The great upgrade – modernising the UK’s railway tunnels

12 September 2016

Even infrastructure ‘built to last’ can’t truly last forever; eventually there comes a time when elements need upgrading and replacing. Colin Sims, principal engineer at Network Rail, discusses the practicalities of upgrading incredible lengths of Victorian tunnels to maintain the UK’s railway network.

The UK’s rail operators are currently investing millions of pounds in new rolling stock. But to reach their destinations, those state-of-the-art train sets will rely on a far older network: 335km of Victorian tunnels, most of which were completed over 150 years ago.

“We have 692 bores on the network, and just about all of them were built between 1840 and 1910,” says Colin Sims, principal engineer at Network Rail. “Even today’s designs will only have a 120-year lifetime.”

Working out the most effective and efficient ways to maintain and upgrade these navvy-dug, brick-lined bores so that they can function as part of a modern network is the responsibility of Sims and his team at Network Rail’s safety, technical and engineering directorate. They provide specialist engineering advice, validate technical standards, and report and analyse asset performance for the operational directorate that manages network operations and upgrades.

“My team provides assurance that the day-to-day maintenance and running of the tunnels is safe,” he says. “We also look to the future in developing new policies, standards, techniques and strategies.”

Monitor and assess

With vibration, high-speed airflow, water ingress and vegetation growth, every tunnel’s structure degrades in time – some slowly, others more quickly. Inadequate original construction methods can cause weak points, like the ‘dogtooth’ bricks bookending most 4.5m tunnel sections that tend to admit water.

Changing ground conditions can produce local overloading, cracking and settlement while water washes out mortar and soaks the lining bricks; combined with frost damage, these bricks can then spall or fall out altogether. To monitor that degradation and understand when it becomes critical, each bore has a tunnel-management strategy, including a risk assessment and an action plan to mitigate any intolerable risks.

Most tunnels undergo a detailed annual examination, with inspectors also checking the ground above for movement. Algorithms developed over the past eight years process each bore’s inspection data to deliver a final score – the tunnel-condition marking index. All of this information informs the maintenance programme and helps accurately prioritise interventions.

Engineers have a range of options to help them judge a condition and then choose the right remedy, such as drilling test holes then inserting an endoscope to look for cavities. Sims wants to move away from wired monitoring devices like strain gauges, however.

“We are developing DISCAM, a contactless monitoring system that moves through the tunnel robotically on a vehicle, using techniques like digital image correlation and laser scanning,” he explains. “We are also working with partners like Omnicom and the National Physical Laboratory to develop other systems for detecting subsurface defects.”

Remedies include improving brickwork’s strength and stiffness through cross-stitching or brick replacement, grout injection, installation of waterproof membranes, relining and – in some rare cases – complete renewal.

Upgrading tunnels to cope with modern traffic often prompts the beginning of larger projects. Faster trains will produce higher transient pressures along with other unwanted aerodynamic effects, for example, which can mean aural discomfort for passengers or high loadings on tunnel fixtures.

“We have to cater to that through measures like pressure-relief shafts, as was the case in the west-coast modernisation,” says Sims.

Electrification is another common element; modern equipment rarely fits the UK’s existing tunnels, so some form of enlargement is the only way forward.

“Normally, we can get away with track lowering, but that impacts the invert [base] part of the tunnel, which is structurally significant,” says Sims. “We have to undertake a significant engineering assessment of the structure and calculate how tolerant it will be to modifying the invert to increase headroom for electrification.”

Service maintenance

Bolton’s recently reopened Farnworth tunnel is one example of more extreme remodelling. “We looked at all sorts of options to add electrification and cater to a line-speed increase,” says Sims. “But the only way was to enlarge one of the bores.”

Network Rail had to increase the size of one tunnel by almost half, while continuing to operate the railway through the adjacent smaller bore tunnel. To do so, engineers first filled the old tunnel with foam concrete to prevent forward collapse, then drilled right through it with a 9m mechanical shield, a mere 2m away from the operational bore.

Even though one bore remained open, local services were vastly reduced. This is the case with most serious major project work; the Severn and Patchway tunnels will also close for most of September and October to allow electrification.

