Contractors on Crossrail’s western running tunnels are to get their first sight of the cutting face of one of its tunnel boring machines since its launch almost a year ago. Claire Symes visited Bond Street to find out more.
The first Crossrail tunnel boring machine (TBM) passed within metres of the southern side of the Bond Street station’s western ticket hall at the end of March after the machine had achieved a record tunnelling rate of 217m per week.
Nonetheless, the joint venture project team of Bam Nuttall, Ferrovial and Kier (BFK) are only just about to its first glimpse of the cutter head of the TBM - named Phyllis - since it was launched from Royal Oak in May last year.
BFK has been constructing sprayed concrete lined (SCL) tunnels out from the bottom level of the Eastern ticket hall box and it is into one of these platform tunnels that Phyllis will break through.
The second TBM - named Ada - was launched in August last year and is just passing to the north of the station’s western ticket hall. It is due to cut into the contiguous piled box of the eastern ticket hall in late April.
BFK site agent Matthew Mule told GE that the current plan was to backfill the excavated shaft in the eastern ticket hall with foamed concrete so Ada could tunnel through it and then on towards Farringdon.
But while the two TBMs will soon be en route to Tottenham Court Road and due to reach Farringdon station by the end of this year, there is still much work to be done at Bond Street.
Once the two TBMs have passed, work to excavate the ticket halls will continue. But construction of the Bond Street station platform tunnels will not start until after the TBMs have passed Farringdon station, the station to the east of Bond Street. Only when the TBM gantries and trains have been removed can work start on enlarging the platform tunnels using SCL techniques.
Bond Street platform construction
Crossrail’s platforms at Bond Street station will be 250m long and will allow passengers to connect to the western and eastern ticket halls, as well as Bond Street’s Underground station that is also undergoing a upgrade programme. The tube station currently handles 155,000 passengers but this is set to grow to 225,000 per day when Crossrail services start in 2018.
Mule said platform construction would start early next year once the TBMs have reached Farringdon and all the gantries and back up trains have been removed. “Once the area is clear, we will enlarge the TBM tunnels over the length of the platform using SCL,” he said.
Eastern ticket hall
Work on the eastern ticket hall is slightly eased by the ability to use Hanover Square for vehicle movements but the site is bounded by offices which are still in daily use, and construction is currently operating 24 hours a day.
The perimeters of the eastern ticket hall and temporary shafts are formed by 462 secant piles constructed by Cementation Skanska last year.
BFK’s excavation at the site has already been extended to the full depth in the 25m diameter shafts and work is underway on the SCL tunnel that will form the cross passages and will be where the first TBM will break through.
“Once the TBMs have passed we will have the site over to CSJV later this year to break out the temporary shafts and slabs to allow the site to be fully excavated and the permanent floor slabs cast,” says Mule.
Construction here will also continue beyond the Crossrail work as the site will be handed over to Portland Estates in 2018 for work to begin on an over site development.
Western ticket hall
Like many of Crossrail’s construction sites, space at the western ticket hall at Bond Street is limited. The development sits on the former site of the University of Arts building, which was demolished at the end of 2010.
“Demolition revealed some substantial 2m by 2m pad foundations that were not on any of the plans and had to be broken out before we could start on the diaphragm walling,” says Crossrail project manager Chris Pateman.
Diaphragm walls, constructed to 50m below ground level, to form the outer wall of the sub surface ticket hall were completed by Cementation Skanska last summer.
The site is tightly constrained by buildings at the surface, and there are also underground structures to deal with - the diaphragm walling work went to within 3m of the Jubilee Line tunnels which pass close to the Crossrail route.
The Cementation Skanska joint venture (CSJV) is now working on building the structure using top down techniques.
CSJV works manager Martin Redsell says: “Excavation of the five levels [to 26m below ground level] is expected to be completed by the end of the year, then the site will be handed over to BFK for 10 months for the fit out stage.”
Floor slabs will be cast in three sections but the pours have been further split into 200 to 300m3 stages to ease the logistics.
Although space is at a premium, ground conditions are good. “We had 4m to 5m of made ground at the top but the rest of the construction is within the London Clay,” says Crossrail engineering manager for Bond Street Hugo Axel-Berg.
“The base slab will be tied into a series of 1,200mm tension piles that extend to 30m below the bottom level but were cast from the surface by Cementation before excavation started. The reinforcement starts at the base slab level and the column above is formed from a weak cement mix. Bearing piles, with a diameter of 2.4m, were also formed to a similar depth in the same way.”
In 2018 the site will be handed back to Grosvenor Estates which will build an over site development on top of the station.