A tunnel is an underground or underwater passageway, enclosed except for entrance and exit, commonly at each end.
A tunnel may be for foot or vehicular road traffic, for rail traffic, or for a canal. The central portions of a rapid transit network are usually in tunnel. Some tunnels are aqueducts to supply water for consumption or for hydroelectric stations or are sewers. Utility tunnels are used for routing steam, chilled water, electrical power or telecommunication cables, as well as connecting buildings for convenient passage of people and equipment.
Secret tunnels are built for military purposes, or by civilians for smuggling of weapons, contraband, or people. Special tunnels, such as wildlife crossings, are built to allow wildlife to cross human-made barriers safely.
A tunnel is relatively long and narrow; the length is often much greater than twice the diameter, although similar shorter excavations can be constructed such as cross passages between tunnels.
The definition of what constitutes a tunnel can vary widely from source to source. For example the definition of a road tunnel in the United Kingdom is defined as "a subsurface highway structure enclosed for a length of 150 metres (490 ft) or more. In the United States, the NFPA definition of a tunnel is "An underground structure with a design length greater than 23 m (75 ft) and a diameter greater than 1,800 millimetres (5.9 ft)."
In the UK, a pedestrian, cycle or animal tunnel beneath a road or railway is called a subway, while an underground railway system is differently named in different cities, the Underground or the Tube in London, the Subway in Glasgow, and the Metro in Newcastle. The place where a road, railway, canal or watercourse passes under a footpath, cycleway, or another road or railway is most commonly called a bridge or, if passing under a canal, an aqueduct. Where it is important to stress that it is passing underneath, it may be called an underpass, though the official term when passing under a railway is an underbridge. A longer underpass containing a road, canal or railway is normally called a tunnel, whether or not it passes under another item of infrastructure. An underpass of any length under a river is also usually called a tunnel, whatever mode of transport it is for.
In the US, the term "subway" means an underground rapid transit system, and the term "pedestrian underpass" is used instead. Rail station platforms may be connected by pedestrian tunnels or footbridges.
Much of the early technology of tunneling evolved from mining and military engineering. The etymology of the terms "mining" (for mineral extraction or for siege attacks), "military engineering", and "civil engineering" reveals these deep historic connections.
Clay-kicking is a specialised method developed in the United Kingdom of manually digging tunnels in strong clay-based soil structures. Unlike previous manual methods of using mattocks which relied on the soil structure to be hard, clay-kicking was relatively silent and hence did not harm soft clay based structures.
The clay-kicker lies on a plank at a 45-degree angle away from the working face and inserts a tool with a cup-like rounded end with the feet. Turning the tool manually, he or she extracts a section of soil, which is then placed on the waste extract.
Regularly used in Victorian civil engineering, the methods found favour in the renewal of the United Kingdom's then ancient sewerage systems, by not having to remove all property or infrastructure to create an effective small tunnel system. During the First World War, the system was successfully deployed by the Royal Engineer tunnelling companies to deploy large military mines beneath enemy German Empire lines. The method was virtually silent, and so not susceptible to listening methods of detection.
Geotechnical Investigation & Design
Conventional desk and site studies may yield insufficient information to assess such factors as the blocky nature of rocks, the exact location of fault zones, or the stand-up times of softer ground. This may be a particular concern in large-diameter tunnels. To give more information, a pilot tunnel, or drift, may be driven ahead of the main drive. This tunnel will be easier to support should unexpected conditions be met, and will be incorporated in the final tunnel. Alternatively, horizontal boreholes may sometimes be drilled ahead of the advancing tunnel face.
Other key geotechnical factors include:
- Stand-up time is the amount of time a tunnel will support itself without any added structures. Knowing this time allows the engineers to determine how much can be excavated before support is needed. The longer the stand-up time is the faster the excavating will go. Generally certain configurations of rock and clay will have the greatest stand-up time, and sand and fine soils will have a much lower stand-up time.
- Groundwater control is very important in tunnel construction. If there is water leaking into the tunnel stand-up time will be greatly decreased. If there is water leaking into the shaft it will become unstable and will not be safe to work in. To stop this from happening there are a few common methods. One of the most effective is ground freezing. To do this pipes are inserted into the ground surrounding the shaft and are cooled until they freeze. This freezes the ground around each pipe until the whole shaft is surrounded frozen soil, keeping water out. The most common method is to install pipes into the ground and to simply pump the water out. This works for tunnels and shafts.
