Engineers are working on a small but complex project to repair a flood damaged bridge pier in Cumbria.
Over seven days in December 2015, Storm Desmond broke UK rainfall records as it pummelled Cumbria with more than 1.15trillion litres of rainfall.
Flood defences designed to withstand a one in 200 year event were overtopped and more than 18,000 properties lost power, while the Army was drafted in to help residents trapped in their homes.
As the initial chaos subsided, the scale of the damage to the county’s infrastructure became clear. Main roads, like the A591, had been half washed away and many historic bridges had crumbled into swollen rivers. More than 2,000 businesses were affected, as Cumbria’s connectivity was severely compromised, costing the county’s tourism-based economy around £500M.
Since early 2016, 452 of Cumbria County Council’s assets, including 278 bridges, have been painstakingly repaired, under the local authority’s innovative £123.6M Infrastructure Recovery Programme (see box).
The programme, managed with partner Mott MacDonald, is due to finish in August 2019. It has encountered its share of challenges. Surveying more than 1,600 assets immediately after the floods, complex procurement and changing supply chain behaviours have all tested the programme’s core team.
Brougham old bridge temp works
But the biggest technical challenge has been the £1.15M project to save Grade II-listed Brougham Old Bridge across the river Eden.
Built in 1812, the bridge sits adjacent to 13th century Brougham Castle and is part of a popular cycle and pedestrian route.
On 15 December 2015 an initial survey showed there had been significant scour damage to the three span bridge. On its the north side one of the two cutwaters at either end of the central span had been destroyed. The cutwaters are sharp pointed masonry noses on the front of the piers. The collapse of the cutwater left a gaping hole revealing serious damage to the foundations.
When the cutwater collapsed, it had taken part of the central arch span with it. A section of the spandrel face had also fallen into the river.
“It left the parapet sort of spanning over a gap, so when we initially looked at it we were very concerned from a structural stability point of view,” says Mott MacDonald project lead Craig Mitchell.
The bridge was closed immediately and the team set about working out whether the historic bridge could be saved. But before any progress could be made, a puzzle had to be solved – why had the whole of Brougham Old Bridge not collapsed?
“We really struggled at first to understand why it was still stood up, which is an issue because unless we could understand the structure and how it’s safe, we couldn’t get people close enough to work on it,” explains Mitchell.
Engineers were perplexed. Although a third of the pier behind the collapsed breakwater had been lost, most of the central arch remained and Mitchell wanted evidence that the load from the arch was being safely transferred into the ground. Over several months engineers scrutinised the bridge but were unable to prove it was in a safe enough condition for work to be allowed to start.
Meanwhile, the bridge was edging closer to a full scale collapse.
“We could gradually see that there was more and more material being lost from the pier and some being lost from the arch barrel,” explains Mitchell.
“We knew at some stage it would go, so it was almost a race against time to understand: was it safe enough at that stage to get in there and do the works before it actually did collapse?”
Eventually engineers at consultant Curtins were able to provide an answer. Using a thrust line analysis, Curtins proved that the force from the arches was still being safely transferred through the remaining part of the pier. A method of work was devised which showed how the reconstruction could be delivered safely.
Once Mitchell and his team were confident the structure could be saved, stabilisation work commenced.
Over two months from September to November 2016, local contractor Metcalfe Plant Hire carried out the vital works.
Firstly, a cofferdam was built around the base of the collapsed pier to keep the area dry and free of fresh silt from the river. Concrete was then pumped into the 80m² hole.
Once the team had pumped the concrete under the pier, engineers were more confident about the bridge’s stability. Construction workers moved closer, to a distance of roughly 5m away, and used a long reach excavator to pick off the overhanging masonry parapet block by block. The stone was then stored to be reused later in reconstruction.
The initial stabilisation finished after a series of concrete blocks were used to build a temporary cutwater, slotted into the gap left by the pier. Monitors were dotted around the structure and programmed to send a warning to team members’ mobile phones if movement was detected.
Work halts for winter
At this point, the team left the bridge for the winter, satisfied it would not collapse.
Away from site, conversations with Historic England were informing designs for reconstruction work. As the bridge is a listed structure it was important to deliver a like-for-like replacement of the lost cutwater and parapet masonry.
Before the reconstruction could start, however, a surprising discovery had to be tackled.
“When we looked back through historic records, we found a record from 1890 that showed the same issue had occurred and that a cutwater had fallen away previously,” explains Mitchell. “That allowed us to do a bit of site investigation and understand that the actual root cause of the problem was a very soft material underneath that single pier.”
Weak cutwater base
On the other side of the central span, the sandstone bedrock runs higher, so the right-hand pier which was founded on it was much less susceptible to scour during the floods. By contrast the collapsed cutwater had been resting on a 1.5m to 2m thick layer of dense sand and very weathered sandstone bedrock which was easily washed away.
