The Severn Bridge became a victim of its own success ….
The original Severn Bridge was built as a key part of the M4 linking South Wales to London and the southern counties of England. Following its opening in 1966, it became increasingly popular. By the early 1980s, traffic was severely congested in the summer and at peak times. It was anticipated that traffic would continue to grow significantly, which it did at a rate of up to 8% per year. High winds meant restrictions and the occasional closure of the bridge, with a long diversion via Gloucester.
Delays cost time and money and hinder development. Enhancing the link to South Wales was seen as both a physical and psychological requirement, to ensure that confidence in development would be sustained. In February 1984, the Secretary of State for Transport announced “a study into how a second crossing of the Severn Estuary might be provided in the general corridor of the existing bridge.” The completion of this study would avoid unnecessary delay in providing a second crossing as soon as it was needed.
The Study was completed in 1986 and, in 1987, the consultants were reappointed to develop the scheme. Project management was transferred to the Department of Transport’s South West office at Bristol, to be taken forward in consultation with The Welsh Office under the direction of a joint Steering Group. The original programme, set out in 1987/88, was to start works in 1992 with completion in 1996. The new crossing was completed to programme and to budget.
The second Severn Crossing was the first major bridge in the world to have its traffic protected from high winds by wind shielding. It is designed so that coping with the wind on the Crossing, high above the estuary, should be no more difficult than on the approach roads.
Simulation studies and wind tunnel testing were used to find the best solutions, both for the traffic and for the aerodynamics of the bridge. The shielding was also tested to ensure that snow and ice would not block the gaps between the horizontal rails, since this could cause the system to be blown down.
The maintenance train
When a bridge is structurally complete, and you can walk the length of the deck, it might appear to be nearly finished. In reality there are many operations that still have to be carried out before it can be opened. The finishing works on this scheme included waterproofing the deck, laying the carriageway surfacing, erecting signs, installing the lighting, providing carriageway markings (such as white lines), installing electrical, water and fire mains and safety fences, and integrating the motorway communications system with the approach roads. And, uniquely for this scheme: the installation of wind shielding and the provision of a maintenance train, suspended by rail under the deck.
Creating the link was not just about building the crossing
The economic benefits, which make the scheme worthwhile, come from reduced journey lengths, leading to substantial savings of time, fuel and other running costs, as well as from greater reliability.
The new motorway approaches connect the crossing to the national network and were therefore equally important in creating a successful scheme. Connections to both the M4 and M5 were provided. In England, the new section of M4, together with the M49 which connects to M5, total 14 km in length. In Wales, the new M4 is 5 km long, while a further 2 km of the existing M4 was widened.
The toll plaza for the Second Crossing is located roughly midway along the Welsh approach road, with the tolls collected from westbound traffic only.
Crossing the floodplain.
The approach roads cross the reclaimed alluvial floodplain of the River Severn and this posed very significant engineering problems. The ground is soft and wet and it moves… a lot! Settlement of up to 1.8 metres was expected beneath some of the highest embankments. What was done?
Extensive vertical drainage systems were installed, deep into the mud, to allow the water that was being squeezed out to escape. Additional material was placed on the embankments to speed up the rate of settlement. The embankments were monitored to check movements and work on the foundations for the bridges over the motorway was delayed until the ground had been stabilised.
On the Welsh side, the expected settlement of the existing peat was so great that it was dug out and replaced with crushed rock. A special reinforced embankment was built on this rock to minimise the area required.
Where the new motorways cross the artificially drained floodplains of the estuary, the flora and fauna that had established itself in this special environment required particular care and attention. Water running off the road had to be kept separate from the natural drainage.
Keeping traffic moving.
29 new bridges were required to carry the local roads and new links over and under the motorways,. Other bridges were extended, while two existing bridges over live motorways were demolished.
Most of the bridges employ steel beams to support thin concrete decks, as these could be lifted straight onto the piled supports. Eight bridges over the busy M4 were constructed with special glass reinforced polymer (GRP) enclosures around the beams. These enclosures were built with sufficient space inside to enable inspection and maintenance to be carried out from within, thus avoiding the need for cones on the motorway. The enclosures also protect the steel beams from aggressive elements in the atmosphere, reducing corrosion and the subsequent need for maintenance.
