The challenge of nature… again!
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 on land, in large units, and transporting the pieces into the estuary for assembly using causeways, jack-up platforms, and large powered barges controlled by GPS (Global Positioning System).
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.
Equipment specifically for the project, such as precast moulds, barges and gantries, was designed and manufactured and there was a worldwide hunt for marine and other equipment. The amount of time available to work on the English Stones at low tide was limited, as was the time to float out large units of the bridge at high tides. Above all, strong teamwork was required from the men and women involved to develop very 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, spans were adjusted and piled foundations were used, because no change in the loading condition could be tolerated where the viaduct crossed the tunnel. 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.
Pier units were also 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.
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 was created. This achieved factory production with repeated modular construction.
The viaducts were built progressively from each shore using a purpose built launching gantry that was supported by the 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 pie
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, to each side of the first unit and these were then held in place by stressing steel strands (pre-stressing). This gave 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 pre-stressing was then incorporated into the units to create a continuous structure.
The Shoots Bridge
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 that were unable to be precast on land. They were made of reinforced concrete. The reinforcement cages were prefabricated on land, transported out to the pylon locations, and lifted into place with one of two tower cranes fixed to the side of the pylon.
The concrete was made in a batching plant that was located on the caisson. It was placed using the 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 allow easy access for maintenance.
For more on Building the Shoots Bridge Pylons, Click Here
The Bridge Deck
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 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 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.
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.
Watch the 12 Minute Video of the Construction Process
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