The £14.8 billion Crossrail project involves a phenomenal amount of tunnelling work, as might be expected of a new rail link crossing a major metropolis like London. The statistics are worth revisiting. The project includes 21 km of twin 6.2 metre internal diameter tunnels and 37 stations, of which eight will be below ground. In addition there will be a new station at Custom House, separately funded and constructed but still integral to the completed railway. There are over 8,000 people employed on 40 worksites, and the project is providing apprenticeships to around 400 employees.
Tunnelling under London
Eight tunnel boring machines (TBMs) will have been used by the time the construction of the project is finished in 2018. Each one weighs around 1,000 tonnes and is 148 metres in length. Their cutter heads are 7.1 metres in diameter and they are expected to average 100 metres/week (though the first two in operation have both achieved twice this rate) working pretty much 24/7. On each shift, these monsters are manned by sixteen people on the TBM itself and eight in the tunnel behind.
At the expected average rate of progress, each TBM produces around 7,500 tonnes of spoil a week while moving forward in 1.6 metre increments – the width of the eight-segment tunnel rings being installed behind them. These segments are manufactured locally from 300mm-thick fibre-reinforced concrete and each ring weighs 24 tonnes.
As is to be expected, the progress of each TBM is monitored by a computer-controlled guidance system using lasers. Their position and direction is controlled very accurately using the four pairs of hydraulic articulation rams incorporated into the front body of each machine behind the cutterhead.
Some six million tonnes of spoil will be generated by the tunnelling, and 85% of this will be removed by rail and water after it emerges from the tunnels, so avoiding an estimated 270,000 lorry movements. At least three quarters of this material will be deposited at Wallasea Island in an arrangement which is enabling the RSPB to create Europe’s largest man-made coastal nature reserve.
The western tunnels are being driven by TBMs ‘Phyllis’ and ‘Ada’. The former began work at the Old Oak Portal in May 2012 and ‘Ada’ in August that year. Their target is Farringdon, and en route they will have passed through Paddington, Bond Street (of which more later) and Tottenham Court Road stations.
The eastern tunnels were begun in December 2012 on the Limmo Peninsula with TBMs ‘Elizabeth’ and ‘Victoria’, also heading for Farringdon. These tunnels are to pass through Canary Wharf, Whitechapel and Liverpool Street stations. Later this year, two further TBMs will start the section of tunnels from Pudding Mill Lane to Stepney Green.
Finally there is the twin-bore Thames tunnel running from the Plumstead portal through a station at Woolwich Arsenal and under the river to the portal at North Woolwich. These TBMs, ‘Sophia’ and ‘Mary’, differ from the others as they are slurry machines. This is because of the particular ground and ground water conditions expected along their drives. ‘Sophia’ began work in January 2013, and ‘Maria’ will follow later this summer.
Bond Street complex
In March 2013, The Rail Engineer was invited to visit the Bond Street Station construction works. This is a major project in its own right. The new Crossrail platforms will be 250 metres in length to accommodate the 10 car trains planned for the line. Two accesses (ticket halls) are being built, one at each end of the intended platforms, and will provide step-free access from street level to the trains of the Jubilee and Central lines and to Crossrail itself.
The 1.2km of running tunnels in the overall complex at the station will be constructed by contractors BFK (Bam Ferrovial Kier joint venture) as part of its contract to deliver the western tunnels. All of the station works are being constructed by a Costain-Skanska joint venture (CSJV) with a phasing of operations between the tunnel and station contractors.
Each ticket hall will have constructed above it an over-site commercial development, the one at Hanover Square is to be some 300,000 square feet in total area.
When visited, the tunnels at the eastern ticket hall at Hanover Square were being constructed by the BFK consortium. The site will be later handed over to CSJV. Meanwhile, two access shafts had been driven 25 metres below ground, down to floor level -5 which is the full depth of the intended construction. Compensation grouting had been applied during the shaft sinking works in order to limit ground movement and disturbance to neighbouring buildings.
