It was finally decided that the section of the route by-passing Helsby and Frodsham should cross the marshland to the north and, following the completion of the statutory procedures, an Advance Works Contract began in May 1968.
There were two peat deposits 9000 feet and 4000 feet long with maximum depths of 30 feet and 18 feet. It was the intention in the Contract to remove 1.3 million cubic yards of the peat by hydraulic means and replace it with sand from a nearby deposit. However, the Contractor chose to carry out the work in a more conventional manner, using 'drag lines'. Sandstone was excavated from a nearby outcrop and used as filling, and the resulting hole was back filled with the peat.
The Contract also included the construction of the 490 feet long seven-span Helsby Junction Viaduct over the Ellesmere Port to Helsby railway, and a branch line.
The works were completed in December 1970.
During the same period, a further Contract was awarded for the construction, in concrete, of the Weaver Viaduct. Designed by Husband and Company, Consulting Engineers, it is 3,186 feet long, and has a 222 feet span over the Weaver Navigation Canal, and a 125 feet span over the River Weaver. The Canal crossing has anchor spans and cantilevers of six cast insitu box beams, stiffened transversely with reinforced concrete diaphragms. Both the 125 feet long suspended span, and the River crossing, are of prestressed beam and slab composite construction with transversely stressed diaphragms. There are 30 approach spans of 90 feet.
The reinforced concrete piers are founded on 17 inch diameter driven cast in place piles up to 70 feet long through alluvium to underlying boulder clay and Keuper Marl.
The Contract also included the construction of a haul road alongside the Viaduct, and temporary bridges over the Canal and the River for use by the motorway Contractor.
Progress on the Contract was severely disrupted by labour disputes which bedeviled the whole of the construction industry in Merseyside and North Cheshire for the following 10 years. It was, however, completed in September 1970.
Under separate advanced contracts three bridges carrying railway lines were designed and the construction supervised by British Rail. Of these, two are similar 300 feet long Tied Arch Bridges, the superstructure consisting of a main span of 207 feet and side spans of 44 feet. The central span comprises two parabolic arch ribs 34 feet 9 inches in height, and joined at the crown. The prestressed concrete deck slab is suspended from the arches by 4 inch mild steel bars at 10 feet centres.
Sutton Fields Farm Bridge was also constructed in advance, to carry an occupation road and large diameter ICI brine mains. It was vital to ICI's activities, in the area, that the mains were fully operational at all times.
Work on the Main Contract for the 8 mile length of dual three-lane carriageway motorway began in December 1968. It included the construction of the two terminal interchanges at the ends of this length of motorway, where it connected with the A5117 at Hapsford and the A56 at Preston Brook. In addition an intermediate junction with the A557 at Clifton, provided a link with Runcorn New Town.
A total of 18 further bridges was required, additional to those within the advance contracts. They, and those required on the M56 contracts which followed, were designed by the Sub-Unit under a policy of bridge standardisation. It had three aims; to reduce costs of construction by standardisation of components; to increase the productivity of the design engineers; and to establish a characteristic appearance. This led to the development of the Cheshire beam which uses a standard form of constant cross-section throughout, the form being filled with concrete to various heights for the various lengths of beam. The manufacture combines the advantages of both pre-tensioning and post-tensioning.
The Contract was, in effect, split into two distinct parts, either side of the Weaver Viaduct. To the west, the finished level across the marshes was generally about 10 feet above ground level. The embankments were to be formed of non-cohesive material separated from the 1.5 million cubic yards of excavation from deep cuttings in Keuper Waterstones and Boulder Clay to the east of the Viaduct. This was to be transported over the temporary bridges crossing the waterways.
However, due to the labour disputes, which affected progress in the construction of certain of the structures, the earthworks were disrupted and a borrow pit was opened-up in a sand deposit on the marsh section. The surplus Keuper Waterstones material was stockpiled on the line of the motorway to the east of Preston Brook, as advanced works for the future extension.
The section between Hapsford and Clifton was completed and opened to traffic in February 1971, thereby achieving the objective of by-passing Helsby and Frodsham. The final section was completed in September of the same year.