The earth under London is composed of a bed of clay, therefore making tunneling much easier. It is one of the most tunneled cities in the world, with tunnels for road vehicles, pedestrians, the Tube and railway lines and utilities. Several tunnels are over a century old.
There is 150km of tube tunnel of which 32km is cut and cover tunnel. The other 55% of the system runs on the surface.
SUB SURFACE NETWORK
The Circle, District, Hammersmith & City, and Metropolitan lines are services that are on the sub-surface network. The railway tunnels are just below the surface and built mostly using the “cut and cover” method. The trains are similar in size to those on the British main above ground lines.
DEEP-LEVEL TUBE LINES
The Bakerloo, Central, Jubilee, Northern, Piccadilly, Victoria and Waterloo & City lines are deep-level tubes; they have smaller trains that run in two circular tunnels. Many of the central stations on the deep-tube lines are higher than the running lines to assist deceleration when arriving and acceleration when departing (Central and Piccadilly lines). The deep tunnels have a diameter of about 3.59m and are lined with cast-iron or precast concrete rungs. They are bored using a tunneling shield.
METHODS OF CONSTRUCTION