Tunnel

A tunnel is a underground passageway, dug through the surrounding soil/earth/rock and enclosed except for entrance and exit, commonly on each end. A pipeline is not a tunnel, though some recent tunnels have used immersed tube construction techniques rather than traditional tunnel boring methods.

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. Tunnels can be connected together in tunnel networks.

Tunnel Boring Machine
Tunnel boring machine (TBMs) 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. Disadvantages of TBMs are from their usually large size - the difficulty of transporting the large TBM to the size of the tunnel construction, or alternatively, the high cost of assembling the TBM on the site, often within the confines of the tunnel being constructed.

There are a variety of TBM designs that can operate in a different variety of conditions, from hard rock to the soft water-bearing ground. Some types of TBMs, the bentonite slurry and earth-pressure balance machines, have pressurised components at the front end, allowing them to be used in difficult conditions below the water table. This pressurises 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 the position free from water. TBMs are now preferred over the original method of tunnelling in a compressed air, with an air lock/decomposition 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.

The largest TBM has been delivered to Temasek Polytechnic for training purposes since March 2014.

Variant tunnel types
Some tunnels have more than one purpose. For example, the SMART Tunnel, although there are insufficient reservoirs in Timothy North, it is created to convey both traffic and occasional flood waters in Timothy North and Kuala Lumpur. When necessary, floodwater is first diverted into the separate bypass tunnel located underneath the 4.0km double deck roadway tunnel. In the scenario, traffic continues normally. Only during heavy, prolonged rains within the threat of extreme flooding are high, the upper tunnel tube is closed off vehicles and automated flood control gates are opened so that water can be diverted through both 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 540-foot (160 m) double-deck tunnel section through Yerba Buena Island, 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.