Welwyn still requiring improvement to be brought up to motorway standard was the section between Rocstock and Stanborough. The aim here was to find an acceptable solution to a difficult problem, which would overcome the conflicting demands of highway, environment and local needs.
The scheme chosen included, on environmental grounds, a 1150 metre long tunnel and in so doing created a redevelopment area of some 25 acres of land on the western side of Hatfield only a 1/2 mile from the town centre.
Bounded on one side by the existing A1 with its heavy volume of traffic, and on the other by houses, space on the flanks of the construction was very limited and work had to be carried out in deep, narrow trenches. In such conditions Tcrram laminated filter drains provided an ideal solution to the problems of protecting the imperméable polythene liner from damage, whilst at the same time providing drainage externally, adjacent to the concrete walls of the tunnel. Traditional methods of structure drainage would have created handling and installation problems, and work would have progressed more slowly.