The Walton Bridge Project’s vision was to replace two life expired and utilitarian bridges with an iconic structure. The resulting slender yet dramatic hexagonal curved ivory coloured steel arches also provide a legacy for the local community.
The new A244 Walton Bridge is far from being the first bridge to be built across the River Thames connecting Walton and Shepperton in Surrey. It is in fact the sixth, although they are not all standing today. The first bridge to stand at the site dated back to the mid-1700s, and was a complex timber construction. However, plagued by decay, it was demolished 33 years later. The second, a stone bridge, opened in 1788 but collapsed in 1859.
Second World War bombing was partially responsible for the demise of the third crossing built on the site in 1864 and which was eventually demolished in 1985. And while a replacement bridge - the fourth -went up in 1953 it was only ever intended to be temporary.
However, it remained in use for vehicles until 1999, at which point another temporary bridge - the fifth - was built to cope with increasing road traffic. The older temporary bridge was retained as a pedestrian and cyclist bridge.
The fifth bridge was built with a limited life expectancy, with planning permission only granted to last until December 2011 (extended until 2014), so the race was on to put a long-term practical solution in place.
Surrey County Council awarded a £32M contract to Costain with consultant Atkins as designer in June 2005 under a design and build contract.
A first public inquiry in November 2006 rejected the original planning application for the bridge due to its perceived imposing impact on the public landscape.
A redesign to lower its height was swiftly undertaken and the application was resubmitted in 2007. Planning permission was given in 2009 and the design progressed through to detailed design stage. This was not to be the end of the story though and, after the 2010 General Election, funding for the bridge was cut by 20%.
Three months of redesign and value engineering followed and full planning permission was eventually given in January 2011.
Situated about 8km down river is Hampton Court Palace, which has its own three-span - and very picturesque - crossing. Its proximity is one of the reasons the potential form of the new structure provoked controversy, according to the design project manager.
“Some of the local people wanted a bridge that looked more like the traditional bridge at Hampton Court, but that’s a very old masonry arch design,” says Atkins chief engineer Rob Wheatley.
Client: Surrey County Council
And, although all of the bridge’s predecessors had supports in the river to break up their spans, the Environment Agency stipulated that this one should not, to enable boats to navigate the river at that point.
“There could have been a number of solutions and one that we looked at very early on in the planning process, was to keep the bridge deck very thin and keep the alignment very close to its existing alignment,” says Wheatley.
A box girder bridge was initially considered. However, the height of the structure required for it to span the 100m between the banks, would have raised the level of the deck and approach ramps too much to match the existing alignments of the connecting roads. Therefore an arch, which by contrast kept the deck as thin as possible to match those connecting road alignments was the preferred solution. After numerous consultation and planning meetings explaining the implications of the scheme public support was won over.
The final solution comprises a thrust arch bridge with two parallel steel arches spanning between the banks of the river. The deck is supported by 20, 100mm diameter high strength hangers ranging between 4m and 9.4m in length, suspended from the arches.
From an aesthetic point of view, the arches are not linked apart from a hidden connection under the bridge deck at both ends. The team ruled out a tied arch solution at an early stage because it was too expensive. Instead, Atkins designed the thrust in the arches to be resisted by the foundations and four reinforced concrete “springings” built up from the abutments. These springings are a complicated mass of post tensioned Macalloy bars and a dense cage of reinforcement. Extensive building information modelling (BIM) ensured that there were no clashes of reinforcement.
Construction of the springings used a combination of steel shuttering and temporary works to achieve the complex geometrical shape prior to site installation.
A balance between aesthetics and cost dictated the form of the main beams, which span between the hangers and support the deck. The client originally stipulated a box section, giving a smooth outside line. However, box sections were expensive to fabricate. Mabey Bridge proposed an I-section , citing the benefits of an automated fabrication process.
“The compromise was effectively a J-[section] because you get the outside appearance of a box, but Mabey could still fabricate it similarly to an I-beam,” explains Costain project manager Andy Bannister.
“The J-beam gave the outward appearance that the client wanted, but with the efficiency and cost effectiveness of being able to fabricate them automatically ”
Andy Bannister, Costain
“The J-beam gave the outward appearance that the client wanted, but with the efficiency and cost effectiveness of being able to fabricate them automatically with their T-[beam] and I-[beam] machine.”
The J-beam has a pair of top flanges, but where an I-beam has a pair of bottom flanges, the bottom of the J-beam is one sided with an extended flange that faces inwards (see drawing).
The steel bars used for the hangers that support the bridge deck presented some challenges. The failure of the hanger on the Clyde Arc bridge in Glasgow was a major concern and the team was keen to get the specification right (NCE 7 February 2008).
The new Eurocodes and the potential for fatigue and failure by brittle fracture produced a rather demanding design, and sourcing steel with the correct strength proved tricky. At the time, only manufacturer Anker Schroeder could provide the fully tested bars of the specified strength.
“We did have some discussions with the fabricator about the appropriate value for the charpy notch [a measure of a material’s ductile-brittle behaviour] on the high strength steel hangers,” says Wheatley. “We had to source the correct material properties that complied with the specification, which took a bit of time.”
Anker Schroeder fatigue tested the entire hanger component, as well as the separate parts - including forks, hangers and turnbuckle connections.
“They were put through a testing regime of 2M cycles of fluctuating fatigue stress to 45% of their characteristic strength and then tested up to their full design capacity, and they passed,” explains director Colin Jacobs.
A specialist machine in Germany carried out the fatigue testing. This speeded up the testing time, taking only five days which is less than half the time it could have taken on traditional testing machines. When the bars were tested, they also included an allowance for imperfections.