The enclosure for the bridge over the railway at Rogiet was fixed to the beams before they were lifted into place. This allowed construction of the deck to proceed, without delaying the trains passing underneath.
People made it happen
The complimentary skills of the British/French joint-venture produced a team that was able to cope with many challenges and deliver the new crossing on time.
Ensuring safety on a large scheme in an aggressive environment is never easy and, in many ways, that presented the greatest challenge – nothing is more precious than life. The excellent news is that there were few accidents during construction, with no fatalities.
Other facts and figures
At the peak, 1000 men and women were employed on the project. Work continued 24 hours a day, seven days a week.
Workers consumed half a million eggs, half a million rashers of bacon, over 10,000 loaves, 12,000 catering size tins of baked beans, and around 19,000 lbs of chips. They also downed 150,000 mugs of tea and coffee, and 150,000 pints of milk. Laid end to end, the half a million sausages served in the site canteens would stretch from Cardiff to Birmingham
The project was built to time, and to budget, from inception to completion. It was formally opened by His Royal Highness the Prince of Wales on 5 June 1996.
The significance and excellence of the project led to it receiving the British Construction Industry’s Supreme Award for 1996.
The building of the Second Crossing faced the same challenges as the original Severn Bridge: the same 14 metre tidal range, eight knot tidal flows, and exposure to 100 mph winds. However, this was a much longer crossing, giving greater concerns for the environment and the aggressive site conditions.
Preparations for Construction
To meet these challenges, it would be necessary to minimise the impact of the tides, winds and weather during the four years of construction. This would be achieved by building as much as possible in large units, on land, before transporting the pieces out on to the estuary for assembly. This would require causeways, jack-up platforms, and large powered barges controlled by GPS (Global Positioning Systems).
Aerial view of the construction yard adjacent to the Second Bridge
Large construction yards were required on each bank of the estuary and there was a great deal of detailed planning, bringing together onshore construction expertise and knowledge gained from the offshore oil and gas industry.
There was a worldwide hunt for marine and other equipment that would be needed. At the same time, a start was made on the design and manufacture of items such as precast moulds, barges and gantries that were specific to this particular project. The length of time available to work on the English Stones between tides would be limited, as would the time to float large units out to the bridge at high tides. These restrictions had to be factored in. Above all, strong teamwork would be required from the men and women involved to develop innovative solutions and then to turn them into reality.
Building the Viaducts
Foundations and Piers
All the foundations were built using pre-cast concrete caissons, which are very large boxes without tops or bottoms. Most caissons were seated directly on the rock of the English Stones but some were on piled foundations.
To avoid any additional loading on the Severn Railway Tunnel, the lengths of all viaduct spans were adjusted to ensure that a longer standard length of span would be available at the point where the viaduct would cross over the railway tunnel. Also bored piles were used to support the caissons on either side of the tunnel because no change in the loading condition on the tunnel could be tolerated. Monitoring devices were fitted to the tunnel lining to check for any movement in the structure of the tunnel. Where movement did occur, it was found to be related to the very different loads imposed by high and low tide, rather than by the new crossing.
The Pier units were pre-cast. They were then transported from the yard and assembled on top of the caissons, ready to support the viaduct. These units were 3.5 metres long and weighed up to 200 tonnes. Approximately 2400 units were required.
Caisson loaded on traction unit in construction yard
Caisson on tractor about to move down on to waiting barge
Caisson on board barge
The concrete batching plant for the Caissons, etc was kept close to the action
Caisson being lowered onto prepared bedrock under floodlights
Preparing to lift caisson off barge
The precast concrete deck units were “match cast” in the yards. Each unit was cast against its neighbour to ensure that it would fit correctly when it was assembled in the estuary.
Adjustments were made to the mould so that the correct curve of the viaduct could be created. This achieved a combination of factory production with repeated modular construction.
The viaducts were built progressively from each shore using a purpose built launching gantry that was supported on units already placed. Units were delivered along the completed part of the viaduct and picked up from there by the gantry.
