Three years seems like an awful long time to complete a marathon. But it’s a pretty impressive feat when you take into account that this is a 26-mile (42km) journey through solid earth, weaving between pipes, sewers, utility tunnels, building foundations and Tube lines, with the added difficulty of weighing around 1,000 tonnes.
Now, eight monster Tunnel Boring Machines (TBMs) have finished their marathon relay across London - from Victoria Dock portal in the east to Royal Oak portal in the west - constructing brand new Crossrail tunnels.
At depths of up to 42 metres underground, it was no easy task. Teams of engineers worked round the clock to complete the tunnelling on schedule, amounting to nearly 70 million work hours so far. And work on the overall project is still only 65% complete, although completing the tunnelling has to be the major milestone.
At about 0530 on May 23 - almost exactly three years after tunnelling began - TBM Victoria broke into Farringdon Crossrail station. It then spent the final three days of the project (up to May 26) constructing the remaining section of tunnel and linking all the Crossrail tunnels together for the first time.
After a few days of checking everything was sound, it was time to officially mark the occasion. Prime Minister David Cameron and London Mayor Boris Johnson descended the 40 metres under Farringdon to congratulate the engineers on the achievement (see panel, page 52).
What they were celebrating was a mammoth engineering project - the Crossrail tunnelling was a complex task to construct 6.2m diameter tunnels. More than 200,000 concrete tunnel segments were used, each weighing 3.4 tonnes. Seven segments and a keystone slotted together to form one complete tunnel ring.
And nearly all were locally made - the 75,000 segments for the western tunnels (Westbourne Park-Farringdon) were made at Old Oak Common in west London, and the 110,000 segments for the eastern tunnels (Docklands-Farringdon) were made in Chatham (Kent). The only ones from slightly further afield were those for the new Thames Tunnel (Plumstead-North Woolwich), which were made in Ireland.
The 110,000 segments from Chatham for the eastern tunnels were moved to their destination in 260 barge movements, saving 10,000 lorry trips.
But the undoubted stars of the show were the affectionately named TBMs (see panel, page 52). The industry has been charting their progress under London for the past three years, but now that chapter is closed, what will happen to the eight leading ladies?
Phyllis and Ada were dismantled after their tunnelling journeys were completed. Their 130m-long trailer systems were removed from the tunnels via the Fisher Street shaft, leaving only the front ‘can’ and cutterhead 30m below the ground in Farringdon.
Victoria and Elizabeth will also be dismantled. Their trailers will be removed from the tunnel via the Stepney Green shaft and returned to their manufacturer in Germany, where some parts will be recycled for future tunnelling projects.
The cutterheads will be cut into small pieces and removed via the Farringdon shaft, and the front ‘cans’ will be left in the tunnel, where Crossrail trains will pass through them when they begin running in 2018.
Mary, Sophia, Jessica and Ellie were dismantled and also returned to Germany, where parts will be re-used.
And it’s not just parts of the TBMs that are being recycled. So far 3.4 million tonnes of spoil have been excavated from the tunnels (that figure will grow to seven million by the end of the project). Three million tonnes of it has been shipped to Wallasea Island in Essex, to create a new wetland nature reserve twice the size of the City of London (RAIL 769).
But while this is a massive achievement to celebrate, it is by no means the end of Crossrail’s hard work. Engineers are still working round the clock, and work is continuing apace to get the stations and fit-out work within the tunnels ready for the 2018 start of services.
Let a new marathon commence - onwards to the finish line!
- This feature was published in RAIL 777 on June 24 2015