To fix the underlying problem the team had to strengthen the ground. Drilling down through the damaged part of the bridge, once it was supported with mass concrete, grout was injected into the soft material to create a more durable foundation.
“Hopefully in 100 years’ time there won’t be someone else sat here telling people about how the cutwater fell down. We hope we’ve dealt with that issue permanently,” says Mitchell.
Once the foundation had been strengthened, work to reconstruct the damaged central arch began. A temporary support system was installed using hydraulic jacks to push it into the shape of the arches. A laser survey was then carried out to identify exactly what replacement stones were needed.
“It’s quite an interesting approach to doing it,” says Mitchell. “A lot of people were interested in the technical side of those works.” Local Penrith sandstone, the same material used for the existing bridge, was pre-cut off-site to save time. Environmental constraints dictate that water-based works in the Cumbrian rivers be carried out over 75 days from June to September.
“We wanted to have the masonry there, cut and ready to slot in place, a bit like a jigsaw type of approach,” explains Mitchell. “We didn’t have time to be messing around cutting bits of stone on site.”
Through meticulous reconstruction and resilience-boosting engineering, the bridge was restored to its loading capacity of 17.5t and on 13 December 2017 reopened to the public.
Although the most technically challenging project in the infrastructure replacement programme, Brougham Old Bridge’s restoration is just one of 1,234 individual schemes in the programme. By July 2019, 754 bridges will have been repaired or rebuilt across the county – including the programme’s “jewel in the crown”, Pooley Bridge (see below).
Cumbria’s flood response
“What we’d never experienced, even in the historic records going back to the 1800s, is a county-wide flood event,” says Cumbria County Council assistant director for highways, transport and fleet Stephen Hall.
“It’s absolutely fair to say we didn’t have a response plan for such a widespread and such a significant flood event.”
The council teamed up with programme partner Mott MacDonald for the £123.6M infrastructure recovery programme set up after Storm Desmond to rebuild the county’s assets. Around 1,600 early surveys identified 1,234 repair schemes, comprising 174 highway jobs, 754 bridge repairs and 306 major civils projects.
Those 1,234 individual assets have been bundled into packages of work, to make procurement easier. So far, more than 70% of the contracts have been awarded to local suppliers, meaning the economic benefits of the programme are felt in Cumbria.
Two years into the programme and 452 repair jobs have been completed so far, with 409 in progress and 373 still to do.
But the IRP is not just about repair works. A quirky new invention has come out of the programme, a sonar system called BridgeCat.
BridgeCat was created by retrofitting a Unimog vehicle with high-definition cameras and sonar equipment to do rapid bridge inspections post-flood events. A crane on the side of the vehicle allows inspections to be carried out where it is too dangerous to put a diver in the fast-moving flood waters.
The Department for Transport (DfT) has given the Infrastructure Recovery Programme team £500,000 to develop the technology further, working with surveyors Gaist.
Trials are being carried out, and if successful, the technology could be rolled out across local authority areas affected by flood events.
The programme also taught the council how to procure work quickly after a flood event, and this is being shared with the DfT. It will share this with other local authorities who experience serious flooding.
Grade II listed Pooley Bridge collapsed as a result of damage sustained during Storm Desmond. Knights Architects has designed a replacement which has been dubbed the “jewel in the crown” of Cumbria’s infrastructure recovery programme.
As the 18th century bridge had been completely destroyed, the recovery programme team was free to come up with a completely new design. Work is at an early stage but the bridge is expected to be completed later this year.
Getting it right in the eyes of local people is the main challenge for the scheme, as the original bridge was admired by the community and holiday-makers. It also forms one of the main gateways into the Lake District, making the replacement scheme particularly high-profile.
Pooley bridge concept
Three different options were created for public consultation, and the most popular aspects of those schemes have been combined in the final design.
The draft design is for a single arched bridge with open spandrels and additional lattice detail in the spandrel voids. It is proposed as a composite structure, with a stainless steel frame and a concrete deck.
Initially local people were sceptical, believing their consultation responses would not be taken seriously. But hearts and minds have been won through listening to the community’s concerns and incorporating them in the design.
The single span reduces in-river working, meaning less construction has to be squeezed into the 75 day, June to September window, when work is allowed to take place in Cumbria rivers. As there is limited space at the site the steel frame will be pre-assembled and slotted in by crane in two sections, with the concrete deck poured on top.
“We’re very constrained down there so we’re already thinking about how we’re going to build the bridge,” explains Mitchell.
Where possible, materials from the old bridge will be used in the new one. “It’s a bit of a step away from what the old structure was, but well received. I think it’s seen as being a bit of a statement structure,” says Mitchell.
A design and build contractor is being procured.