A tunnel is an underground passageway. The definition of what constitutes a tunnel is not universally agreed upon. However, in general tunnels are at least twice as long as they are wide. In addition, they should be completely enclosed on all sides, save for the openings at each end. Some civic planners define a tunnel as 0.1 miles (0.16 kilometers) in length or longer, while anything shorter than this should be called a chute. For example, the underpass beneath Yahata Station in Kitakyushu, Japan is only long and therefore should not be considered a tunnel.
A tunnel may be for pedestrians or cyclists, for general road traffic, for motor vehicles only, for rail traffic, or for a canal. Some are aqueducts, constructed purely for carrying water — for consumption, for hydroelectric purposes or as sewers — while others carry other services such as telecommunications cables. There are even tunnels designed as wildlife crossings for European badgers and other endangered species. Some secret tunnels have also been made as a method of entrance or escape from an area, such as the Cu Chi Tunnels or the tunnels connecting the Gaza Strip to Egypt. Some tunnels are not for transport at all but are fortifications, for example Mittelwerk and Cheyenne Mountain.
The longest canal tunnel is the Standedge Tunnel in the United Kingdom, over three miles (5 km) long.
In the United Kingdom a pedestrian tunnel or other underpass beneath a road is called a subway. This term was used in the past in the United States, but now refers to underground rapid transit systems.
The central part of a rapid transit network is usually built in tunnels. To allow non-level crossings, some lines run in deeper tunnels than others. Rail stations with much traffic usually provide pedestrian tunnels from one platform to another, though others use bridges.
It is essential that any tunnel project starts with a comprehensive investigation of ground conditions. The results of the investigation will allow proper choice of machinery and methods for excavation and ground support, and will reduce the risk of encountering unforeseen ground conditions. In the early stages, the horizontal and vertical alignment will be optimised to make use of the best ground and water conditions.
In some cases, conventional desk and site studies will not produce sufficient information to assess, for example, the blocky nature of rocks, the exact location of fault zones, or stand-up times of softer ground. This may be a particular concern in large diameter tunnels. To overcome these problems, a pilot tunnel, or drift, may be driven ahead of the main drive. This smaller diameter tunnel will be easier to support when unexpected conditions occur, and will be incorporated in the final tunnel. Alternatively, horizontal boreholes may sometimes be used ahead of the advancing tunnel face.
Tunnels are dug in various types of materials, from soft clay to hard rock, and the method of excavation depends on the ground conditions.
Two basic forms of cut-and-cover tunnelling are available:
Shallow tunnels are often of the cut-and-cover type (if under water, of the immersed-tube type), while deep tunnels are excavated, often using a tunnelling shield. For intermediate levels, both methods are possible.
Large cut-and-cover boxes are often used for underground metro stations, such as Canary Wharf tube station in London. This construction form generally has two levels, which allows economical arrangements for ticket hall, station platforms, passenger access and emergency egress, ventilation and smoke control, staff rooms, and equipment rooms. The interior of Canary Wharf station has been likened to an underground cathedral due to the sheer size of the excavation. This contrasts with most traditional stations on London Underground, where bored tunnels were used for stations and passenger access.
Tunnel boring machines (TBMs) and associated back-up systems can be used to highly automate the entire tunneling process. There are a variety of TBMs that can operate in a variety of conditions, from hard rock to soft water-bearing ground. Some types of TBMs, bentonite slurry and earth-pressure balance machines, have pressurised compartments at the front end, allowing them to be used in difficult conditions below the water table. This pressurizes the ground ahead of the TBM cutter head to balance the water pressure. The operators work in normal air pressure behind the pressurised compartment, but may occasionally have to enter that compartment to renew or repair the cutters. This requires special precautions, such as local ground treatment or halting the TBM at a position free from water. Despite these difficulties, TBMs are now preferred to the older method of tunneling in compressed air, with an air lock/decompression chamber some way back from the TBM, which required operators to work in high pressure and go through decompression procedures at the end of their shifts, much like divers.
Until recently the largest TBM built was used to bore the Green Heart Tunnel (Dutch: Tunnel Groene Hart) as part of the HSL-Zuid in the Netherlands. It had a diameter of 14.87 m.
Nowadays even larger machines exist: two for the M30 ringroad in Madrid, Spain, and two for the Chong Ming tunnels in Shanghai, China. These machines are 15,2 m and 15,4 m in diameter respectively. The two machines for Spain were built by Mitsubishi/Duro Felguera and Herrenknecht. The TBMs for China were built by Herrenknecht.
By special monitoring the NATM method is very flexible, even at surprising changes of the geomechanical rock consistency during the tunneling work. The measured rock properties lead to appropriate tools for tunnel strengthening. In the last decades also soft ground excavations up to 10 km became usual.
For water crossings, a tunnel is generally more costly to construct than a bridge. Navigational considerations may limit the use of high bridges or drawbridge spans intersecting with shipping channels, necessitating a tunnel. Bridges usually require a larger footprint on each shore than tunnels. In areas with expensive real estate, such as Manhattan and urban Hong Kong, this is a strong factor in tunnels' favor. Boston's Big Dig project replaced elevated roadways with a tunnel system to increase traffic capacity, hide traffic, reclaim land, redecorate, and reunite the city with the waterfront. Examples of water-crossing tunnels built instead of bridges include the Holland Tunnel and Lincoln Tunnel between New Jersey and Manhattan in New York City, and the Elizabeth River tunnels between Norfolk and Portsmouth, Virginia and the Westerschelde tunnel, Zeeland, Netherlands. Other reasons for choosing a tunnel instead of a bridge include avoiding difficulties with tides, weather and shipping during construction (as in the 51.5 km Channel Tunnel), aesthetic reasons (preserving the above-ground view, landscape, and scenery), and also for weight capacity reasons (it may be more feasible to build a tunnel than a sufficiently strong bridge).
There are particular hazards with tunnels, especially from vehicle fires when combustion gases can asphyxiate users, as happened at the Gotthard tunnel in Switzerland in 2001. One of the worst railway disasters ever occurred when a train stalled in the Armi tunnel in Italy in 1944, killing 426 passengers.
The two major segments of the San Francisco – Oakland Bay Bridge (completed in 1936) are linked by a double-deck tunnel, once the largest diameter tunnel in the world. At construction this was a combination bidirectional rail and truck pathway on the lower deck with automobiles above, now converted to one-way road vehicle traffic on each deck.
A recent double-decker tunnel with both decks for motor vehicles is the Fuxing Road Tunnel in Shanghai, China. Cars travel on the two-lane upper deck and heavier vehicles on the single-lane lower.
Overbridges can sometimes be built by covering a road or river or railway with brick or still arches, and then levelling the surface with earth. In railway parlance, a surface-level track which has been built or covered over is normally called a covered way.
Snow sheds are a kind of artificial tunnel built to protect a railway from avalanches of snow. Similarly the Stanwell Park, New South Wales steel tunnel, on the South Coast railway line, protects the line from rockfalls.
Common utility ducts are man-made tunnels created to carry two or more utility lines underground. Through co-location of different utilities in one tunnel, organizations are able to reduce the costs of building and maintaining utilities.
The use of tunnels for mining is called drift mining.
Snow tunnels are created by voles, chipmunks and other rodents for protection and access to food sources. Larger versions are created by humans, usually for fun.
For more information regarding tunnels built by animals, see Burrow
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