Dorset County Council has said that it expects to be able to reopen the Beaminster Tunnel in late July, more than two months later than planned.
Access issues delayed the start of work and difficult ground conditions have called for a redesign of part of the slope stabilisation meaning that the council has missed its originally planned May reopening of the route.
The tunnel on the B3066 to the north of Beaminster has been closed since 7 July last year when a landslide above the northern portal killed two people.
DCC has said that its specialist contractor Can Geotechnical has installed 300 out of 1000 soil nails planned for the stabilisation work so far. The soil nailing work on the slopes above the northern portal is nearing completion but work on the southern side has been delayed by the discovery of an unexpected area of saturated ground.
DCC has said that it expect to open the tunnel under traffic light control in late July, which will allow the work to continue but reduce the impact of the road closure on traffic over the busy summer period.