Work was well under way opening out the excavation at this level and lining it with steel-fibre-reinforced sprayed concrete to make a reception chamber in preparation for the expected arrival of TBM ‘Phyllis’ on its 6.8km drive. It subsequently passed through successfully during the last few days of March. ‘Ada’ was to pass through the site later, actually going through the base of the north-west access shaft that is also used to provide access to the worksite.
A curtain wall of secant piles had been previously installed by CSJV around the perimeter of this site and the station box was to be excavated within this perimeter. Five reinforced-concrete floor slabs, to be installed at the designed levels as this excavation proceeds, will prop the piled walls, providing the necessary restraint to them. This construction method was selected for the site due to the very close proximity of sensitive listed buildings on neighbouring sites. These buildings could not tolerate significant ground movement. Real time monitoring of these structures takes place to record the expected movement so as to ensure that agreed limits were not exceeded.
The station tunnels are being constructed under a 30 month contract which began in 2012, and work is taking place 24 hours a day, 7 days per week below ground. The remotely-controlled concrete spraying machine that was being used was a very impressive device, quite a contrast to the equipment that the writer encountered when using fibre-reinforced sprayed concrete to strengthen Arley Tunnel, on the Birmingham – Nuneaton railway, back in the 1970s.
Then, it was normal for the operative spraying the concrete to have to manipulate the spray nozzle manually, and blockages in the system, caused by the steel fibres clumping together, were a regular cause of delays and came with significant safety risks. There appeared to be no such difficulties with the current system. The ability of the operative to stand to one side of the robotically manipulated nozzle, freed of the physical struggle and able to better observe the work in progress, appeared to be a dramatic improvement, aiding the quality of work, the rate of production and the safety of the workforce.
Similar construction using sprayed concrete will be used in the construction of the passenger and service tunnels as they are driven for the new ticket hall once the two TBMs have both passed through this site.
The western ticket hall at Davies Street, at the other end of the planned platforms, was being constructed by CSJV using ‘top down’ construction methodology as the works come to within three metres of the existing Jubilee Line tunnels. The ticket hall is to have five levels reaching down to 25 metres below street level, and construction began with 1.2 metre thick diaphragm walls around the perimeter down to below the intended bottom floor and a substantial temporary steel deck at street level. The latter was needed because the ground floor slab of the final works was not adequate to carry the temporary loads of construction including such items as plant and the arisings from the excavation.
Work on the diaphragm walls commenced in November 2011 and was completed in June of last year. Top down construction of the station started in November 2012 following completion of the capping beam installation to the diaphragm walls. When The Rail Engineer was on site, the access from the street was down one of two ‘mole hole’ shafts, the other being used to bring up the excavated spoil from below. Internal bearing piles and plunge columns had been installed to their full depth within the perimeter of the site to provide the necessary additional support to the floor slabs, and at that time work was ten metres below ground at floor level -2.
At one end of the site, preparations were well advanced with the shuttering and steel fixing for the next floor slab. This was to be cast onto the carefully levelled and smoothed ground at the then base of the excavation. A separation membrane had been laid over the ground to ensure a clean soffit to the slab which would be exposed later by the excavation of the next level below. The reinforcement required was substantial, as might be imagined, and careful detailing was clearly an issue to ensure that the steel could be fixed and that concrete could be poured and compacted satisfactorily around it.
In contrast, at the opposite end of the site the excavation was still proceeding to complete the dig for the floor. The way that the site had been 1926 693000 divided to ensure safe separation between the excavation plant and the personnel working on the slab preparations was impressive. In addition,
a system called “my zone” was in operation to stop workers entering the unsafe area close to the excavators. This comprises a transmitter on each machine and receiver systems on the hard hats of each worker. Should an individual encroach too close to any machine, they received a warning by means of a vibrating alarm triggered by the system.
Work on the western site is subject to more restricted hours because of the possible effect upon neighbours, with the site starting work at 8.00 and closing down each evening at 18.00 on weekdays with permissions available to work until 10pm on occasions, and tighter restrictions at weekends.
The CSJV consortium is very proud of its health and safety system, and it appears that it has been very effective, having reached over 1 million man hours worked without any reportable accidents.
The new station is an integral part of the entire Crossrail project. Once complete, Bond Street will be only 26 minutes from Heathrow Airport and 15 from Stratford.