- Tunnel shape is very important in determining stand-up time. The force from gravity is straight down on a tunnel, so if the tunnel is wider than it is high it will have a harder time supporting itself, decreasing its stand-up time. If a tunnel is higher than it is wide the stand up time will increase making the project easier. The hardest shape to support itself is a square or rectangular tunnel. The forces have a harder time being redirected around the tunnel making it extremely hard to support itself. This of course all depends what the material of the ground is.
Choice of Tunnels vs Bridges
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. In Hampton Roads, tunnels were chosen over bridges for strategic considerations; in the event of damage, bridges would prevent U.S. Navy vessels from leaving Naval Station Norfolk.
The 1934 Queensway Road Tunnel under the River Mersey at Liverpool was chosen over a massively high bridge for defence reasons: it was feared aircraft could destroy a bridge in times of war. Maintenance costs of a massive bridge to allow the world's largest ships to navigate under were considered higher than for a tunnel. Similar conclusions were reached for the 1971 Kingsway Tunnel under the Mersey.
Water-crossing tunnels built instead of bridges include the Holland Tunnel and Lincoln Tunnel between New Jersey and Manhattan in New York City, the Queens-Midtown Tunnel between Manhattan and the borough of Queens on Long Island, and the Elizabeth River tunnels between Norfolk and Portsmouth, Virginia, the 1934 River Mersey road Queensway Tunnel, the Western Scheldt Tunnel, Zeeland, Netherlands, and the North Shore Connector tunnel in Pittsburgh, Pennsylvania.
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-kilometre or 32.0-mile 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).
Some water crossings are a mixture of bridges and tunnels, such as the Denmark to Sweden link and the Chesapeake Bay Bridge-Tunnel in Virginia.
There are particular hazards with tunnels, especially from vehicle fires when combustion gases can asphyxiate users, as happened at the Gotthard Road Tunnel in Switzerland in 2001. One of the worst railway disasters ever, the Balvano train disaster, was caused by a train stalling in the Armi tunnel in Italy in 1944, killing 426 passengers.
Cost Estimates & Overruns
Government funds are a major factor in the creation of tunnels. When a tunnel is in the process of being constructed, economics and politics play a large factor in the decision making process. This division of the project is part of the construction/project management aspect of civil engineering. The project duration must be identified using a work breakdown structure (WBS) and critical path method (CPM). Understanding the amount of time the project requires, the amount of labors and materials needed is a crucial part of the project. Also, the amount of land that will need to be excavated and the proper machinery that is needed is also very important. Since infrastructures require millions, or even billions of dollars, acquiring these funds can be challenging.
The need for an infrastructure such as a tunnel must be identified. Political issues are bound to occur as it was shown in 2005 when the US House of Representatives approved a $100 million federal grant to build a tunnel in the New York Harbor. However, the Port Authority of New York and New Jersey was aware of this bill and had never asked for a grant or for such a project. The current state of the economy reflects on the amount of money the government can give for public projects. Since taxpayers money goes to projects such as the creation of tunnels, or any other infrastructures, increasing taxes may cause issues.
Tunnels are dug in types of materials varying from soft clay to hard rock. The method of tunnel construction depends on such factors as the ground conditions, the ground water conditions, the length and diameter of the tunnel drive, the depth of the tunnel, the logistics of supporting the tunnel excavation, the final use and shape of the tunnel and appropriate risk management.
There are three basic types of tunnel construction in common use:
- Cut-and-cover tunnels, constructed in a shallow trench and then covered over.
- Bored tunnels, constructed in situ, without removing the ground above. They are usually of circular or horseshoe cross-section.
- Immersed tube tunnels, sunk into a body of water and sit on, or are buried just under, its bed.
Cut & Cover
Cut & Cover is a simple method of construction for shallow tunnels where a trench is excavated and roofed over with an overhead support system strong enough to carry the load of what is to be built above the tunnel. Two basic forms of cut-and-cover tunnelling are available:
- Bottom-up method: A trench is excavated, with ground support as necessary, and the tunnel is constructed in it. The tunnel may be of in situ concrete, precast concrete, precast arches,or corrugated steel arches; in early days brickwork was used. The trench is then carefully back-filled and the surface is reinstated.
