For the last few years, several 1,000 tonne, 150m long drilling machines have been boring away beneath London. More than 155 years since the first trains – steam locomotives with rickety wooden carriages – started trundling along the Metropolitan Railway (the first line of what would eventually become the infamous London Underground), the Crossrail line will open in 2018 with 42km of new running tunnels, stretching from Essex and East London, all the way across the city to Heathrow and Reading in the west.
Seven million tonnes of earth has been excavated from underneath an already crowded city, 98 per cent of which has been used to create new areas of agricultural land, nature reserves and recreational facilities in the South East of the UK, including a new 1,500 acre RSPB nature reserve in Wallasea Island, Essex. Over 2000 years of history has also been uncovered during the dig, including Roman and post-medieval structures and burial grounds.
One key element of the Crossrail project is the Thames Tunnel between Woolwich (south of the river) and Custom House (north) in Southeast London. This pair of tunnels (one going each way) is the only section of Crossrail which crosses under the river Thames, and each one is almost 2 miles (3km) long – 460m of which is under the river itself. A pair of bespoke giant boring machines – named Mary and Sophia, after the wives of Isambard and Marc Brunel, constructors of London's first Thames tunnel – each spent a year tunnelling through the clay, sand and chalk which comprises the geological base of the city.
At it's deepest point the tunnel sits 15m below the bed of the river Thames. However, at it's closest point it runs only 2m above the tunnel carrying the DLR (Docklands Light Railway), which also runs underground as it crosses the river in this part of the city. The whole tunnel is well below the water table, therefore the chalky soil around the tunnel is fully saturated. This means that once a month a pump has to be activated in order to remove any water which has leaked into the tunnels (not something to be concerned by, Geographical was assured!)
Walking down the completed section is an unusually light and breezy experience. Relatively recent innovations in tunnel-building means the walls are smooth, consisting of ring after ring of eight concrete blocks, fitted together like a giant circular jigsaw in the wake of the enormous drilling machine. These two tunnels combined now contain 3400 concrete rings, with each ring containing one known as the 'key', which is the last one fitted, and holds the whole ring in place. It's unusual wedge-shape means the direction of travel of the blocks can be subtly altered with each ring, enabling the tunnel to go straight forward, or bend to either the right or left, depending on which direction the tunnel is being constructed.
When the concrete blocks are molded, it is impossible to remove all the water used in the production process 100 per cent. Everything used to construct a modern tunnel is non-flamable, however when an intense fire inside the Channel Tunnel caused water in the concrete to heat and turn into steam, it expanded and broke apart blocks in the near vicinity of the fire. So the Crossrail blocks contain microfibres designed to melt under intense heat, which would allow the steam to escape without destroying the blocks themselves.
The tunnels will now be fitted with features such as train tracks and permanent lighting, before then enduring 18 months of rigorous testing prior to a grand opening in late 2018. Once operational, Crossrail will carry over 200 million passengers annually, adding an extra 10 per cent capacity to London's already substantial rail capacity.
This article was published in the August 2015 edition of Geographical Magazine