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?