The project to upgrade Bond Street Underground station so it can cope with extra passengers from Crossrail is a hugely complex logistical and engineering challenge. Katherine Smale finds out more about the work beneath London’s busy Oxford Street.
Upgrading Bond Street Underground station has been no mean feat. When looking at the 3D model, you could be forgiven for confusing the network of tunnels under the station for an extremely complex piece of plumbing work.
The £320M project is necessary to increase the capacity of the station by 45% to accommodate forecast growth in passenger numbers and the influx of people interchanging with the new Crossrail station a few streets away. To do this, the station is undergoing a radical transformation. A new ticket hall is being built on the north side of Oxford Street, new escalators will connect to the Central and the Jubilee lines and step free access from street level down to the Jubilee and Central Lines will also be created. There will also, of course, be a connection to the new Crossrail station.
The logistics for the site are truly complex. There is only one entrance and exit for materials on nearby Marylebone Lane; busy Oxford Street splits the entrances between the existing ticket hall and the new ticket hall construction. There is also a five star hotel adjacent to the new ticket hall. As a result, the team have had to tread carefully to ensure the smooth running of the project.
The team approached these problems head on. The new six storey building which will house the new ticket hall has been fitted out with cladding which attenuates the noise of the construction work below. At night, a roller shutter door seals the entry and exit point allowing 24 hour working while not disturbing the sleeping hotel guests next door.
Within the new ticket hall building, the first and second floors have been left out to create a crane hall to lift all of the excavated material out and lift in the construction equipment and materials.
“There are two 17.5t gantry cranes and it’s more like a factory environment than a building site,” explains London Underground project manager Richard Watts. “Once we’ve finished tunnelling, we’ll remove the cranes. The [colour coded red structure] is temporary steelwork, and this will be taken out and we’ll put in the first and second floors.”
Underground, the new network of tunnels and the large open space for the ticket hall have had to thread their way through an extremely complex, spaghetti junction of existing tunnels (see diagram). Central Line and Jubilee Line tunnels, the Post Office mail rail tunnel, and a Victorian Bazalgette sewer all had to be avoided, not to mention the linking passageways and service tunnels. To allow construction of the new routes, the tunnels are all of varying diameter.
“The reason this is the most complex spayed concrete lining [SCL], tunnelling project in the UK is that none of the tunnels are the same size,” says Transport for London project manager Andy Grant. “They are constantly changing in diameter so it’s a constant engineering challenge.
“A bigger diameter tunnel cannot be built from a smaller [tunnel] therefore if you’re going to build one tunnel and then dig out of it, you have to make the first tunnel bigger than it probably needs to be to take the equipment out of it.”
The tunnels themselves are excavated with a variety of 360 degree excavators, bespoke tunnelling equipment and modified conventional excavators. It was then lined with a sacrificial sprayed concrete lining which contains 38mm long steel fibres.
A smoothing coat which protects the waterproofing layer from being punctured by the steel fibres is then applied, on top of which is the waterproofing layer. A permanent structural layer of sprayed concrete varying from 250mm to 500mm thick is then applied. Two hundred and fifty tonnes of dry concrete is stored in large silos on site and then mixed with water when needed.
“Tunnels are formed by digging a bit and then spraying it with concrete, those increments are usually about a metre,” says Watts. “The face also has to be sprayed with 75mm thick of concrete to stop it drying to maintain the moisture content of the clay.”
Most of the tunnels were excavated at a rate of 1m per 12 hour shift in the sprayed concrete lining tunnels. The larger diameter tunnels were dug in two or three stages, so the rate of progress varied accordingly.Passengers entering the station from the existing Bond Street escalators, currently encounter a blue hording at the end of the corridor.
Once this is removed the corridor will open up to form the central hub and route down to the Jubilee Line. From here, more escalators take passengers down to the platforms.
Nothing is ever that simple though and this new connecting tunnel has to dip under the famous Royal Mail railway tunnel above, and then subsequently rise up again to cross above one of the Jubilee Line tunnels.
To the south of the existing Bond Street ticket hall is the new Crossrail station which the new works will link into.
The connecting interchange between the two stations has to thread its way through the most congested part of the underground labyrinth, feeding under the old Victorian Bazalgette sewer and over the Central Line tunnels.
Due to space restraints the tunnel cross section has to be square, not circular as it is elsewhere. As such, more traditional mining techniques are being employed to build it. Horizontal piles were installed under the sewer tunnels to strengthen the ground, and the tunnels have been painstakingly dug underneath using spades and small pneumatic drills.
Steel frames to support the excavation are installed at around 500mm centres with timber boards spanning in between.
Work on this tunnel was slower, averaging at around 1m per week.
Step free access tunnels have also been constructed in square cross section.
“Trains kept running,” says Watts. “We had to monitor the track to make sure the track didn’t dip and derail the trains, but we didn’t have any problems.”
Escalators also pose a challenge to install. Space is at a premium and so the team has to lift them into place in 4.5m long sections.
“We need to slide them down, and then down the incline 30m, we need to lift the escalators up and then level them accurately,” says London Underground premises manager Andy Koniotes. “So it’s working in very tight spaces with equipment which has to be installed to within 1mm.”
Where the new openings into the existing tunnel linings have been made, “picture frame” supports have been constructed around the aperture to take the loads and to stop the existing tunnels collapsing. When breaking through to one of the existing tunnels, it was found to be 150mm higher than anticipated and so adjustments had to be made to the frame to accommodate this. Despite all of the challenges the construction team has been working 24 hours a day, seven days a week with 90 workers on site for each shift. The project is on budget and expected to finish on time by 2017.