“The real challenge over the next 30 years is how we cater to the inevitable ramp-up of maintenance activities as assets get older and use increases,” Sims says. “How do we upgrade them quickly and safely while maintaining the service?”

"The real challenge over the next 30 years is how we cater to the inevitable ramp-up of maintenance activities as assets get older and use increases."

Extending the use of single-line working during structural work is one answer. A limited single-track service can help extend the normal night dead time or ‘white’ periods used for tunnel maintenance by a couple of hours, improving engineering productivity.

Working-area protection

New single-line working equipment is a big part of this solution, and the latest creation from Chris Scott of Innovative Support Systems (ISS) and contractor AMCO is a great example. It employs segregation screens that isolate the working area of a tunnel from the operational side, countering the aerodynamic effects of passing trains and filtering out any dust or contamination from ongoing works.

The screens sit on one side of standard shipping containers carried on flatbed wagons, unfolding hydraulically when deployed. Platforms also unfold to support maintenance personnel who, located on the far side of the works train, are protected from passing traffic on the other line.

Somerset’s Whiteball tunnel could be the venue for this new system’s first proof of concept. A Brunel tunnel built wide to accommodate the broad gauge, it would allow the new lining to be installed within the existing lining. For the same reason, it offers more working room.

ISS has created numerous other specialist tunnel-maintenance machines, able to simultaneously drill multiple holes in roofs or remove the thick layers of Victorian soot that impede today’s maintenance crews. To aid the Severn tunnel’s 2004 relining, ISS built bespoke mobile crash decks, protective workforce shelters and a 10t bogie for transporting materials. Another earlier development is RamArch: curved panels of wire mesh that bolt together to form an arch.

“We had a lot of input on the development of RamArch,” says Sims. “It lets us rapidly deploy a supporting system if we find large areas of degraded brick lining. That removes any emerging risk and we can come back and spray it with concrete.”

RamArch is currently enabling the rapid shotcrete reinforcement of a Merseyrail tunnel into Liverpool Central. As there is plenty of headroom, maintenance crews can spray concrete on to the top half of the tunnel while trains run below. A crash deck prevents any tools, components or materials falling on to the tracks.

This kind of birectional working is the optimum solution for network efficiency and a long-term goal for Sims. But achieving it in smaller tunnels is a tough ask. “That would need relining methods that sit outside the operating envelope; something like tunnel-boring machines that trains could run through the middle of,” he says. “Germany is attempting this method at the moment for some smaller-diameter tunnels.”

Remodelling remedies

In Italy, existing motorway tunnels are also being relined without stopping traffic; engineers construct a protection canopy, behind which they build the new lining.

“We need to look at the feasibility of importing those sorts of methods into the UK railway network,” says Sims, noting that Southampton tunnel was relined in cast iron 30 years ago while trains continued to pass through. “They moved the track to the middle of the tunnel to allow rebuilding outside of the operating envelope. We just need the will to take this forward and make it work using modern techniques.”

Developing new materials and enhancing existing ones so they can be more readily applied or installed, or reach operating condition more quickly will aid this efficiency drive. Network Rail already uses spray waterproof membranes, for example, but only for small-scale patching.

“You have to remove some or all of the lining first and then put a new lining on top afterwards,” notes Sims. “But the membrane takes a long time to go off and that means its use is constrained by our access times.”

Another research direction is finding alternatives to rebuilding multiple layers of brick lining following remedial work. As its cost-effectiveness increases with volume, spray concrete is a prime candidate for large areas. For smaller lining sections, sheet material would be far quicker to apply than bricks.

According to Sims, extra effort is needed to develop the tunnel materials and machinery to work faster, safely and with less network interruption. His team’s headcount has quintupled in recent years, showing the rising scale of the challenge.

“If we don’t start thinking hard, it will be a problem in the next 20 years,” Sims says. “We need that time to introduce innovations and let them mature to the point where we are using them regularly. Everyone is very excited about new tunnel builds like Crossrail 2 and HS2, but they pale in significance when you have infrastructure at this scale that the country is becoming more reliant on.”

Farnworth Tunnel saw reconstruction completed in December 2015, as part of a wider investment to electrify around 300km of the rail network in the north-west of England.
A soot scabbler pulverises soot along the inside of a railway tunnel.

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