stonework.
Beaminster Tunnel is expected to remain closed until the 22nd July.
Beaminster Tunnel in Dorset is to remain closed until the 22nd July. The tunnel was damaged in a landslip caused by heavy rain in the area. The A3066 had to be completely closed between B3163 Clay Lane and B3164.