Tunnelling has begun for a new southbound Northern line tunnel as part of Transport for London’s £607M Bank station capacity upgrade.
Bank is Transport for London’s biggest Tube station, serving around 100,000 people over three peak hours each morning. Measures to regulate the flow of passengers, 40% of which change trains at the station, are often required. These measures affect journey times at other stations, spreading delays.
The station is getting a £607M upgrade to boost capacity by 40% when the work finishes in 2021 to cater for an increase in demand of more than 50% in the last 10 years and for further increases in London’s population. At the moment, one line in particular is causing capacity problems.
“Where we’re got issues on the station is mainly around the Northern line,” said programme delivery manager Andy Swift.
“We get big capacity issues on the platforms with people pouring into the ends, and then we can’t get them onto the trains and out quick enough.”
Design and build contractor Dragados is using a mechanised excavator to construct a new section of Northern line southbound tunnel as part of the upgrade. An existing tunnel will become part of a new central concourse between the north and southbound platforms.
A tunnel boring machine (TBM) was discounted for the job. At around 600m, the drive is not long enough for a TBM and the upgrade area is surrounded by 31 listed buildings, meaning there are too many pre-existing foundations to create an easy path.
Instead, a rotating excavator is being used to construct the 4m to 8m diameter running tunnel. Sprayed concrete lining will be used for the 27m deep tunnel, which will run through London Clay.
All plant for the project will be lowered via a shaft at Arthur Street, near nearby Monument station, due to strict heritage protections encompassing the upgrade area. The shaft itself took a year to build as complex utility cables had to be rerouted.
New passenger tunnels between Northern line platforms will be hand dug due to the danger of exposing and damaging the existing tunnel linings.
“This is 100 year old infrastructure. We have to be very careful about how we work around that infrastructure,” said Swift.
After being hand-dug, the tunnels, which will average around 3m diameter, will be lined with concrete.
One of the main challenges in tunnelling for the southbound Northern line tunnel will be intersecting piles for existing buildings, many of which are listed. Although tunnelling has just started, at a later stage the team expects to encounter load-bearing piles in the densely populated ground.
When that occurs, the tunnel lining will be thickened to provide extra support.
At its peak the upgrade will have 250 people on the job. The Northern line is also being extended to the Nine Elms development on London’s South Bank.
Tunnelling is set to last 18 months, finishing at the end of 2019.