How Are Tunnels Dug? Boring Machines, Blasting and the Channel Tunnel

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 patented in 1818 and first used under the Thames in 1825

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.

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 tunnel under the Strait of Dover was proposed in 1802, begun in 1880 and abandoned in 1882 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 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.

A tunnel produces an enormous volume of broken rock and soil, called spoil, and getting rid of it is a planning problem in its own right. Every cubic metre removed from the ground swells when it is dug, so the heap is bigger than the hole. On big projects the spoil is increasingly treated as a resource. Chalk marl from the British side of the Channel Tunnel was placed at the foot of Shakespeare Cliff near Dover, creating a new stretch of land now called Samphire Hoe, and millions of tonnes of clay from London's Crossrail were shipped to Wallasea Island in Essex to build a coastal wetland for birds.

Once a tunnel is open, the air inside becomes the next concern. Trains and vehicles push air ahead of them like a piston, and road tunnels fill with exhaust, so long tunnels need ventilation shafts, large fans or both. Fire is the greatest danger, because smoke in a confined tube spreads faster than people can walk. Serious tunnel fires in the Alps around the turn of the century led to stricter European rules on escape routes, fire detection and smoke control. This is part of why the Channel Tunnel has its central service tunnel, kept at a higher air pressure so that smoke is pushed away from people sheltering there.

The first tunnel known to have been driven successfully beneath a navigable river was Marc Brunel's Thames Tunnel between Rotherhithe and Wapping, and its story shows why soft ground was so feared. Work began in 1825 with his rectangular iron shield, in which miners worked in separate cells, removing the face a few boards at a time. The river bed above was thinner and looser than expected, and the Thames broke in more than once. In 1828 a flood killed six men and nearly drowned his son, Isambard Kingdom Brunel, who was serving as resident engineer.

Money ran out and the tunnel was bricked up for seven years before a government loan allowed work to resume with an improved shield. It opened in 1843, and because the planned ramps for carriages were never built it served only pedestrians, drawing crowds of sightseers and stalls selling souvenirs before it declined. From 1869 trains ran through it, and it still carries passenger trains today as part of London's rail network. Eighteen years for a tunnel of under 400 metres is a measure of how much the later pressurised and mechanised methods changed the work.

Tunnel boring machines are traditionally given female names before they start digging, a custom often linked to St Barbara, the patron saint of miners and tunnellers. Workers in many countries place a small statue or image of St Barbara at the tunnel entrance, and the machine is not supposed to begin work until it has been named.

London's Crossrail project, now the Elizabeth line, used eight machines named after notable women connected with London, including Ada, after the mathematician Ada Lovelace, and Phyllis, after Phyllis Pearsall, who created the London A to Z street atlas. Naming a giant machine is a small human touch on an enormous engineering project, and the names are often chosen by public vote or by local schools.

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.