- Top-down method: Side support walls and capping beams are constructed from ground level by such methods as slurry walling, or contiguous bored piling. Then a shallow excavation allows making the tunnel roof of precast beams or in situ concrete. The surface is then reinstated except for access openings. This allows early reinstatement of roadways, services and other surface features. Excavation then takes place under the permanent tunnel roof, and the base slab is constructed.
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, owing 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 Machine (TBM) and associated back-up systems are used to highly automate the entire tunnelling process, reducing tunnelling costs. In certain predominantly urban applications, tunnel boring is viewed as quick and cost effective alternative to laying surface rails and roads. Expensive compulsory purchase of buildings and land, with potentially lengthy planning inquiries, is eliminated.
There are a variety of TBM designs that can operate in a variety of conditions, from hard rock to soft water-bearing ground. Some types of TBMs, the 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 over the older method of tunnelling 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 deep-sea divers.
In February 2010, Aker Wirth delivered a TBM to Switzerland, for the expansion of the Linth–Limmern Power Stations in Switzerland. The borehole has a diameter of 8.03 metres (26.3 ft). The four TBMs used for excavating the 57-kilometre (35 mi) Gotthard Base Tunnel, in Switzerland, had a diameter of about 9 metres (30 ft). A larger TBM was built to bore the Green Heart Tunnel (Dutch: Tunnel Groene Hart) as part of the HSL-Zuid in the Netherlands, with a diameter of 14.87 metres (48.8 ft). This in turn was superseded by the Madrid M30 ringroad, Spain, and the Chong Ming tunnels in Shanghai, China. All of these machines were built at least partly by Herrenknecht. As of August 2013, the world's largest TBM is "Big Bertha", a 57.5-foot (17.5 m) diameter machine built by Hitachi Zosen Corporation, which is digging the Alaskan Way Viaduct replacement tunnel in Seattle, Washington (US).
A temporary access shaft is sometimes necessary during the excavation of a tunnel. They are usually circular and go straight down until they reach the level at which the tunnel is going to be built. A shaft normally has concrete walls and is usually built to be permanent. Once the access shafts are complete, TBMs are lowered to the bottom and excavation can start. Shafts are the main entrance in and out of the tunnel until the project is completed. If a tunnel is going to be long, multiple shafts at various locations may be bored so that entrance to the tunnel is closer to the unexcavated area.
Once construction is complete, construction access shafts are often used as ventilation shafts, and may also be used as emergency exits.
Sprayed Concrete Techniques
The New Austrian Tunneling Method (NATM) was developed in the 1960's, and is the best known of a number of engineering solutions that use calculated and empirical real-time measurements to provide optimised safe support to the tunnel lining. The main idea of this method is to use the geological stress of the surrounding rock mass to stabilize the tunnel itself, by allowing a measured relaxation and stress reassignment into the surrounding rock to prevent full loads becoming imposed on the introduced support measures. Based on geotechnical measurements, an optimal cross section is computed. The excavation is immediately protected by a layer of sprayed concrete, commonly referred to as shotcrete, after excavation. Other support measures could include steel arches, rockbolts and mesh. Technological developments in sprayed concrete technology have resulted in steel and polypropylene fibres being added to the concrete mix to improve lining strength. This creates a natural load-bearing ring, which minimizes the rock's deformation.
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 kilometres (6.2 mi) became usual.
In Pipe jacking hydraulic jacks are used to push specially made pipes through the ground behind a TBM or shield, commonly used to create tunnels under existing structures, such as roads or railways. Tunnels constructed by pipe jacking are normally small diameter tunnels with a maximum size of around 3.2m.
Box jacking is similar to pipe jacking, but instead of jacking tubes, a box shaped tunnel is used. Jacked boxes can be a much larger span than a pipe jack with the span of some box jacks in excess of 20m. A cutting head is normally used at the front of the box being jacked and excavation is normally by excavator from within the box.
There are also several approaches to underwater tunnels, the two most common being bored tunnels or immersed tubes. Submerged floating tunnels are a novel approach under consideration; however, no such tunnels have been constructed to date.
During construction of a tunnel it is often convenient to install a temporary railway, particularly to remove excavated spoil, often narrow gauge so that it can be double track to allow the operation of empty and loaded trains at the same time. The temporary way is replaced by the permanent way at completion, thus explaining the term "Perway".
