Cramped sites and complex underground work characterise the project to expand Bank Tube station in the City of London.
In the centre of London’s fast-paced financial district, Bank Tube station often grinds to a halt. As the capital’s third busiest interchange station Bank serves around 100,000 passengers over a three hour peak each morning.
Crowd control measures are common: passengers are held behind barriers and trains queue in tunnels to give the clogged-up platforms a chance to clear, delaying journeys across the network.
Demand at Bank has risen by more than 50% in the last 10 years. If nothing is done, temporary closures will increase and trains will have to run through the station without stopping, further inconveniencing passengers.
To combat the crisis London Underground (LU) is carrying out the six-year, £623M Bank Station capacity upgrade.
By 2022 a new Cannon Street entrance will provide a new, central exit for the congested station. A spacious Northern line concourse will boost capacity. At the same time, 12 new escalators and two travellators will provide direct routes between Bank’s four Tube lines, plus the District and Circle line station at neighbouring Monument station.
Bank station (black) 1
To make space for the new concourse, the Northern line southbound running tunnel will be replaced with a new one, currently under construction. This will allow the existing running tunnel to be converted into a wider concourse between the north and southbound platforms.
For the most part Bank will remain open to passengers throughout the works, making the upgrade a fiendishly challenging scheme. This, coupled with the fact that the project is pioneering the use of a new procurement approach – an approach that was not without its controversy – means that LU programme manager Andy Swift feels the intense industry gaze keenly.
“We are under scrutiny a lot. We do therefore feel that we need to demonstrate that we are delivering value. But I think the proof is in what we are achieving,” he says.
LU’s pioneering procurement exercise, known as Incentivised Contractor Engagement (ICE), attracted attention when it was used in 2013 to select and contractor. Selection was based on promised added value to customers in the form of reduced journey times through the station, rather than lowest cost, or other traditional output measures.
It definitely delivered an improved solution on LU’s reference design, but it was also very time consuming and expensive for bidders. To compensate for that it also pledged to pay unsuccessful bidders for useful innovations developed during procurement, but that payment was a fraction of the cost of bidding.
In the end design and build contractor Dragados won LU over with its simple travellator solution, which modelling shows will speed passengers through the station. It was the only bidding team to suggest travellators rather than lifts, which have less capacity.
“Dragados came up with that solution because it understood the problem that we were trying to solve. How do you move people within the station environment quickly?” explains Swift.
For the new section of Northern line running tunnel, Dragados was also the only bidder to suggest squeezing in an access shaft at Arthur Street, a narrow lane just off the northern approach to London Bridge. This was chosen as it provided optimum access for tunnelling and would sit outside a strict conservation perimeter.
20180125 bscu kw002 (002)
But when the team started at the site in early 2016, the challenges of working in such a confined space set the project back and helped push up costs by £19M to reach an estimated £642M. Live utility cables were discovered and rerouted. Archaeological discoveries also disrupted construction.
“Without a doubt sinking the shaft – just the start of the shaft – was the hardest part,” says Dragados construction manager John Commins.
Although the shaft is constrained at the top by buildings on either side, it bells out underneath to provide more space for plant.
The shaft is an 11.4m by 7.3m rectangular opening supported by sheet piles. This gives way to an ellipsoidal excavated space with a maximum height of 14.6m and a maximum depth of 9.5m.
It takes a while, but now we’re in full tunnelling mode we have plenty of space
To begin with, the team encountered a chicken and egg situation: it was difficult to begin tunnelling without all the necessary plant, but to get the plant down the shaft, enough space was needed at the bottom.
“You need space, and to have space you need to dig the tunnel,” explains Commins. “It takes a while, but now we’re in full tunnelling mode we have plenty of space.”
Since tunnelling began in May last year, the programme is back on track and work has progressed smoothly along the 600m drive, which was thought too short for a tunnel boring machine (TBM).
Instead, excavators are digging a 4m diameter tunnel 27m deep, lined with sprayed concrete.
But a major challenge is looming. This summer, the running tunnel will intersect with what Swift describes as “a forest” of 3.4m diameter, end-bearing piles underneath 6-8 Prince’s Street, an eight-storey building opposite the Bank of England.
Constructed in the 1980s, the end-bearing piles are close to the Northern Line running tunnels. They were chosen to avoid spreading loads into the surrounding ground, possibly affecting the Northern line tunnels.
