Since the dawn of civilization, people have been building tunnels for accessing tombs or underground quarries, or in the hill slopes for allowing the flow of water from porous rocks. Romans were skilled tunnel builders, who made several kilometers long underground passages using the work of slaves. They made a 5.6 km (3.5 mi) long tunnel for draining the Fucino Lake, east of Rome. The project, aimed for impeding the flooding of the area, required the work of 30,000 people for 10 years.
Modern construction of tunnels started in the 1760's in England, when channels were constructed for inner transport. If hills were in the way, the top could be overcome by modifying it in platforms and building of locks, so that ships reached the next level. This method depended on a good alimentation with water to the top, as water descended through the system each time locks were used. That's why the first channels tended to eschew hills, making the journeys very long and therefore the technique of building tunnels through the versants emerged.
In 1825, the opening of the Stockton Rail Way and Darlington, England, founded a new era in transports. The knowledge got through making the channel tunnels was soon applied in digging railway tunnels. By then, most of the work was done manually, and the dug material was removed using horses. For constructing long tunnels, wells were made along the route, so that work could be started in several places, reducing the time for ending the project.
One of the first under river tunnels was built beneath the Thames River (London): the Rotherhithe Tunnel, connecting Rotherhithe and Wapping. Digging started in 1825, but a great flow of water and mud delayed the ending of the job until 1843.
The first idea of a tunnel under the English Channel was advanced by the French engineer Albert Mathieu in 1802, but work started as late as the end of 1870's, using a machine for excavating tunnels. But the work was stopped in 1882, as the British feared a French invasion through the tunnel. Excavations restarted in 1973, but were stopped by 1975 due to a rapid increase of the costs. The work was resumed by the end of 1980's and finished in 1994. Now the EuroTunnel ensures a link between France and England in both directions through railway and vehicles.
Before building a tunnel, detailed ground analyses and probe drills are made. Past movements of the ground could have perturbed and crashed rocks even in regulated formations. Rock crevices not only make the digging more difficult, but the tunnel can experience sudden and incontrollable infusions of water during the construction, especially if it's deep or built under water.
Sometimes, small exploration tunnels are made near the main route of the tunnel, so that the rock layers can be checked and investigated, but currently, the builders use to drill just a little further from the main front of the tunnel for testing eventual signs of possible problems.
Small tunnels are excavated generally using tunnel digging machines with a simple rotative head. Larger tunnels are carved using a road header, having a cutting rotative head on a spire with hydraulic command, which allows it to reach all the parts of the tunnel's front.
Tunnels dug in stone are excavated through perforation and dynamiting. The roof of the tunnel can be left unsustained for a while, but the tension readjusting amongst small rocks can cause stone collapses, that's why bolstering is ensured soon after excavation. A common method is the use of steel vaults sheathed with wood. Instead, reinforced concrete can be used, or concrete can be pulverized on the rocks. Some rock formations are so strong (that) they do not require bolstering.
Tunnels dug in soft ground require bolstering as close as possible to their front, to avoid collapse. One technique is to advance 60 cm (2 ft) with the front, and then place a sustentation ring made of cast iron or concrete. Alternatively, moisturized concrete can be applied, and temporary steel bolsters can be fixed until hardening.
The method of the immersed tube is used especially in the case of undersea tunnels. On the surface, tunnel portions are made and then sunk into a dredge groove. Articulations between the sections are then tightened to make them resistant to water and then the construction is covered by sand. Then, water is pumped out of the tunnel so that inner works can start. This method was employed for example in the case of the tunnel connecting Hong Kong to Kowloon (mainland China).
The world's longest tunnel (54 km or 32 mi) connects the Japanese islands Honshu and Hokkaido, under the Tsugaru Strait. It consists in a larger railway tunnel and two smaller road tunnels, also employed for maintenance, draining and ventilation.
The EuroTunnel (under the English Channel) is 50 km (30 mi) long and each of its twin tunnels are 7.6 m (25 ft) wide. It was built with an average speed of 12 cm (5 in) per minute.