Force of nature behind Tunnel’s punishing cycle of maintenance

Mar 29 2008 by Rhodri Clark, Western Mail

The Severn Tunnel closes today for yet another engineering programme which will disrupt weekend rail passengers – including Cardiff City fans heading to Wembley – for several months. Rhodri Clark descended 200ft underground to view the tunnel and ask engineers why it must close again

THERE are plenty of world firsts on Wales’ CV, but being a pioneer can have drawbacks. When the Severn Tunnel was constructed in 1873-86 the Great Western Railway had no precedents to show how to tunnel beneath seawater.

The 4.5-mile tunnel was a breathtaking achievement. Its builders accepted the tunnel would drain the land either side of the estuary. A smaller tunnel beneath the main one brings water from England and Wales to Sudbrook, near Chepstow, for pumping out – a river beneath an estuary.

The tunnel also broke into a spring, or “underground river”. More than seven million gallons of freshwater are pumped out daily, some piped to a brewery and local homes.

Standing beside the track in the tunnel beneath Sudbrook, I felt a change in air pressure as a train entered the tunnel two miles away but the growl of ventilation equipment drowned the train’s noise. Even this small Sprinter train made a sudden roar as it passed at 90mph.

Lower down, the scene was Dickensian. Water, displaced by the air pressure of a train passing above, sprayed into a narrow brick passage from a vault in which other brick structures were arranged. I peered into a void where water collects for pumping to the surface.

The control room at Sudbrook is a short lift ride and a technological world away. There’s a computer monitor and a board on the wall where lights indicate which pumps and fans are working. There are spares on standby. Emergency generators are ready to start within 30 seconds of a power cut.

The contrast is a reminder that we want 21st-century service from an asset predating the motor car. Computers may control the pumps and fans, but water still gushes in the tunnel.

Exhaust fumes from passing diesel trains combine with the moisture to corrode the steel rails so quickly that the track must be replaced every five years. Outside the tunnel, the same track carrying the same trains lasts 25 years.

Then there are many miles of brickwork and cables to preserve in this harsh environment.

The programme starting today and continuing on weekends until September is mainly to replace brackets made 50 years ago to hold cables along the tunnel walls. The new ones are like super-size shelf brackets. Just two bolts secure each one. Why close the tunnel for so long to install them?

Project manager Mike Palmer gives several reasons. More than 14,000 must be installed. Moving the brittle cables from old to new brackets will be a delicate operation to avoid breaking cables and severing communications.

Getting workers and materials into the tunnel will take two hours at the start of each weekend closure. The same time is needed to get them out at the end. Three trailers have been specially designed and built, enclosing the men in cages for safety and keeping them at the right height to fix the brackets.

Around 750 brackets, 12 tonnes in total, will be fitted each weekend.

Theoretically, the work could be done by closing only one of the two tracks. But Mr Palmer says that’s impractical because trains on the other track would have to travel slowly, for the workers’ safety and comfort.

Couldn’t next weekend’s work have been postponed once Cardiff City were heading for Wembley?

“Nobody could have predicted they would get to the FA Cup semi-final,” says Mr Palmer. “We spent two years planning this work.”

Sunday-only instead of two-day closures are scheduled for two weekends in May when pre-planned events occur, including the Heineken Cup final in Cardiff.

When it takes two years to plan such a project, rescheduling in less than a month to accommodate City fans was impossible, says Mr Palmer. “The train-operating companies would have to reschedule their train times. It’s not just the work we’re doing in the tunnel – when we’re working there we don’t close the alternative line through Gloucester.”

Work that closed the main line near Swindon was finished last weekend, avoiding clashes with the Severn Tunnel project. When the tunnel is closed, London trains will run via Gloucester, with three extra trains for City fans next Saturday.

The new brackets should last decades, but the track still needs replacing every five years. If we want to break this cycle, the solution is electrifying the railway to banish diesel fumes and extend the rails’ life – or building a 21st-century tunnel lined with concrete and free of water.