Cross examination

3 min read

The Crossrail link project could lead to costly and disruptive deformation of existing tube lines, unless the potential stress caused by construction of new railway connections under central London is properly evaluated.

This is the fear of Imperial College engineers who are involved in a £1.1m government-funded research programme to help the developers of London's new east-west tunnels avoid damage to existing underground structures.

The programme is receiving support from Cross London Rail Links, London Underground and Morgan Est, a UK infrastructure specialist.

Jamie Standing, a civil engineering lecturer at Imperial College, will lead a team of fellow investigators who hope to have results that can be implemented before tunneling construction begins on the Crossrail project in October 2010.

His team will conduct a series of laboratory tests on sample grey cast-iron tube tunnel linings and examine how they deform under stress.

He said: 'We will know what the stress of existing tunnels is before any tunnelling starts and that is something at the moment no one has any idea about.'

The group will also perform field experiments in Hyde Park, where the Crossrail tunnels will pass beneath the existing Central Line.

Those experiments will involve traditional measuring instruments, such as rod extensometers and electrolevel inclinometers, installed at set depths and positions around existing and new tunnels. These instruments will predict and measure ground movements around the tunnels.

Within the tunnels themselves, Standing has proposed installing fibre optics for measuring continuous strains at points along and around the existing linings. The data will be downloaded through wireless techniques being developed at Imperial.

Finally, the group will also perform monitoring at a neutral nearby location, where there is no influence of existing tunnels, to compare the two scenarios.

Standing said all of the results will be evaluated through numerical analysis and fed into Imperial's bespoke finite-element analysis software, called ICFEP.

The results will indicate to developers whether construction around these existing tunnels will cause them to deform in a way that makes it difficult for trains to pass through.

Standing explained that trains operate within a narrow 'kinematic envelope', which is the maximum amount of space for rolling stock to move around the track. 'When a train goes around corners you get centrifugal forces that tend to move the train outwards so it needs a certain amount of space to run without clipping the sides of the lining,' he said. 'Sometimes that's quite a small space — an order of centimetres or millimetres. What we need to make sure is that kinematic envelope isn't impinged on. If the tunnel does deform, then they will slow the train down so the centrifugal forces aren't so severe.'

Standing said there currently is no understanding of the stresses within London's tube tunnels. 'It's a very complex structural interaction problem,' he added. 'There hasn't been an understanding of it because people tended to avoid constructing near existing tunnels.'

The work that bears the most resemblance to this current project, Standing said, was Imperial's involvement with the Jubilee Line extension in the mid-1990s. The effort, which was headed by his co-investigator John Burland, looked at the effects of tunnelling near existing foundation, geology and a much smaller number of tube tunnels compared with the Crossrail project.

'The Crossrail will go under 43 tunnels,' he said. 'Those tunnels are of different forms and construction materials. Some are cast-iron segments, some of them masonry, some of them pre-cast concrete segments.'

The Crossrail project, which was given parliamentary approval in July 2008, is expected to be one of the most challenging engineering projects ever undertaken in the UK. It is claimed that when it is completed in 2017, it will be the biggest addition to the transport network in London and the south east for 50 years.

The construction project will involve driving five tunnel-boring machines beneath London to create 22km of twin 6m-diameter tunnels. The new line will include east-west tunnels connecting the Great Western Main Line near Paddington to the Great Eastern Main Line near Stratford. A second eastern branch will diverge at Whitechapel and will run through Docklands, under the River Thames and terminate at Abbey Wood.

The Imperial research programme, which concludes in 2013, will have an immediate impact on the Crossrail project. Standing said the results of his team's work could also be applied elsewhere.

'We will be in a much better position to judge how tunnelling beneath existing tunnels is going to influence future works,' he added. 'Some of it will go directly to Crossrail but probably a good deal of it will be for future projects. Not just in London but internationally.'

Siobhan Wagner