“We allowed for an angular deviation of about half a degree,” explains Wheatley. “There’ll be some small tolerances in construction. The bridge expansion also provides some further deflections on the hangers and because the bridge deck is moving, it also moves the hangers around. So that half a degree - which is in the manufacturers allowances - we had to test
The bridge lies on a flood plain and as such the top layer of the banks are alluvial deposits. The bridge itself is founded on the river terrace gravels below the alluvial deposits. And during construction the soft top layer presented a problem.
To create a stable platform on which the construction vehicles could work, the original idea was to create a mat of stone. This would have entailed excavating the top layer of the alluvium and replacing it with imported and subsequently compacted stone. The stone would then be removed and the alluvium re-laid at the end of construction.
When Costain was brought on board, it proposed that a soil stabilisation technique be used. The soft alluvium layer was mixed with RoadCem, an additive made by PowerCem, and cement. Once mixed, the ground hardened sufficiently, allowing the construction vehicles to drive over it. When construction was complete, the ground was simply rotavated and returned to its previous state.
“Doing this saved about 3,000 lorry movements and avoided digging out and replacing about 14,000m3 of poor quality material, which wouldn’t have been strong enough to support the temporary works which we needed to build the bridge,” says Bannister.
Another of the early cost savings the team, working with the steelwork contractor, was able to bring was to streamline the erection sequence. It was proposed that each of the steel arches be lifted into place over the river in two parts rather than three. As a consequence, only two temporary supports, placed at the centre of the span, had to be built in the river.
“There were fewer temporary works in the river as a result, which had lots of benefits, including being a lot less disruptive to river users,” says Wheatley. “And the river was pretty much in use for the whole time, apart from when we were lifting heavy steelwork.”
“There could have been a number of solutions and one that we looked at very early on in the planning process, was to keep the bridge deck very thin”
Rob Wheatley, Atkins
The temporary props were screw piled into the riverbed from a barge. A grillage was built on top and a trestle tower supported off this, ready to assist with the lifting sequence. Bannister describes the screw piles as “low noise, low vibration” and generating “very minimal disturbance to the riverbed”.
“All of these things were important so we couldn’t pile drive,” he adds. “The [Environment] Agency was obviously pleased we weren’t disturbing the river bed otherwise we would have had to go for a cofferdam approach that would have been disruptive to the local residents and businesses.”
The screw piles were able to be completely removed and recycled when they were no longer required.
Mabey Bridge fabricated the steel arches in Chepstow, in Monmouthshire and delivered them to site in quarters, half on each of the respective banks. They were then welded into halves and each section was lifted into place by a 600t crawler from the banks with the mid-point of the arches supported by the temporary trestle in the river.
A fully enclosed scaffold platform was then built up around the joint and the two halves were welded together. Pin joints connected the hangers to the arches and were subsequently welded in place. The bridge deck was erected in sections to enable it to be lifted between the arches.
The sections were lifted lengthwise and then rotated by 90° once clear of the arches and lowered on to a ladder arrangement of beams below.
“We were looking to create a contrast against most typical skylines so that the birds would see there was a structure there and avoid it”
Andy Bannister, Costain
“The deck was made up of a permanent formwork, which were glass reinforced plastic (GRP) panels and then a 250mm reinforced concrete deck cast on top of it,” explains Bannister.
As subsequently demonstrated by the floods of 2014, which hit the whole area, the Environment Agency was highly sensitive about the loss of
Due to the additional footprint of the new bridge and while the two previous bridges remained in place, the area lost had to be compensated for during construction . As the river floods, water flows into the tributary, Engine River. A temporary stream excavated on land purchased by the client, replaced the land lost to the bridge.
In the wake of the government-imposed 20% funding cut and as part of the subsequent value engineering exercise, the excavated ground was used as reinforced earth structural fill in the abutments.
“The river was pretty much in use for the whole time, apart from when we were lifting heavy steelwork”
Rob Wheatley, Atkins
As part of the consultations about the project, Costain even sought advice from the local swan sanctuary on the colour of the bridge.
“[Swans] can’t see very well and they’re not very good at spotting contrasts,” explains Bannister. “So by painting [the bridge] the colour it is, an ivory colour, which the client chose, we were looking to create a contrast against most typical skylines so that the birds would see there was a structure there and avoid it.”
The bridge across the river is the showpiece, however, as the wider plan reveals, it was very much part of a larger project. Swathes of approaches surrounding and leading to the bridge were reconfigured and revitalised. A new 92m long, four span cast insitu concrete
approach viaduct was built alongside a listed Victorian brick arch viaduct. The team refurbished that existing viaduct, which now forms part of a bridleway leading down onto the revitalised Thames Path area.
New areas of planting and wetland environments increased the biodiversity in the surrounding area and improved river water quality.
At the completion ceremony in July 2013, transport secretary Patrick McLoughlin called it a “magnificent new bridge” and praised the project team for delivering it on time and to budget.
The bridge is an elegant addition to this historic setting on the Thames, worthy not only for its looks but also for creating a vital connection across the river.
- Click here for a three minute time lapse video documenting the construction sequence
- Walton Bridge won the British Construction Industry Awards 2014 best practice award.
BCIA judges’ view
A great team spirit, within which everyone showed great pride in the job, created an iconic structure without spending a fortune. And the project breathed best practice.
The project delivered the first UK road bridge designed and fabricated to the final Eurocodes using the advanced provisions for economy of design.
Building information modelling ensured buildability and precise control of the fabrication of complex structural steelwork. Early specialist subcontractor involvement was key with regular whole supply chain workshops and quality reviews held to identify and collaboratively resolve technical challenges.
It all meant that when the budget was reduced by 20%, the cost savings were effectively absorbed.
Project innovation included the use of screw piles in the temporary support, and sustainable soil stabilisation to support heavy plant. The project also achieved a great safety record and ultimately provided a delighted client with a striking new bridge.