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M4 in Wales. Lon Las to Llangyfelach (J44 to J46)

In 1969 the Chief Engineer at the Wesh Office, invited Swansea Borough to prepare this section of the M4 known locally as the Morriston Bypass. It was a unique invitation not bestowed on any other Borough.

The scheme had been protected both pre-war and post-war, as was the Newport Bypass first managed by the MoT and later the Wesh Office. It was programmed in 1969 and only later was it designated to be built to motorway standards as part of the M4. The protection of the route proved to be an advantage, for very few properties were affected and there were few objections. Efforts could therefore be centred on technical preparation and contractual work.

Public Consultation

A special design team was created to see the scheme through both design and construction, and Bill Ward oversaw the works with his customary energy and often lateral thinking. One teams task was to ensure public awareness at a time when formal public consultation was only just being discussed. Here the scheme benefited from being in the hands of a Local Authority, as it had the full backing and involvement of local councillors. Apart from statutory requirements, many local public meetings were held which resolved problems before they arose, and talks were arranged for schools in the area of the scheme.

Advance Contracts

Advance contracts were let, as on other sections. In this heavy industrial area they included for the resiting of statutory apparatus, such as several large diameter water mains and CEGB overhead power lines, as well as for crude oil pipelines serving the Llandarcy oil refinery, and a British Steel effluent main from Velindre Tinplate Works.

Rock Cutting West of the A4067 - Swansea to Clydach Road

Advance Contracts were also let to enable large volumes of pennant sandstone and spoil to be excavated from the edge of the escarpment and moved eastwards across the Clydach Road, to form an embankment across the River Tawe flood plain.

The Contractor, R M. Douglas Ltd, had allowed for crossing the Clydach Road with heavy Euclid dump trucks. They did not have a road licence however, and could not legally cross the highway. A bridge was impractical due to both cost and level, so that finally all the Authorities, including the

Police, agreed to temporary traffic signals with a banksman and traffic controller present at all times when operational. Operation was limited to daylight hours.

During excavation work coal was found immediately west of the Clydach Road. The locals recall that it was of low quality, only worked by the unemployed during the 1926 Strike. This did not deter a certain officer from trying it at home however, but the iron pyrites content resulted in a firework display and burnt holes in his fireside rug.

The Embankment East of the A4067

The quarter mile(0.5Km) embankment and the strata below it contained a peat bed ten feet(3 metres) below ground. Rather than remove and replace the peat, the cost was reduced at Bill Ward's insistence, by forming a 3 feet (0.9 metres) rock blanket. The embankment was then built initially to an 18 ins (0.46 metres) depth, the anticipated settlement, as surcharge over the peat area. The extent of settlement and amount of heave either side of the embankment was regularly monitored. After one year settlement was negligible and only 3 to 4ins(7to10cms) of the embankment needed trimming.

This decision was not readily accepted by the Welsh Office since faster progress was wanted. The Welsh Office was not prepared to risk future settlement however, and proceeded as planned. He was perhaps lucky or intuitively aware that the peat was not a type that fully retained its water, but contained drainage paths (aquifers), created by sand replacing decaying vegetation allowing more rapid consolidation. Results proved them right.

They were also very conscious of settlement at the back of bridge abutments, and insisted on continuous close inspection by supervisory staff. Consequently there has been no requirement for remedial work since the road was opened to traffic in 1972 except for a length adjacent to the eastern edge of the Clydach Road viaduct.

Borrow Pit and Lean Mix

In the Felin Fran area of Llansamlet at the eastern end of the project, there was a very unusual feature. A large area of moraine had been left following glaciation, with an excellent gravel, 1.5 inch (3.8cm)down in particle size. It was surprisingly free from contamination, clean and evenly graded, and eminently suitable for use as a subbase for the new road.

The Contractor, John Laing, set up a batching plant on site and mixed the as dug material with cement to produce a lean mix used throughout the site.

The moraine was on private land so that the Contractor's staff negotiated with the owners to create a borrow pit. The site was finally restored by controlled generally using hard waste.

Salt Depot at Ynysforgan

Land for a special, motorway type, winter maintenance facility sited adjacent to the A4087 Interchange, was included within the CPO. The design was prepared with the new road, and the necessary contracts let. The site contains offices, telephone and wireless facilities, vehicle repair and servicing facilities for gritters and patrol vehicles, a large salt storage bin and a number of steel hoppers for loading the salt. Rest rooms for men on night shift were also included, and a 24 hour stand by service provided from November to February.

Structures

The structures on this length were of conventional construction but quite varied. Morriston Hospital footbridge was of three span parabolic reinforced concrete construction, with a 90 ft (27.4 metres) centre span. The bridge carries both services and pedestrians. It has a number of interesting features, including sloping legs, void formers in the main span, and concrete hinges. The behaviour of the concrete hinge had just been clarified at that time. It could carry very large compressive stresses in the throat, and was very efficient. The slopes beneath the bridge supports were faced with natural stone to match the adjacent stone cutting face.

The concrete hinge was also used at the supports of the Cwm Rhyd-y-Ceiriw bridge, which is of cantilever and suspended span construction. Rock anchors were stressed through a hinged joint at the end abutments to counter tensile reactions i.e.uplift forces. The centre span is 138,6ft (43 metres) and has the appearance of a shallow arch as the counter balance span is only just clear of the ground.

The Ynysforgan Viaduct is of conventional prestressed boxes transversely pretensioned. The trestle head supporting the beams is sunk into the deck through half joints so as to give an open aspect under the viaduct. The foundations carrying the reinforced columns are seated on long precast Hercules piles.

The other bridges were either of conventional steel beam and composite construction or reinforced concrete. Measures taken to avoid settlement back of abutments created a particular problem at the River Tawe Bridge, a three span, central parabolic span, with the abutments and piers founded on Hercules piles. To eliminate settlement at the back of the abutment the Contractor offered to install lean concrete rather than compact backfill to the abutments. This was a mistake as the greater weight of the concrete fill caused the abutment to turn backwards about the slender piles, causing the expansion joint to open. The lean concrete was removed, and the joint returned to its original width. Compacted fill was then installed.

Details of all the other bridges are contained in the Archive.