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M4 in Wales. Newhouse to Castleton (J2(M48) to J29)


This first length of the M4 in Wales was divided into two sections, Newhouse to Coldra and Coldra to Tredegar Park. Both sections were built in the 1960s, commissioned by the Ministry of Transport (MoT) prior to the creation of the Welsh Office in 1964. The Groes to Baglan (Port-Talbot ByPass) section was also built in this period. The Welsh Office thus became the client for all schemes thereafter.

There was little objection to these early schemes, and processing was a fairly easy task. Overall it seemed that there was a public wish for their provision. Locations had been laid down at earlier times.

Newhouse to Coldra (J2(A48(M)) to J24)

The Newhouse to Coldra Scheme was prepared by Maunsell and Partners and Ossy Osbourne was the Partner in charge. The Coldra Viaduct, the most notable structure on the scheme, carried the M4 over the previously at grade roundabout junction with route A449 (giving access to the Midlands), and the A455 (giving access to Newport), and was a late addition (Ref.B2-2).

The viaduct was of continuous beam construction in which the capping beam, joining the piers with a top sliding joint and bottom articulation transversely, is hidden in the depth of the longitudinal beams. In order to achieve this appearance the lateral capping beam, cast as an inverted T, links the top of the piers. Scaffolding towers support the pier and capping beam to ensure stability. Beams with half joints are then positioned on the capping beam.

Macalloy bars are then threaded through the beam end blocks and the leg of the inverted T capping beam, and tensioned. Once achieved for the complete viaduct length the viaduct is secure, and the towers may be removed. The work is then completed by the provision of an in-situ reinforced concrete slab, which also provides for the reverse moment at the support.

However the structure is vulnerable to any instability in the scaffold towers until the Macalloy bars are threaded and tensioned. Unfortunately one tower buckled during the construction of the viaduct and the capping beam, piers and beams, became a mechanism. Loads were thrown on to other parts of the viaduct, particularly the falsework, and progressive collapse occurred as in a pack of cards.

Fortunately there were no fatalities, but there was a significant delay to the completion date. This

was well before the collapse of the Lodden Bridge, and Industry were not so alert then to the general poor state of the design for, and construction of, temporary works, and the lack of clear cut responsibility for them.

There was no significant cut or fill in this length and the earthworks were constructed without problems.

Coldra to Tredegar Park - the Newport Bypass (J24 to J28)

The original location and designs were first completed in 1946, but then shelved. When preparation recommenced in 1961 the location was maintained but virtually all the structures had to be redesigned because standards had changed so radically in the meantime. The design and construction came under the direction of Sir Owen Williams and Partners (SOWP), and drew the personal attention of both Sir Owen and his son O.T.Williams.

The early procedures to establish the line and purchase the land are now lost, but in any event they were determined at a time prior to receipt of significant objection to road construction. Car ownership was increasing rapidly and people were pleased with any effort to reduce congestion. In addition routes other than that located through the Crindau ridge had greater impact on people and property.
crindau tunnel Newport

M4. Crindau TunnelThe work was split into two contracts. The first comprised the tunnels through the Crindau ridge and the Usk River Bridge. The early completion of this work was intended to give access between East and West, across the river and through the ridge, so that earth could be moved from one side to the other in order to optimise the earthworks.

The route commences at the Coldra Interchange, constructed under the previous contract, and then runs westwards across Christchurch Hill largely in cut to connect to the A4042 and B4596 linking to Newport and Caerleon at the Caerleon Interchange (J25/25A.) It then crosses the River Usk on two separate decks (flanked by two further decks carrying the A4042) before continuing in the twin Brynglas tunnels through the Crindau Ridge to connect almost immediately with A4051 Malpas Road providing access to Newport, and north to the first of the Welsh Valleys containing Cwmbran and Pontypool.

Continuing westwards the route at its highest point links with the local road system, connecting to Risca and providing access to extensive residential areas north of Newport. The motorway then terminated originally at a large at grade roundabout at Tredegar Park.

The river crossing and tunnel complex was a crucial factor in trying to resolve the traffic flow problems in and around Newport. In the 1946 design the tunnel was a single entity but in the final design and construction, twin bored tunnels were built with the bores separated enough to allow them to be constructed independently.

