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Rail: De-risking the design

Bank station entrance

London Underground hopes that planning carried out at the design stage will reduce the risks of carrying out the Bank Station Capacity Upgrade.

Timeout magazine once described Bank tube station in the City of London as a “hellish labyrinth” and many, including those who work for London Underground, would agree that it is a fitting description. The station, first opened in 1884, was last updated when the Docklands Light Railway (DLR) terminus was added in 1994 and now handles 337,000 passengers – many interchanging between lines – a day.

Work has been underway to plan a £560M upgrade to add 40% more capacity to the station for several years and, after securing a Transport and Works Act order at the end of last year, contractor Dragados is expect to start on construction soon. Over the last three years London Underground has worked closely with Dragados to build on past experience and de-risk the work which, despite the challenging site, means no compensation grouting and less monitoring.

Bank Station is a key interchange between the District and Circle, Waterloo and City, Central, Northern and DLR with 15 entrances/exits and three ticket halls. Typical issues at peak times include congestion at crossflow areas, queues onto platforms and narrow staircases.

“The station is already at capacity and demand is forecast to increase so doing nothing is not an option”

Paul Dryden, tunnel engineer, London Underground

“The station is already at capacity and demand is forecast to increase so doing nothing is not an option,” says London Underground tunnel engineer Paul Dryden.

London Underground lead civil engineer Mark Dewhirst adds: “Modelling of the station just emphasises the turbulent and conflicting flow in the triplication area. It just shows how overcrowded the existing Bank station is, and I for one avoid interchanging here when possible.”

Small incidents can have a disproportionate effect on services but running trains straight through to alleviate issues at Bank has an immediate effect on services at London Bridge and Moorgate stations.

The solution is to build a new southbound running tunnel so the existing tunnel can be used as a pedestrian concourse and construction of a new station shaft with lifts and escalators between the new and existing running tunnels. The plan also includes a travellator to speed up interchange between the Northern and Central lines.

In total 1km of new running tunnel will be constructed.

Bank station web version

The solution may sound simple but delivering the work calls for a significant amount of tunnelling close to operational tube lines and within the City of London. LU aimed to address these challenges and reduce the risks associated during the design stage.

According to Dewhirst, the project will call for significant tunnelling works are required underneath some of the most historic buildings in London. “Extensive use of modelling and investigation has been used to de-risk the design and construction by assessing the condition of neighbouring assets and modelling the impact of construction,” he says.

One of the major issues was existing buildings with deep foundations on the alignment and some of these obstructions only became known late on in the design process.

Dryden describes the collection of 1970s under-reamed piles at the northern end of the new tunnel as “forest”.

LU worked with Dragados to re-sequence the work in this area to de-risk it by taking it off the critical path.

A review of existing ground investigation data and targeted new work was carried out. Only six new boreholes were undertaken to add to the existing ground investigation work. Dewhirst says that the locations of these were severely limited by existing utilities and buildings.

Dryden adds that there were also challenges above ground with a large number of heritage structures around the site. In the UK around 3% of buildings are listed but around the Bank Station upgrade site the total is nearer 45%. These structures include the 18th century Mansion House, which lies close to the travellator and escalator construction sites and has been previously impacted by tunnelling, and St Mary Abchurch was disturbed during construction of the District and Circle line and has also been affected by bomb damage.

“In total there are 70 buildings within the planned construction zone and 31 have a heritage interest, while six are Grade I listed and a further 25 are Grade II listed,” says Dryden.

Unlike other recent London Underground upgrades, the work at Bank will not use compensation grouting to manage the risk of settlement. Instead there has been a focus on revising the tunnel design to reduce the risk at source.

“The feedback from other projects was that compensation grouting does not necessarily solve the problem”

Mark Dewhirst, lead civil engineer, London Underground 

According to Dewhirst, not using compensation grouting has significant programme benefits for Bank. “The feedback from other projects was that compensation grouting does not necessarily solve the problem and can create ground movement risks that wouldn’t have been realised, if compensation grouting wasn’t used,” he says.

“So we compared the risks associated with having compensation grouting against not having it, and with the conclusions of the damage assessment reports we were able to conclude that the best solution was not to use compensation grouting at Bank.”

Instead the profile of the tunnel has been revised from a 10.71m diameter circular tunnel that was planned at tender stage to a 9.9m wide and 8.7m high invert and arch flattened profile to reduce the excavation face. The tunnel lining will also be 40% thinner than conventional design through use of the primary sprayed concrete lining as a permanent structure.

“We have challenged our own standards on platform tunnels to achieve this,” says Dryden.

As well as the risk to third party assets, the work presents a significant risk to London Underground’s own assets. Over 280 were identified as being potentially at risk but by using a staged approach only a quarter were taken through to stage three assessment.

“Some assets fail assessment at stage three,” says Dryden. “We will be carrying out some mitigations to strengthen some structures but no escalator jacking is required. By comparative risk assessment we found it would be preferable to observe some assets rather than mitigate even though assessment expects these to be working beyond their theoretical capacity.”

The project team calibrated their predictions against previous projects and the nature of movement around London Underground assets from work at Tottenham Court Road and Green Park.

According to Dewhirst, one of the key mitigations for any tunnelling project is monitoring.

“In previous projects I have worked on such as Crossrail, monitoring has been installed before even the completion of stage three assessments which has often led to excessive use of instrumentation and monitoring,” he says.

“For Bank, we have been selective about what tunnels to monitor dependent on the risk profile and the contractor has developed its monitoring database which will be able to show predicted and real time settlement contours.”

Dewhirst says that the mantra for Bank is “less is more” with a focus on the objective in determining the monitoring requirements. “Trigger levels are both prediction based for design verification and asset control limit based,” he adds.

Both Dewhirst and Dryden believe that the approach taken by London Underground at Bank shows that the organisation is becoming more intelligent as a client. “As our experience grows of tunnelling near operational assets and historical buildings, it is our duty to refine and make use of best practice, using new and existing technology to reduce uncertainty and increase confidence,” adds Dewhirst.

While it is clear that considerable planning has gone into the design, the proof of the work will come when construction starts in the near future and over the six year build period.

 

 

 

 

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