Another view of the yard, with the mobile launching gantry at the eastern end of the viaduct, having already placed viaduct units either side of the first pier.
A view into the deepest viaduct unit that sits directly over the pier
The gantry is bringing the next viaduct unit to its intended location, adjacent to the unit against which it had been match-cast in the construction yard
Building started by placing a deck unit on the first pier and holding it in place with temporary steel ties. Successive units were then added, one at a time, first to one side, then the other, to keep the two sides in balance. These additional units are held in place by steel strands positioned within the units that are tensioned horizontally. This provided a “balanced cantilever” form of construction.
Units were added until they reached half way to the next pier. The gantry was then moved forward over the completed work, so that it could be supported by the next pier. This sequence was repeated and the gaps behind were closed. Further longitudinal pre-stressing was then incorporated into the units to create a continuous structure.
The gantry has been moved forward to start building the viaduct out in both directions from the new pier, keeping the new section in balance over the pier
The upper cross beam of the pylon is being lifted into place
A pylon is under construction, with the lower cross beam already in place
A side view of the top of a pylon. A pair of cables, one from each pylon, will support the most distant end of an individual deck unit
The first section of bridge deck is being lifted into place, on top of the lower cross beam
The foundations for the two large pylons of the cable stayed bridge were built by using caissons in exactly the same way as for the viaduct. These two caissons were both in excess of 2000 tons and were among the first to be placed, in the Spring of 1993.
The pylons were the most significant parts of the crossing for which the general policy of precasting on land, proved unfavourable. They were therefore constructed of reinforced concrete, in situ. The reinforcement cages were prefabricated on land, transported out to the pylon locations, and lifted into place. The concrete was made in a batching plant located on the caisson. It was placed using one of two cranes and then, when the concrete was strong enough, an ingenious “self climbing” formwork moved up the pylon. Precast cross beams were lifted into place as the pylons were being built.
The pylons are approximately 150 metres high, which is equivalent to a 50 storey block of flats. They have warning lights for aircraft at the top and they are hollow and have lifts, inside, to provide access for maintenance.
The long span of the main bridge required lighter deck units, which were made of steel lattice girders. A concrete slab on top carries both the traffic loads and the large compression force introduced by the cable stays.
The final deck unit, that will close the gap between the bridge deck and the viaduct, is being lifted from the barge into its final resting place
A close-up of the final deck section approaching the readied gap
The steelwork for the units was fabricated off-site, brought to the site and assembled in the construction yards, complete with the concrete deck. The units were 7 metres long and full deck width. Unlike the viaduct units, which were progressively taken out to the launching gantry along the viaduct itself, the main bridge units were taken out to the site by barge.
Like the viaduct, the deck construction started directly over the pier and units were placed alternately on each side. The units were lifted off the barge by cranes located at the ends of the deck and were tied back to the pylon with inclined steel support cables to take the load. This required simultaneous work both at deck level and high up on the pylons.
Construction started at the eastern pylon and, once the eastern part of the deck was complete, construction of the western deck followed. A critical stage was when the eastern part of the deck was complete but was free to sway in the wind until the western side came to meet it, as if holding hands. The final deck unit, that closes the gap between the bridge deck and the approach viaduct, is seen being lifted from the barge into its final resting place.
A long view of the bridge deck approaching the end of the waiting viaduct
The style of support gives the cable-stayed form of construction its name. It is a key difference from suspension bridge construction, seen upstream on the Severn Bridge, where the deck is hung from the main suspension cables.
The overall sequence for placing the deck units took from September 1994 until 12th of November 1995 when the middle section was lifted into place, with 6 mm to spare on each side.
Design work on the new crossing included further detailed studies, a hydraulic model to test pier positions, mathematical modelling, a bathymetric survey, and geotechnical and topographical surveys for the route corridors. Extensive research was carried out into wind shielding and also into climate change issues, which indicated a potential rise in sea levels. The viability of engineering concepts and innovations were confirmed, together with the buildability and quality of the scheme.
Extensive consultations were undertaken with all those affected to ensure that concerns were fully understood and positively addressed in the development of the scheme. Wide ranging studies were carried out into the existing environment, potential impacts were identified and removed where feasible, and proposals were developed for reducing remaining adverse impacts. The consultations also included navigation interests, industry, landscape advice, and the Royal Fine Arts Commission regarding the main bridge and other structures.