The early work consisted of pilot boxes, about three metres in diameter. Only a short time into the construction a failure of some temporary 'I' sectioned steel frames activated a rock fall of quite serious proportions. As a result all tunnelling work was stopped, and whilst the work was re-appraised families were evacuated from their homes above the tunnels and placed in hotels

An idea put forward by the Consultant to use short shields of heavy steel construction to provide full support for the rock at the rock face, and give safe working platforms at various heights for drilling and excavation works, was adopted after much argument. With trailing bars it would also support a few feet of the roof rock while the ring was formed.
Newport tunnel shield

As far as staff knew this method had never been used before, and several very sceptical views were expressed. In the event the method proved to be eminently successful, and four shields were employed to drive from each end of each tunnel bore. The tunnels restarted, homes were repaired, and residents resettled.

Newport TunnelThe tunnels were constructed to an accurate profile, with the concrete arch lining following very closely behind each shield, in a Jumbo moving shutter. There was no ground movement. Those involved favoured mass concrete lining, as reinforcement would have severely complicated the work..

Because of the lateral separation of the tunnel bores the Usk bridge was a twin deck design. The depth of construction was limited in order to provide essential clearance under the railway bridge which had to be built at the Eastern end of the tunnel to carry existing tracks (at that time), and to meet navigational clearance over the River Usk at high tide.

The R.C beams and slab deck, of conventional cross section, had widened beams at the supports to avoid the need for haunching i.e. varying the depth. A number of hinges were formed in the deck which whilst transmitting lateral and longitudinal forces allowed flexure in the vertical plane. All calculations were carried out on a slide rule, and it is asserted that all the moments and shears were accurately predicted, as were the effects of differential movement at the piers.

Rapid traffic growth, changes in land use, and population increase, resulted in congestion in the tunnels. The above construction was therefore modified in the 1990s to form basically a single complex of the Caerleon and Malpas Interchanges. Westward movements entering and eastbound leaving the M4 at Caerleon, were removed to reduce traffic volumes in the tunnels. A new dual carriageway was constructed to the east of the tunnels, connecting south into Newport and north to Cwmbran, to relieve the additional loading which would have resulted on the A4042 Malpas road. The system increased the capacity effectively, but due to steadily increasing traffic growth congestion is again becoming an increasing problem. Proposals exist for diversions to the south of Newport.

Many of the bridges were constructed as 'integral'. Following the construction there was a move away from integral construction for almost thirty years until maintenance became a dominant issue. Recently 'integral bridges' have been recognised as providing much improved durability, and are now generally a requirement for new construction of up to sixty metres in length.

To create a hinged 'drop-in' span, whilst providing the necessary horizontal forces in the deck to create an integral bridge, Sir Owen devised a unique horizontal hinge in reinforced concrete. Diagonal bars were placed in sets to cross a narrow concrete throat. It was a detail very similar to that used in vertical members to give a hinge, but here used horizontally.

Referred to on the drawings as a 'Thrust Hinge'' this detail was used by Sir Owen on over one hundred motorway bridges, mainly in the Midlands but also at Newport in South East Wales. Although apparently performing well for nearly forty years, this detail is now the focus of investigations to better estimate its condition in the field, and hence its residual life. This is now necessary information for use in the unified approach to motorway maintenance based on resource accounting and budgeting.

Generally this hinge detail exists in bridges with concrete slab desks, but in the special case of the Newport Bypass, the crossing of the River Usk on a beam and slab viaduct also uses the hinge detail. Each carriageway is separately supported on a four beam deck over five continuous spans. A large version of the hinge detail is incorporated into the beams at four points over the length of the structure, to give a degree of articulation whilst maintaining horizontal force continuity. It is believed that the use of the 'Thrust Hinge' in a beam and not a slab, is unique to this structure.

Tredegar Park to Castleton (J28 to J29)

Sir Owen Williams and Partners were also the Consultants appointed for this 4.08 mile section. From a large grade separated interchange, constructed over the existing at grade roundabout at Tredegar Park (Ref B6-1), the route extends westwards to link into the Cardiff Eastern Bypass at St.Mellons. The location was determined by the existing bypass and by a possible route for a dual carriageway to the north of Cardiff identified by Glamorgan County Council. There was nothing of major civil engineering interest on this route.