Reconstruction of the Erwood Bridge in Wales was completed in three months while the structure remained open to pedestrians. Katherine Smale reports on the project which won a place on last year’s British Construction Industry Awards shortlist.
Set in an area of outstanding natural beauty, Erwood Bridge provides a vital link across the River Wye just north of the Brecon Beacons National Park in Wales.
Built in the 1960s, the bridge spans 80m and is supported by abutments on both banks and three equally spaced piers in the river. At the base of each pier there is a 1.4m wide and 4.2m deep concrete breakwater, a structure which protects the bridge structure from the force of the water flowing in the river. Built up off each of the breakwaters are two 1m wide octagonal reinforced concrete columns which support a 1m wide by 1m deep concrete crosshead beam. Steel beams span between bearings on the top of the crossheads and support the 8m wide concrete bridge deck.
Inspections on the bridge by client Powys Council, revealed that steel supporting the bridge deck needed maintenance and the concrete in the columns and the crossheads, was in very poor condition. Fearing that the concrete could fail if left, it was decided that the columns, cross heads, bearings, cantilevering sections of the bridge deck forming the pedestrian walkways and the waterproofing and surfacing had to be replaced.
Residents and local businesses relying on the bridge for passing trade and a 24km diversion to connect to the main A470 road, it was essential that the work was carried out as quickly as possible.
Initially the council obtained a flood defence consent from the Environment Agency for the bridge to be demolished using a technique called hydro demolition.
“Hydro demolition is blasting the concrete with a high power water jet which removes the concrete,” explains Dawnus contracts manager John Evans. “The problem with that method is that it breaks the concrete into little pieces and would have generated about 330,000l of water.”
“The challenges were that there was about 60m3 of concrete which had to be demolished and it was all above the river in an environmentally sensitive area,” says Evans.
“There were also migrating shad [fish] in the river so there was a restriction in our working hours - we had to down tools for two hours every day so we wouldn’t disturb them with any vibration or too much noise. But the real challenge was that we had to keep the bridge open for pedestrians; we could close it to traffic, but not to people.”
Dawnus eventually abandoned plans to use hydodemolition because of the risk that too much debris and concrete would end up in to the river.
Working in conjunction with temporary works specialist Mabey Hire, the team devised a method of deconstruction which enabled the removal of the deteriorating parts of the bridge without polluting the river.
The work had to be done quickly as the bridge is a vital road route. It also had to be kept open for pedestrians while work took place
Dawnus prioritised the works below the bridge deck because the time to replace the column and the crossheads was limited. Work could not start until July because of the need to keep the bridge open to traffic for the Royal Welsh show in nearby Llanelwedd. Work in the winter was also ruled out because higher river levels made it a potentially unsafe environment to work in. Painstaking construction planning started in May 2013 with work on site starting two months later.
Mabey Hire erected a large temporary support tower around each of the three piers, building them off the breakwater. Jacks at the top of the towers allowed the bridge deck to be lifted from its bearings and onto the tower, freeing the crossheads from the structure above. Chains were then attached to the crossheads so they could suspended from the deck after being cut free from the piers.
A working platform, hung from the deck, was created under the crossheads. With the crossheads suspended from the bridge deck, a small, 100mm cut was made in the columns below them, freeing the crossheads from the existing structure.
The crossheads, which were suspended by manually operated chain blocks, were then lowered onto rollers on the work platform, allowing them to be slid out from under the deck. They were then cut into 1m long sections using a diamond cutting wire which was wrapped around the beam and pulled and rotated, working its way through the hard material. The blocks were then winched upwards and removed by a crane on top of the bridge.
When the beams had been removed completely, the columns were then cut in 1m long sections and the blocks removed by lifting them up through the stucture to the work platform and then moving them out from under the deck.
In total, 60, 1m3 blocks were cut and removed and are now being used as ballast on other construction projects.
With the columns and crossheads demolished, reinforcement cages were installed, shuttering was built up and the structures were recast. The temporary working platform suspended from the underside of the deck was then used as falsework when building the new concrete crossheads.
New bearings replaced the old and the bridge deck was lowered onto the new structure. With the main support structures now complete, the temporary support structure was removed in October 2013, only three months after demolition began.
The works were not without complications, not least because of the forces involved in moving the crosshead out from under the deck.
“Normally if you use the hydro demolition method, the structure would only need to support the bridge deck above it,” says Evans. “But because we were doing sliding backwards and forwards of the heavy weights [crossheads and columns] and lifting things up, it introduced a lot of additional forces into the structure. The problem was the dynamic horizontal load in the structure.
The large temporary support structure had to be designed to withstand the large horizontal loads induced in the structure. In addition, the space within the temporary tower was extremely limited so every part of the deconstruction and reconstruction process had to be planned to take this into account.
Access to the first two piers was created by constructing a new walkway between the abutments and the breakwaters. But the third pier could not be reached in this way, so access had to be created from the top of the bridge deck.
With work under the bridge complete, the temporary towers were dismantled on programme in October 2013.
To grit blast and repaint the steelwork on the underside of the bridge deck, a temporary platform was suspended from the underside of the deck, avoiding the need to work in the less predictable winter river conditions.
As part of the work, the 1m wide cantilevers forming the pedestrian walkways on each side of the deck, had to be replaced.
“Using a jack hammer, we cut a slot in the deck and used concrete shears, which are basically an attachment on the end of the dipper arm of an excavator which nibbles away the concrete in between the steel,” explains Evans.
A crash deck was installed under the cantilever and the concrete was captured in that and taken away. The 1m wide sections were then recast using the existing, straightened, bridge reinforcement.
Using the existing reinforcement in the cantilever produced a large saving, compared to retrofitting resin anchors into the existing structure.
Originally the client had planned for a 12 month construction period, but with the method of demolition developed by Dawnus, the time was reduced to an impressive nine months on site completing in March 2014 with a £1M construction cost. This included a seven week delay to allow a water main to be diverted.