SELECTION OF METHODS

This article discusses how to select the appropriate method to construct a tunnel. Two main factors that must be considered are the size and shape of the tunnel, and the ground in which it is to be built. All other parameters necessary to ensure technical and economic success, such as the best types of equipment, support systems and working procedures, follow from this initial choice, which is of vital importance. The technique chosen must be able to perform, or adapted to perform, all the tasks that it may be required to do. The cross section and span of a tunnel determine how work on the tunnel is approached, and they are normally determined by the intended use. Modern road tunnels, often twin bore, require a span between 10m and 17m; railways need 6m to 8m, and mass transit systems need slightly less; the diameters of water transport tunnels depend only on the quantities of water or sewage to be transported. Method selection in soft ground tunnels, and to a lesser extent in rock tunnels, is dominated by the problem of groundwater. Rock tunnels can usually be drilled rapidly and economically by tunnel- boring machines; only for very strong rocks is drill+blast required. Shield and ring methods are often used to excavate soft ground tunnels, but a wide range of methods is now available. For small-diameter soft ground tunnelling, pipe jacking is most often used. Various tunnel support systems are in use, including the New Austrian Tunnelling Method that seeks to use natural ground strength.