Now the team is facing the challenge of navigating the tunnel through these piles without affecting the building above.
Without a doubt sinking the shaft – just the start of the shaft – was the hardest part
Initially the team attempted to align the tunnel to miss as many of the piles as possible, but this would have required a tight curve on the track. Line speed would have had to be slower, affecting plans to increase Northern line services.
“There’s a balance between the short-term risk around working on that building’s piles, versus the long-term train speed on the Northern line,” says Swift.
“It’s all about track alignment, and you can’t have sharp curves because that slows the trains down, therefore you’ll never be able to get your 32 trains per hour on the Northern line.”
Instead a complex and unique engineering solution was required. The tunnel will crunch through four of the end-bearing piles, but will be used as a transfer structure to carry the building’s loads through the tunnel.
To make this work, the normal running tunnel diameter of 4m will be enlarged to 6m at this point. The space will be filled with extra-large reinforced concrete ring beams to provide additional strength.
It will not be straightforward.
“That is quite a complex piece of work as we try to mine through the piles, and then break the piles out and support them, to make sure that we maintain the loading capacity of the building above us,” says Swift.
Tackling the piles
The piles will be tackled one at a time to avoid leaving that part of the building unsupported for too long, as this could lead to movement and damage to the structure.
While detailed modelling has been carried out to give the team the best idea of how to approach the piles, there are a number of unknowns – such as the exact location of the piles – which will be dealt with as work progresses.
“The trouble is, because it’s never been done before, you’ve got no actual data to compare it to,” explains Swift.
It means, he adds, that you cannot say, ‘well we know if we do this, that happens’, so we’re modelling it very theoretically.”
Swift says contingency plans are in place in case things do not go to plan.
“If we find when we do the cut it’s not behaving as we thought, we would then stop, rethink what we’re doing, and then we’d take a very different approach.”
By the end of this year 90% of the tunnelling will be complete. The last 10%, to link the running tunnel to the rest of the southbound line will be carried out by hand during a five-month blockade of the Northern line in 2020.
Cannon Street station entrance
Robert Bird Group was engaged by Dragados to develop the Bank Station structural works from concept stage through to detailed design and on to the construction phase. A key early design challenge for Robert Bird was an efficient construction sequence for the complex Cannon Street station entrance structure.
Bscu dra dpe n133 z cm w 4000
The structural scheme for the new station entrance involves a combination of raft slabs, piled shoring walls and vertical shafts cascading downwards from Cannon Street. The resulting piled “basement box” and tunnelled shafts will provide improved passenger access to the Northern line platforms and ultimately to the Docklands Light Railway 35m below street level.
Working under the Incentivised Contractor Engagement contract, Robert Bird collaborated closely with Dragados, tier two contractors Byrne Brothers and Keltbray, and other specialist design consultants to deliver a coordinated solution within this challenging area.
The baseline construction sequence inherited by Robert Bird sought to complete excavation as quickly as possible by limiting the amount of structure constructed during the first phases of the project.
Excavation was to be enabled through provision of temporary soil ramps and platforms to give haulage vehicles access to the constrained site. Excavators were to be positioned adjacent to the excavation and haulage vehicles would have had to enter the site backwards and reverse down ramps for loading. This approach resulted in a significant amount of excavation and construction being deferred until late in the programme. In particular, removing the temporary soil ramps and platforms to enable construction of the permanent structure would only have been possible after most of the excavation work was complete.
After interrogating the construction programme alongside Dragados and its sub-contractors, Robert Bird identified opportunities to accelerate the construction of the site entrance structure to ground level early in the programme.
The ground level slab and associated vertical and foundation structures were redesigned to accommodate construction vehicles and to act as a ground level working platform. A temporary platform was designed to extend the ground level platform over the future escalator void. This prefabricated steel platform extension allows a long reach hydraulic excavator to be positioned directly over the excavation.
A swept path analysis of the excavator bucket was performed to optimise the location of the platform and temporary shoring props below, within the excavation.
Using this revised construction sequencing strategy will allow haulage vehicles to enter the site, receive loads from the excavator and exit the site with minimal reversing.
Consequently, the revised sequence will improve excavation efficiencies, and will improve site safety by minimising complicated vehicle movements. The elimination of soil ramps and platforms also helps limit the spread of muck by construction vehicles and avoids double handling of materials. The construction of the ground floor and working platform are currently progressing well on site.