Groundwater control techniques can be used to avoid or mitigate groundwater problems that can affect below ground construction works, both in the temporary case during construction, and under permanent or long-term conditions. There are two main categories of groundwater control techniques: pumping methods (also known as construction dewatering), whereby groundwater levels are lowered by pumping from an array of wells or sumps; and exclusion, methods where low permeability barriers (cut-off walls or zones of ground treatment) are installed around an excavation or structure to prevent or reduce groundwater inflows. Often a combination of these two approaches is appropriate. This Chapter describes the characteristics and application of groundwater control methods.

excavation to become unstable and risking collapse. The most common way to control groundwater is to install dewatering pipes into the ground and to simply A tunnel is an underground or undersea passageway. It is dug through surrounding soil, earth or rock, or laid under water, and is usually completely enclosed except for the two portals common at each end, though there may be access and ventilation openings at various points along the length. A pipeline differs significantly from a tunnel, though some recent tunnels have used immersed tube construc 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 groundwater 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 the shape of the tunnel and appropriate risk management. There are three basic types of tunnel construction in common use. Cut-and-cover tunnels are constructed in a shallow trench and then covered over. Bored tunnels are constructed in situ, without removing the ground above. Finally, a tube can be sunk into a body of water, which is called an immersed tunnel. The Moffat Tunnel, opened in 1928, passes under the Continental Divide of the Americas in Colorado. The tunnel is 10.0 km (6.2 mi) long and at an elevation of 2,816 m (9,239 ft) is the highest active railroad tunnel in the U.S. (The inactive Tennessee Pass Line and the historic Alpine Tunnel are higher.) Williamson's tunnels in Liverpool, from 1804 and completed around 1840 by a wealthy eccentric, are probably the largest underground folly in the world. The tunnels were built with no functional purpose. The Chicago freight tunnel network is the largest urban street tunnel network, comprising 97 km (60 mi) of tunnels beneath the majority of downtown Chicago streets. It operated between 1906 and 1956 as a freight network, connecting building basements and railway stations. Following a 1992 flood the network was sealed, although some parts still carry utility and communications infrastructure. The Pennsylvania Turnpike opened in 1940 with seven tunnels, most of which were bored as part of the stillborn South Pennsylvania Railroad and giving the highway the nickname "Tunnel Highway". Four of the… Some tunnels are not for transport at all but rather, are fortifications, for example Mittelwerk and Cheyenne Mountain Complex. 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. Another use for tunnels was for the storage of chemical weapons [1].