Constructed in 1852, the Loughor viaduct is a 220 metre long, eighteen span railway structure, constructed mainly of timber, which carries a single track railway line over the Loughor Estuary between Swansea and Llanelli in South Wales.
Originally one of Brunel’s once numerous timber viaducts, the superstructure has since been entirely replaced and it has received substantial re-designs and strengthening works in subsequent years, last being refurbished in the mid 1980s.
Now, detailed site investigations and conditional surveys have determined that the viaduct has reached the end of its lifespan. Network Rail and Carillion are replacing the complete structure, including the existing piers within the current track alignment, with a new viaduct capable of supporting two tracks as part of the larger, £50 million Gowerton Re- doubling Scheme. The work is jointly funded by the Welsh Government and Network Rail.
One of the primary considerations for Tony Gee and Partners in designing the replacement for Loughor Viaduct has been the construction methodology and how the new viaduct can be constructed within the limited 250 hour blockade provided by Network Rail in March 2013. As there is only a limited period of rail disruption available, the vast majority of the construction had to be carried out in advance, allowing the switch over from the existing to the new viaduct during the shut down period.
This resulted in the new bridge being designed to be constructed in sections and launched longitudinally from the west bank alongside the existing viaduct and slid transversely into position during the blockade in March.
The existing viaduct is a Grade 2 listed structure, so listed building consent to demolish it was required before work could commence. Network Rail and Carillion planned to retain as much of the existing structure as possible and the east and west abutments have been incorporated into the new design to retain some of the heritage of the original viaduct.
Although the viaduct was originally double track, it was single tracked in 1986 by British Rail due to structural concerns. In addition, a speed restriction was imposed for both freight and passenger trains. Such capacity constraints were having a negative impact on the immediate and wider economy of the region as the bottleneck was causing unacceptable delays. The new viaduct at Loughor will therefore not only be beneficial to the local community but will contribute greatly to the economic regeneration of the region and Wales as a whole.
With Loughor being a designated conservation area, ecological impact assessments were carried out in the form of desk studies, surveys and consultations with stakeholders such as the Carmarthenshire County Council and the City and County of Swansea planning and environmental departments. The existing viaduct is situated within or adjacent to several sites of European importance for nature conservation. These include the Carmarthen Bay and Estuaries Special Area of Conservation (SAC) which was designated in 2004 for the presence of important habitats, SSSI (sites of special scientific interest) and Ramsar wetlands.
Consequently, prior to commencement of the project, flood defence licences and a marine consent had to be obtained. Several months of intense consultation were undertaken with the Environment Agency for Wales, the Marine Consents Unit, the Countryside Council for Wales and the environmental departments of local councils during which the sensitivity issues of the area concerned were addressed.
The surrounding area was also designated as a Special Protection Area (SPA) as it is an important wildfowl overwintering site, regularly supporting over 20,000 birds. As a result, mitigation measures were taken to minimise disruption by carrying out works such as piling for the foundations of the new structure outside the winter period between November and February. This reduced the potential for any significant impact on bird migration.
As part of the agreement between Network Rail, Cadw (the Welsh Government’s historic environment service) and local authority planning and environmental departments, some of the existing timber trestles will be retained. Four will stay in their original place as part of the new structure, while several others will be re-erected close by as an acknowledgment of the heritage of the viaduct.
The design of the new bridge is for a steel and concrete deck supported on new piers 36 metres apart – three times the span distance of the old structure. Twelve 1200mm diameter permanent steel cased piles will form the foundations for the new structure while six temporary piles will facilitate the launch of the new bridge on the north side of the existing viaduct. In order to minimise disruption in the estuary, Carillion employed a method of installing the piles using cranes and piling rigs situated on jack-up barges which were towed into position by four tugs.
Despite adverse weather conditions throughout the operation, and having to deal with one of the highest tidal ranges in the UK with river flows reaching up to eight knots on high tides, this phase of the works was successfully completed in July 2012.
Constructing the crosshead beams in situ working below the existing structure would have been difficult. It was therefore decided to pre-cast the concrete side panels and lift them into position using a 350 tonne crane working off the jack-up barge. Once in position, the main beam reinforcement was fixed and concreting completed to each crosshead.
On completion of the permanent crosshead beams, temporary steel crossheads and slide rails were constructed to facilitate the launching of the new steel deck sections.
Steel deck – and concrete
The bridge deck also had to be manufactured and placed in position without interfering with either the live railway or the local wildlife. Specialist manufacturer Mabey Bridge held a series of planning meetings with designers Tony Gee and Partners and main contractor Carillion to establish the suitability of steelwork to launch and slide. Design discussions at these initial meetings covered the number of launches, nose and tail design, splice design and positioning.
The main design consideration was that the steelwork had to be detailed to allow it to be launched and then slid sideways into position.
Following agreement on design, Mabey Bridge began a three month programme of fabrication of the 1200 tonnes of structural steelwork and walkways, with a total length of 236 metres, at its modern manufacturing facility in Chepstow, South Wales.
As well as fabrication, Mabey Bridge was contracted to oversee site assembly, including temporary pier cross beams, launch of the structure in four phases and slide.
The finished steelwork was transported to site by road in girder sections each 24 metres long. These were assembled in a laydown area on the west side of the estuary. After the assembly of each of the four phases, the structure was launched over the river onto the temporary piers. Following completion of the launches, in December 2012, the steelwork was jacked down onto its permanent bearings.
Once reinforcement was in place, the concrete deck slab was cast in bays to a designed pour sequence using a mobile concrete boom pump and rigid delivery located on the west and east banks of the estuary. Once cured, the new deck was waterproofed using an approved membrane system.
The track goes on
Ramps at each end of the viaduct were constructed to allow vehicular access. Track ballast was delivered by road and placed on the new deck using rubber- tyred excavators. G44 sleepers and rail is also coming in by road before being placed into position.
Once completed, track protection matting will be placed for the full length of the deck in preparation for access for the demolition of the existing viaduct.
The existing bridge will be completely closed for 250 hours between 24 March and 3 April 2013. During this time the existing bridge will be removed and the new structure slid into position on the same track alignment.
Demolition will start with the removal of the existing ballast, rail and deck timbers which will be loaded by excavator and transported from the bridge. The remainder of the bridge, which includes the steel girders and trestles, will be demolished using 65-tonne excavators equipped with mechanically operated shears. These will cut the structural members into short sections for ease of handling and transportation.
Whilst demolition of the existing bridge is going on, abutment works will be carried out on both the east and west ends of the viaduct. New reinforced concrete abutment base slabs will be cast and pre-cast retaining wall units placed to accommodate the new structure.
When the old structure has been completely removed, the new deck can be slid into place from its resting place on top of the temporary piers. Trial slides will be carried out during possessions in advance of the main blockade to test the slide system, which will consist of semi-continuous pulling rams mounted on brackets attached to the deck and the end of the slide tracks. The hydraulic power packs will be coupled up to a central control. This system allows for differential pressure to be applied to the pulling arms for directional control of the new bridge whilst moving into position. Once in place the new deck will be locked into its final position.
And there it shall remain. Will it carry trains over the River Loughor for 160 years like its predecessor? Only time will tell.