The contract for this 11.1 mile length of Motorway was awarded to Tarmac Civil Engineering (now Carrilion Construction) for a tender sum of £12.49m. The scheme commenced at the Chelston Interchange on the Taunton By-pass and ended at the north end of the Cullompton By-pass where the contract included the removal of the slip roads which connected to the A38 road at Willand. Tenders had been invited for alternative forms of pavement construction and Tarmac's (now Carrilion Construction) successful bid was for an 11inch unreinforced concrete slab laid on 11ins of sub-base.
The earthworks included 2,400,000 cu.yds of excavation in marles and gravels. There were 42 bridges and culverts and one interchange, at Sampford Peverell, which served as connection to Tiverton and subsequently the North Devon Link Road. An area of alluvial plain in the Lyner Valley where the Motorway is on embankment had to be stabilised by placing a layer of rockfill directly onto the un-removed vegetation. Elsewhere, north of the Sampford Peverill Interchange, the route passed through a wooded area. At the design stage bore holes were taken out on either side of the wood as part of the soils investigation: because of the difficulty in gaining access in the wood and because the two bore holes showed similar soils on either side no samples were obtained in the wood. However when the earthmoving machines traversed the wood it was found to be highly unsuitable and very wet, so that much material was excavated and replaced. This incident is reported as an illustration of the importance in soil survey of understanding the significance of types of vegetation and local names. The wood was called, on the Ordinance Sheet, "Aller Wood"; now in Somerset "Aller" is a corruption of "Alder" and alder trees only grow in boggy ground or near to water. Hence had either of these facts been known to the soil investigators their suspicions would have been aroused and maybe the added costs caused by disruption would have been averted.
Tarmac (now Carrilion Construction) used a conventional AGB concreting train operating on rails to pave the full 36 ft. width of each of the carriageways. There were no expansion joints and contraction joints were formed at 20 ft. spacing by vibrating in dowel bars, and sealing strips. On this contract the softened value of the marl sub-grade was assumed to be of a CBR value of 2 . As a result there were no major difficulties with the construction of the carriageway. With the passage of time the riding quality of the carriageway has deteriorated somewhat, particularly due to joint failures, but no major reconstruction has been necessary after 24 years which largely vindicates the concerns expressed by the designers about the inadequacy of the specification on the previous contract.
Nineteen bridges were built on this part of the Motorway the largest being crossings of the A38 on heavy skews at Maidendown and near the end of the Cullompton Bypass. An unusual feature in 1974 was the construction of a 24 inch diameter culvert beneath a large embankment to accommodate a well used badger run. In order to make the atmosphere acceptable to badgers a member of the engineering staff crawled through the pipe towing a dead badger skin.