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Bank Station Capacity Upgrade: Planning the Bank job

Appointing a contractor before gaining planning consent may be unusual but ground investigation is currently underway aims to ensure it is a smooth transaction for the Bank Station Capacity Upgrade

Current ground investigations will be used to confirm existing data

Current ground investigations will be used to confirm existing data

London Clay is widely considered to be ideal tunnelling conditions and there is plenty of available data on the horizon for preliminary design work. Nonetheless, London Underground (LU) and its contractor Dragados are taking no chances on the Bank Station Capacity Upgrade project and further investigation is underway.

The scheme involves splitting the Northern line as it currently runs through the Bank area and put the south bound trains into 600m of new tunnel to the west of the existing station. The upgrade will also create a new interchange with extra passageways to improve passenger flow. In addition there will be new entry and exit points on Cannon Street, 12 new escalators, two new lifts and refurbishment of an existing one and step-free access onto platforms.

“The principal reason for the project is to relieve congestion at Bank,” says London Underground Bank Station Capacity Upgrade project manager Simon Addyman. “The station has interchange issues as it is an east-west and north-south intersection.”

At peak times 100,000 people use the station – the capacity for the station – and forecasts suggest that this is set to grow.

Although LU hopes Dragados can start work on site in 2016 and complete in 2021, the scheme still needs to gain a Transport and Works Act (TWA) order and current ground investigation has a key role to play in ensuring this is achieved this summer.

Concept Consultants is trying to thread boreholes through the tight congested streets of the City of London to provide samples for advanced geotechnical testing that will provide vital data for design.

There are 65 buildings within the potential settlement zone above the new tunnel

There are 65 buildings within the potential settlement zone above the new tunnel

Utilities and access has cut the original ground investigation down from 12 to eight boreholes but the focus is on undertaking the work safely and recovering quality samples.

“There is a lot of historic information already as much of the work will be undertaken in the London

Clay,” says London Underground profession head for deep tube tunnels Keith Bowers. “However much of the available data is old and we don’t know the provenance. It’s a confidence issue and we hope the results from this investigation will validate the information we already have.”

According to Concept director Anastasia Savidu, the boreholes will extend to depths of up to 55m to recover class A1 samples. “In addition to standard testing, the advanced testing will include small strain stiffness, anisotropic consolidated undrained triaxial tests, small strain and shear wave velocity measurements of suction and shear wave velocity on unconfined specimens,” she says.

LU is aiming to submit its TWA this summer and the results from the ground investigation work will be used to assess the potential settlement ahead of the application.

The reason why settlement is a concern is that the new tunnels will pass below two Grade I listed buildings – Mansion House and St Mary Abchurch – and 14 Grade II listed buildings. In total there are
65 buildings in the settlement zone.

The upgrade will add a second tunnel on the Northern Line

The upgrade will add a second tunnel on the Northern Line

“It is a small number of properties but they are large properties and there are a high proportion of heritage structures,” says Bowers. “Some of the buildings are rebuilt behind facades and the structures have a range of foundations- there is a marvellous historical mix.”

London Underground and Dragados are using the three stage building assessment approach that has

been used successfully in London on other tunnelling projects.

“Giving certainty through stakeholder engagement and a strong relationship with the contractor has been a key aim in developing this scheme,” says Addyman. “It was a key decision to delay the TWA until after the contractor was appointed in order for the scheme to benefit from the input of both the winning contractor and to use the ideas of other bidders.”

According to Addyman, going for TWA after contractor appointment means that the scheme will benefit from industry innovation applied before this process rather than trying to engineer a solution to the scheme put forward in the TWA. “Dragados demonstrated the capability to deal with the challenges on the scheme and a good approach to solving problems,” he says. “It is fair to say that since contract award the contractor has demonstrated good behaviour which is borne out of the approach to procurement we have taken,” he says.

Addyman says that Dragados’ input to the TWA process has helped increase the business case for the scheme. “Some of the improvement has come from journey times but they have also helped increase the social value of the work too,” he says.

The logistics of actually carrying out the work are almost as big a challenge as the tunnelling itself and demonstrating the ability to manage this aspect was a large part of the procurement, along with the approach to stakeholder management.

Rotary and shell and auger rigs have been used to cope with the limited space

Rotary and shell and auger rigs have been used to cope with the limited space

“I believe we are developing a strong case for the TWA and this ground investigation will help ensure we have covered all possibilities,” says Addyman. “The TWA being done with all options considered is different from previous schemes and we created Ground investigation the option to buy proposals from other bidders too. The bids for the work resulted in four very different schemes being proposed with the key differences being station layouts, logistics and tunnel alignments.”

Addyman confirmed that LU had bought innovations from losing bidders but not all of these will be applied. “We have the benefit of having options to ensure that we have the most effective product in the market,” he says.

Addyman estimates that the TWA has been delayed by two years and the investment in the procurement will benefit the scheme with an estimated £65M saving in costs and 18 month programme reduction. “There is still potential for management of risk and opportunity,” he says. “This may not result in a time or cost saving but could deliver a higher degree of safety and lower risks.”

While it may be another seven years before passengers will see the benefit of the Bank Station project, it is clear that every effort is being made to ensure that the value of the current investment will be felt throughout the design life and not just in the cost of construction.

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