While historically the Underground’s tunnels were constructed through primitive manual labour, today huge 1,000-ton, 150-metre (490-foot)-long tunnel-boring machines (TBMs) do most of the hard graft. Each machine has a rotating cutterhead at the front and a long series of trailers behind that hold all the mechanical and electrical equipment. The cutterhead rotates at approximately three revolutions per minute, carving set diameter tubes out of the clay, sand and gravel situated under much of London.
Interestingly, unlike other tunnel-building projects through more rural terrain, in London all manner of obstacles need to be taken into consideration and bypassed. These include other Underground lines, utility supplies, sewer systems and even hidden subterranean rivers that run down to depths of 40 metres (130 feet) below the surface. For this reason, extensive geophysical maps of the project area are created both before and during an excavation.
Once the Tube tunnel is built, it is lined with concrete and then installed with the electrical cabling and support systems that will power the line’s lights and ventilation systems.