This contract, which was 5.1 miles long, was awarded to W.C. French and Co. for the sum of £7.51 m. The starting date was February 1971 and it was opened in August 1973. As with the previous two contracts much of the route was on the compressible Somerset Levels, at places 100 ft. deep, and the embankments had to be built in PFA. At Dunball the route passed through the end of the Polden Hills in cutting adjacent to the main Bristol-Exeter railway line which was also in cutting. The design provided for excavating into the hillside to improve the appearance, remove the two cuttings and generate filling and sub-base materials.
Filling material was also obtained for advance construction of embankments on the adjacent length of motorway further south.
As on previous contracts, on the alluvium, a drainage layer was laid directly onto the unstripped turf and topsoil and to prevent damage to the surface crust no heavy plant was permitted to cross the surface before the drainage blanket was in place; the blanket being placed by end tipping across previously laid material.
The construction of the Motorway posed potential problems for the drainage of the Somerset Levels which are the second largest area of fenland in the country. The drainage of the area is achieved by a complex system of ditches and rhynes (or water courses) with the general gradient falling inland to pumping stations where the water is pumped into the rivers Brue, Huntspill and King's Sedgemoor Drain. The rhynes also serve as water supply to animals and as field boundaries. In order to eliminate the need for frequent crossings under the motorway and side roads the drainage was reorganised so that parallel water courses were constructed on each side of the motorway. At Dunball an interchange was constructed to provide a connection with the A39 and the A38. There were 14 bridges constructed on this contract including major bridges over the rivers Brue, Huntspill and King's Sedgemoor Drain.