Tunnel construction

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. Tunnel construction is a subset of underground construction. There are three basic types of tunnel construction in common use: Cut-and-cover tunnel, constructed in a shallow trench and then covered over. Bored tunnel, constructed in situ, without removing the ground above. They are usually of circular or horseshoe cross-section. Some concepts of underground mining apply. Modern techniques include Shotcrete used in the New Austrian tunnelling method, use of a tunnel boring machine (TBM) or tunnelling shield. But still tunnels are constructed which are secured with pit props and shoring and then are steined or timer supports are set. Techniques known from barrel vaults are helpful. Immersed tube tunnel, sunk into a body of water and laid on or buried just under its bed. In 2017 experiences show that city subway TBM tunnels cost approximately 500 Million EUR per kilometer. In Switzerland a kilometer of motorway tunnel was roughly calculated at 300 Million CHF, at the time 200 Million Eur. The undersea tunnel between Denmark and Germany is planned for 425 Million per km, in 2015. Cut-and-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.