The vehicles or traffic using a tunnel can outgrow it, requiring replacement or enlargement. The original single line Gib Tunnel near Mittagong was replaced with a double-track tunnel, with the original tunnel used for growing mushrooms. The Rhyndaston Tunnel was enlarged using a borrowed TBM so as to be able to take ISO containers.
The 1836 double-track mile-long tunnel from Edge Hill to Lime Street in Liverpool was totally removed, apart from a 50-metre section at Edge Hill. Four tracks were required. The tunnel was converted into a very deep four-track cutting, with short four-track tunnels in places. Train services were not interrupted as the work progressed. There are other occurrences of tunnels being replaced by open cuts, for example, the Auburn Tunnel.
Tunnels can also be enlarged by lowering the floor.
Open Building Pit
An open building pit consists of a horizontal and a vertical boundary that keeps groundwater and soil out of the pit. There are several potential alternatives and combinations for (horizontal and vertical) building pit boundaries. The most important difference with cut-and-cover is that the open building pit is muted after tunnel construction; no roof is placed.
Other Construction Methods
- Drilling and blasting
- Hydraulic splitter
- Slurry-shield machine
- Wall-cover construction method
Double-Deck & Multipurpose Tunnels
Some tunnels are double-deck, for example the two major segments of the San Francisco – Oakland Bay Bridge (completed in 1936) are linked by a double-deck tunnel through Treasure Island, San Francisco, the largest-diameter bored 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.
In the UK, the 1934 Queensway Tunnel under the River Mersey between Liverpool and Birkenhead was originally to have road vehicles running on the upper deck and trams on the lower. During construction the tram usage was cancelled. The lower section is used for cables, pipes and emergency accident refuge enclosures.
In China, the Lion Rock Tunnel, built in the mid 1960's connecting New Kowloon and Sha Tin in Hong Kong, carries a motorway and an aqueduct. A recent double-deck tunnel with both decks for motor vehicles is the Fuxing Road Tunnel in Shanghai. Cars travel on the two-lane upper deck, and heavier vehicles on the single-lane lower level.
Multipurpose tunnels exist that have more than one purpose. The SMART Tunnel in Malaysia is the first multipurpose flood control tunnel in the world, used both to convey traffic and occasional flood waters in Kuala Lumpur.
Common utility ducts or utility tunnels are carry two or more utility lines. Through co-location of different utilities in one tunnel, organizations are able to reduce the costs of building and maintaining utilities.
Over-bridges can sometimes be built by covering a road or river or railway with brick or steel arches, and then leveling 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.
Safety & Security
Owing to the enclosed space of a tunnel, fires can have very serious effects on users. The main dangers are gas and smoke production, with even low concentrations of carbon monoxide being highly toxic. Fires killed 11 people in the Gotthard tunnel fire of 2001 for example, all of the victims succumbing to smoke and gas inhalation. Over 400 passengers died in the Balvano train disaster in Italy in 1944, when the locomotive halted in a long tunnel. Carbon monoxide poisoning was the main cause of death. In the Caldecott Tunnel fire of 1982, the majority of fatalities were caused by toxic smoke, rather than by the initial crash.
Motor vehicle tunnels usually require ventilation shafts and powered fans to remove toxic exhaust gases during routine operation. Rail tunnels usually require fewer air changes per hour, but still may require forced-air ventilation. Both types of tunnels often have provisions to increase ventilation under emergency conditions, such as a fire. Although there is a risk of increasing the rate of combustion through increased airflow, the primary focus is on providing breathable air to persons trapped in the tunnel, as well as firefighters.
When there is a parallel, separate tunnel available, airtight but unlocked emergency doors are usually provided which allow trapped personnel to escape from a smoke-filled tunnel to the parallel tube.
Larger, heavily-used tunnels, such as the Big Dig tunnel in Boston, Massachusetts, may have a dedicated 24-hour manned operations center which monitors and reports on traffic conditions, and responds to emergencies. Video surveillance equipment is often used, and real-time pictures of traffic conditions for some highways may be viewable by the general public via the Internet.
Some tunnels are not for transport at all but rather, are fortifications, for example Mittelwerk and Cheyenne Mountain. Excavation techniques, as well as the construction of underground bunkers and other habitable areas, are often associated with military use during armed conflict, or civilian responses to threat of attack. One of the strangest uses of a tunnel was for the storage of chemical weapons.