Passageway through a mountain, under a body of water, or underground. Tunnelling is a significant branch of civil engineering in both mining and transport. The difficulties naturally increase with the size, length, and depth of tunnel, but with the mechanical appliances now available no serious limitations are imposed. In the 19th century there were two major advances: the use of compressed air within underwater tunnels to balance the external pressure of water; and the development of the tunnel shield to support the face and assist excavation. Granite or other hard rock presents little difficulty to modern power drills. In recent years there have been notable developments in linings (for example, concrete segments and steel liner plates), and in the use of rotary diggers and cutters and explosives.
Process An important preliminary operation is the survey work. The centre-line of the tunnel is ranged out on the surface and a series of shafts are sunk, from 100–300 m/330–985 ft apart along the line. To transfer this line underground, two marks are made in the cross-timbers, in the centre-line, at the bottom of each shaft and prolonged in both directions when the tunnel is being opened out. When the tunnels are of great length, such as those of the Alps, and can only be driven from both ends, the setting out is much more difficult. In this case the centre-line is determined by a triangulation survey, and ranged out from marked bases.
Small-section tunnels are usually driven from one end to the other at their full dimensions. Large-section tunnels are often driven in two stages; a pilot heading is excavated in advance which is afterwards enlarged to the full section of the main tunnel.
The normal procedure in tunnelling in rock is as follows. Power drills are used to bore holes in the face to take rounds. Each round is fired and the broken rock removed by hand shovels or mechanical loaders. The section is trimmed to its proper size by further blasting or by pneumatic picks, and timber or steel supports are erected. Sometimes side and top lagging boards are required. In loose ground the top laggings are driven in in advance of the last supporting set (fore-poling) before the debris is removed. In sand or gravel the problem is one of support rather than excavation, and fore-poling is necessary. The poling pieces are driven along the sides and top of the tunnel to protect the workers from sudden falls or ‘runs’ of ground.
Types of tunnelHard rock tunnelling In hard rock, blasting is necessary to break down the material. The blasting holes are usually from 2–3 m/6.5–10 ft long, but in some recent tunnels 5-m/16-ft lengths have been adopted. The explosive often used is 60% low-freezing gelatine and sometimes stronger, up to 80%. Liquid air or liquid oxygen has been used as an explosive, and has the advantage of leaving no blasting fumes. The cementation process has been employed successfully for dealing with water from rock fissures. The process consists of injecting liquid cement, at high pressure, through advance boreholes into the water-bearing fissures. The holes radiate outwards so as to intersect fissures on the outside of the tunnel area. After the cement has set and the water been sealed off the tunnel is excavated and lined in the ordinary way. A second length is then cemented, excavated, lined, and so on until the water-bearing deposit has been passed.
Delay-action firing This technique, recently introduced for tunnelling work, possesses many advantages over ordinary electric firing. The various shots in the tunnel face, constituting the complete round, explode either instantaneously or with a time lag of 2, 3, or 4 seconds according to their position in the rock face. Delay-action firing produces a large heap of well-broken rock which can be efficiently dealt with by power loaders. Ventilation is provided by a forcing fan delivering through air tubes to the tunnel face. The problem of dust suppression is given close attention on account of the danger of lung ailments, such as silicosis. Wet boring is done in practically all dry tunnels and the broken rock is copiously sprayed during loading.
Rings of reinforced and pre-stressed concrete have gradually superseded cast-iron rings as tunnel supports. The concrete segments are bolted together as with cast-iron segments, but the former possess the advantage of being lighter.
Soft ground tunnelling Soft ground tunnelling has developed considerably since Marc Brunel drove a tunnel under the River Thames 1843, which established shield tunnelling as a British invention. Shields provide overhead protection, and the support given to the face helps to reduce air losses when air pressure is used to keep water out of the workings. The shield is more easily pushed ahead when the ground immediately adjacent to the shield is replaced with soft, puddled clay. Bentonite slurry under pressure is being used to drive sewer tunnels in Hamburg, West Germany, and Warrington, Cheshire; this new development ensures the stability of the ground about to be excavated, and provides lubrication to help with pushing the tunnelling mole forwards. Alignment is laser-controlled to keep the tunnels within a tolerance on position of 30 mm/1.18 in vertically and 300 mm/11.8 in laterally.
A severe problem associated with the use of compressed air for evacuation of water from the workings is decompression sickness (the bends, caisson sickness), which arises when the shifts in the pressurized area are too long or the workmen come out of the compression area too rapidly. Cases occurred in the 19th century as soon as the caisson method of bridge and tunnel construction was introduced.
Pipe jacking An alternative form of tunnelling is pipe jacking, in which steel and concrete pipes up to 3.6 m/11.8 ft in diameter are forced through the ground by jacks. Short lengths are driven from a jacking station which has the thrust or reaction blocks for the jacks to react against. Longer lengths of up to 96 m/315 ft are driven using an intermediate jacking station; the tunnel is driven in a concertina fashion so that the leading section of pipe is pushed forward by the jacks at the intermediate station, then the pressure at these jacks is released whilst the rear section of pipe is pushed forward by the jacks, thrusting on the reaction blocks until the two sections of pipe have closed up again. Major tunnels include: Orange–Fish River (South Africa) 1975, irrigation tunnel, 82 km/51 mi; Chesapeake Bay Bridge-Tunnel (USA) 1963, combined bridge, causeway, and tunnel structure, 28 km/17.5 mi; Laerdal (Norway) 2000, currently the world's longest road tunnel, 24.5 km/15.2 mi; Zhongnanshan (China) 2007 (scheduled), road tunnel, 18 km/11.2 mi; St Gotthard (Switzerland–Italy) 1980, road tunnel, 16.3 km/10.1 mi; St Gotthard Base (Switzerland–Italy) 2010 (scheduled), rail tunnel, 57 km/35.4 mi; Seikan (Japan) 1964–85, currently the world's longest rail tunnel, Honshu–Hokkaido, under Tsugaru Strait, 53.9 km/33.5 mi, 23.3 km/14.5 mi undersea; Lötschberg (Switzerland) 2005, currently the world's longest rail tunnel on land, 35 km/22 mi; Simplon (Switzerland–Italy) 1906, overland rail tunnel, 19.8 km/12.3 mi; Rogers Pass (Canada) 1989, rail tunnel, 35 km/22 mi, through the Selkirk Mountains, British Columbia; Channel Tunnel rail tunnel linking England and France (begun by the French and British governments 1986, and opened 1994), 50 km/31 mi.