Rehabilitating ageing existing tunnels is set to become an equal technical challenge to new tunnel building.
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Network Rail is putting out a call to arms for engineers to come up with new and innovative solutions to upgrade its network of ageing brick lined Victorian tunnels.
Across Britain’s railways there are around 700 such tunnels totalling around 335km in length. These old assets need to be inspected and maintained, but on a railway network where demand is growing exponentially, the window within which these crucial works can take place is becoming narrower and narrower.
For engineering companies willing to invest in this not so glamourous side of tunnelling, the prize is substantial says Network Rail head of mining and tunnels Colin Sims.
“At some point in the future that railway tunnel renovation will be a much bigger industry than the highs and lows that we currently get out of new build tunnels,” he says. “The work will be constant for a number of years to come.”
Railway tunnel renovation will be a much bigger industry than the highs and lows that we currently get out of new build
The problem however, is not simple.
At present around 60% of the 700 tunnels, ranging from 50m to 7.5km in length with some on key strategic lines such as the east and west coast main lines, are required to have a detailed inspection once a year. The remaining 40% range from two to four yearly inspections, depending on their condition.
These inspections are currently carried out during night shifts with longer work potentially able to be completed during weekend possessions, but these have to be booked around two to three years in advance.
The challenge, says Sims, will come in the not too distant future when growth on the railway prohibits night working, weekend working or any possessions on the lines.
There are a number of defects that examiners look out for during the inspection – water ingress or misalignment of the brickwork, face spalling and loss of mortar.
But the defect that causes the most time to map is ring separation. This is a debonding and consequential delamination of the layers of brickwork in the arches. In some tunnels, the layers of brickwork lining them can be up to eight bricks thick and to spot if there are any openings developing between them, the walls have to be hit with a hammer and any changes in the responding sound noted.
Carrying out this work is not easy, access is often limited to foot only, it is dark, slow, tiring and time pressured. So with safety of the examiner at the forefront of its mind and a need to collect the data faster with diminishing windows of opportunity to physically spend in the tunnels, Network Rail is working on a new solution.
In conjunction with the National Physical Laboratory and remote surveying developer Omnicon Engineering, Sims and his team are developing an examination system based on laser scanning and digital imagery with supporting comparative software.
“We have a project on the go which marries DIC [digital image correlation] and laser scanning to provide a set of high accuracy data sets on defects, their location, their extent and the severity,” says Sims.
“My vision is that we are working towards a high objectivity, highly reproducible, high output data gathering system.”
The project is currently at prototype stage, and is still a couple of years away from completion, however Sims’ aspiration is that the system will produce consistent results, far quicker than previously.
“There are still a number of work streams to develop, but we’re on a road of development for it and once complete it’ll vastly decrease the time it takes to carry out the examinations,” says Sims. “This will allow for a far more optimum use of the examiners expertise and target their areas of concern.”
We are working towards a high objectivity, highly reproducible, high output data gathering system.
However, inspecting the tunnels is only half the story and the real challenge will lie in carrying out any repair work that might be necessary.
“On a structure which is degrading and on which we should be spending more time, how do we repair them in less time so we don’t disrupt the network?” asks Sims. “We have to be innovative.
“The big call to arms is the methodology for repair.”
The team are now building up a huge database to document the condition of the tunnels and Sims’ aspiration is that this will enable a model to be built to give an indication of the serviceability life of each tunnel. Based on this it can establish whether patch repairs are appropriate or more serious interventions are required.
Interventions such as relining the tunnel is invasive work and can involve closing a line, which may not be an option in the future.
“We need to think about how we mechanise tunnel lining with a high output and methods. Those methods could be improvements in materials and equipment and methodology,” he says.
Sims says that he has been following the progress of trials which are being carried out in Germany to repair its similarly lined tunnels.
There, they are developing a system to over cut the existing tunnel and therefore allow the tunnel to be relined while trains are still allowed to pass through, albeit at lower line speeds. However, he says that while this is interesting, the trials have been carried out in single track tunnels with no other infrastructure such as overhead line equipment, drainage and other cables to complicate matters. He now wants engineers to scale up the solutions to include twin track tunnels with overhead line equipment.
It is this overhead line equipment which is proving to be one of the biggest problems.
“Electrification is happening on quite a few of our lines, which is going to have a further complication on methodology of examination and repair,” says Sims. “There are assets in there which we have to avoid and protect such as overhead line equipment.”
Sims is, however, very positive about the problem and thinks that it is not insurmountable but that it could take five to ten years to develop a solution and therefore it is necessary to start thinking about it now. Engineers assemble.