A series of public exhibitions was held in England and Wales in areas affected by the proposals, initially showing the results of the study and then the changes adopted as the design was developed to take account of local concerns and the results of surveys
In April 1989, tenders were invited for the main crossing and toll Plaza. The tender details included highly detailed technical requirements, contractual/financial issues, constructional aspects, and environmental monitoring.
Separate bids were sought for two possible scenarios:
(a) to design, construct and finance the crossing, and to assume responsibility for operating and maintaining both it and the existing Severn Bridge during a concession period, in return for the toll revenue from both bridges during that period, and
(b) to design and construct the new crossing in return for staged payments from the government.
In 1990, following a rigorous assessment of the tenders, the Government accepted, in principle, the proposal of Severn River Crossing Plc to design, construct, finance and operate the second crossing. Severn River Crossing Plc was a consortium set up specially to bid for the project. It included major investment banks, a British contractor, John Laing Plc, and a French contractor, GTM Entrepose.
Obtaining Parliamentary Approval 1990 to 1992
Authority to build the scheme was obtained through Parliament. A hybrid Bill was used to seek the powers required to construct the estuary crossing and the approach roads, to compulsorily purchase the land, and to charge tolls.
The Severn Bridges Bill was lodged in November 1990 and, after thorough examination of the scheme by Parliamentary Committees, Royal Assent was granted in early 1992. The immense value of the extensive and detailed consultations, with over 40 affected parties, was shown by the small number of formal objections that were presented against the Bill.
A Concession Agreement, between the Government and Severn River Crossing Plc, was signed and construction was started in Spring 1992.
Final Design for the Second Crossing. 1992 to 1993
The Second Crossing is comprised of a cable stayed bridge spanning the main navigation channel, with a two kilometre length of approach viaduct on either side. At 5 kilometres, it is the longest river crossing of this type in the country.
There are 20 spans of approach viaduct on either side of the main bridge and each span is made up of 27 separate units of hollow concrete box girder, tensioned together using high tensile steel strands.
The centre-piece of this crossing of the Estuary is the cable stayed bridge over the main navigation channel, known as the Shoots. The main channel resembles a steep sided trench at this location and, although it is only about 300m wide at the base, the pylon legs had to be set back, well away from the top edge of the trench, to ensure stability. After careful consideration, a main span of 456m was agreed upon. At the time of its design, there were no cable stayed bridges operating anywhere in the world with a longer span, although the Pont de Normandie in France was well under construction with a span of 856m.
In parallel with these activities, the detailed design of the motorway approach roads was undertaken and tender documents were prepared. This work included the resolution of many issues affecting areas local to the roads, dealing with environmental, landscaping and community issues, and incorporating a wide range of mitigation measures.
Tenders for the approach roads were invited in October 1992 and contracts were awarded in time for construction to start in Spring 1993. The challenge was to construct the second crossing and the approach roads in time for an opening in 1996.
Plan showing the options considered for an additional crossing of the estuary
In 1984 the government commissioned a detailed feasibility study into a second crossing. The challenge was to investigate the need for a new crossing and to identify the best type of crossing and its location.
The study area extended 8 km up and downstream of the Severn Bridge. Different types of bridge and tunnel were examined. Road connections to the existing motorway network were a vital part of the study.
The study considered all key issues, from the traffic demands to the environmental impacts and from engineering feasibility to economic viability and planning issues. Both local and wider regional effects were considered on both sides of the Severn estuary.
To find the right solution, the study needed to balance the expectations of society at that time for new infrastructure against costs and economic analysis and against the impacts upon both people and the natural environment. It needed to weigh benefits against disbenefits whilst reducing disbenefits where practical.
The study was completed in 1986. It recommended a bridge at the English Stones, with viaduct approaches 5 km in length, together with 20 km of approach roads linking to the M4 and M5. This was the scheme that promised best value for money. The Government accepted this recommendation and so the concept of a crossing at the English Stones, first actively considered in 1935, would now be brought to fruition.