How Are Tunnels Dug? Boring Machines, Blasting and the Channel Tunnel
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Every tunnel has two problems, getting the ground out of the way and stopping the ground that remains from falling in, and the history of tunnelling is the history of solving the second. The Romans drove aqueduct tunnels through hills by lighting fires against the rock face and quenching it with water to crack it; the Victorians dug under London with a cast-iron shield pushed forward by screws; and the machines that drove the Channel Tunnel in the 1990s cut, lined and advanced through the chalk at 75 metres a day without anyone touching the rock. The method depends on what the ground is, and the ground is the whole difficulty.
Three ways to cut
The choice is set by the ground and the length:
- •Drill and blast: holes are drilled into the rock face in a pattern, charged with explosive, fired in sequence from the centre outward, and the broken rock is mucked out before the cycle repeats; it is the method for hard rock and short or irregular tunnels, and advances a few metres per round
- •Tunnel boring machines: a rotating cutting head the full diameter of the tunnel, fitted with disc cutters for rock or picks for soil, grinds the face while the machine shoves forward on hydraulic rams, conveys the spoil back and, in soft ground, erects a lining of concrete segments behind itself as it goes; slow to set up and fast thereafter, and the method for anything long
- •Cut and cover: a trench dug from the surface, the tunnel built in it and the ground replaced, which is how the first underground railways were made and how shallow ones still are
- •Shield tunnelling in soft ground: a steel cylinder holds the ground while miners or a machine excavate inside it, the lining is built within the shield, and the shield is jacked forward off the lining, the method Marc Brunel invented under the Thames in 1825
Holding the ground
Rock that stands on its own needs only rock bolts and a spray of concrete, shotcrete, to stop loose pieces falling, and the New Austrian Tunnelling Method of the 1960s made a science of letting the rock carry itself with a thin, flexible lining applied as it deforms. Soft ground and water are the enemy. A tunnel below the water table in sand or silt will flood and collapse unless the face is held, and modern machines hold it with pressure: an earth pressure balance machine keeps the excavated soil in its chamber as a plug at the same pressure as the ground outside, and a slurry machine pumps bentonite mud against the face. Compressed air, which the Victorians used, kept the water out and gave the miners the bends. The lining, bolted rings of precast concrete, is the tunnel's permanent strength and goes in a metre behind the cutting head.
Knowing where you are
A tunnel driven from both ends must meet, and surveying underground with no view of the sky was the tunneller's oldest headache; the Roman engineer who cut the Saldae aqueduct in Algeria in 152 AD left an inscription complaining that the two gangs had nearly missed each other. Modern drives use laser guidance, gyroscopes and a chain of surveyed stations back to the portal, and the Channel Tunnel's two ends, driven from Folkestone and from Sangatte, met in December 1990 with a horizontal error of 358 millimetres after 38 kilometres. Ground movement is watched from above by instruments on every building along the route, since a tunnel beneath a city draws the ground down slightly as it passes, and the settlement must be predicted and, where necessary, countered by injecting grout.
The Channel Tunnel
The tunnel under the Strait of Dover was proposed in 1802, begun and abandoned in 1880 when Britain took fright at invasion, and finally dug between 1988 and 1991 by eleven boring machines from the two shores. It is three tunnels, two for trains and a service tunnel between them, 50 kilometres long of which 38 are under the sea, at an average of 40 metres below the seabed, and it follows a single layer of chalk marl, soft enough to cut fast and impermeable enough to keep the sea out, which the geologists had mapped by boreholes and which the machines steered along by watching for the harder chalk above and the wet greensand below. The British machines were driven into the ground and buried when they had finished; the French ones were driven back. It opened in 1994 at a cost of about twice the estimate, and it carries 20 million passengers a year.
The longest and the deepest
The Gotthard Base Tunnel through the Swiss Alps, opened in 2016, is the longest railway tunnel at 57 kilometres, driven through 2,300 metres of rock overhead where the temperature reached 45 degrees; the Seikan Tunnel between Japan's main islands is 54 kilometres and runs 240 metres below sea level; and London's Crossrail put 42 kilometres of new tunnel through a city's foundations, threading between existing lines, sewers and the piles of buildings with a clearance in places of under a metre. Tunnels are now proposed under the Baltic, the Bering Strait and the Strait of Gibraltar, and the machines that would dig them are the descendants of a shield that a French emigre engineer copied, in 1818, from the way a shipworm bores through timber, lining its tunnel behind it as it goes.
The takeaway
Tunnels are dug by drilling and blasting hard rock, by boring machines whose rotating heads cut the face and erect a concrete lining behind them, or by cut and cover from the surface, and the governing problem is holding the ground and the water back, which soft-ground machines do by pressurising the face. Guided by lasers and gyroscopes to meet within centimetres, the method drove the Channel Tunnel along a single layer of chalk marl under the sea and the Gotthard through 57 kilometres of Alps.