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 0.08 miles (0.13 km) 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 road tunnel is the ----- Tunnel in the United Kingdom, over --- miles -- 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.
Tunnels are dug in various types of materials, from soft clay to hard rock, and the method of excavation depends on the ground conditions.
Cut-and-cover is a simple method of construction for shallow tunnels where a trench is excavated and roofed over. A strong overhead support system is required to carry the load of the covering material.
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 within. The tunnel may be of in situ concrete, precast concrete, precast arches, corrugated steel arches and such, with brickwork used in early days. The trench is then backfilled, with precautions regarding balancing compaction of the backfill material, and the surface is reinstated.
- Top-down method: In this method, side support walls and capping beams are constructed from ground level, using slurry walling, contiguous bored piles, or some other method. A shallow excavation is then made to allow the tunnel roof to be constructed using 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 machinery is then lowered into the access openings, and the main excavation is carried out under the permanent tunnel roof, followed by constructing the base slab.
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.
Machine Bored Tunnels Edit
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.
The New Austrian Tunneling Method (NATM) was developed in the 1960s. The main idea of this method is to use the geological stress of the surrounding rock mass to stabilize the tunnel itself. Based on geotechnical measurements, an optimal cross section is computed. The excavation is immediately protected by thin shotcrete, just behind the excavation. This creates a natural load-bearing ring, which minimizes the rock's deformation. (part of the Heathrow express Tunnel being built using this method in the 1990s colapsed)
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.
Pipe Jacking, also known as pipejacking or pipe-jacking, is a method of tunnel construction where hydraulic jacks are used to push specially made pipes (these can be Rectangular as well as round) through the ground behind a tunnel boring machine or shield. This technique is commonly used to create tunnels under existing structures, such as roads or railways. This is often used in the UK for Building new Bridges under existing roads and Railways were closures are not acceptable. A Large Concrete Box with a cutting edge attached is built off to the side then Slid on a base to the bank and pushed (Jacked) through digging out the spoil as it advances.
Other tunnelling methods include:
- Drilling and blasting (Used in Wales on the A55 coast road)
- Slurry-shield machine (Used in some wet ground conditions)
- Wall-cover construction method. (Not a true tunnel used were unstable rock or snow falls occur, a wall is built at one side and a roof put on to cover the road, thus protecting the traffic).
Choice of tunnels vs. bridgesEdit
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).
Some water crossings are a mixture of bridges and tunnels, such as the Denmark to Sweden link and the Chesapeake Bay Bridge-Tunnel in the eastern United States.
UK Road TunnelsEdit
- Conway Tunnel
- Dartford Tunnel - Under the Thames river A282 (M25)
- Lime House Link Tunnel - Under the Thames river
- Mersey Tunnel
- Rotherhithe Tunnel - Under the Thames river
- Tyne Tunnel
Wikipedia extract for explanation of terms.
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