Driving of tunnels in rock can be carried out by means of the New Austrian Tunneling Method (NATM) and by Tunnel Boring Machines. When applying the NATM it is possible to adapt the amount and thickness of the temporary support as well as the thickness of the internal lining to the different rock conditions to be expected along the tunnel alignment. Furthermore in case rock conditions encountered during construction differ from the predictions it is easy to modify support and lining accordingly. Also support of the temporary face can be carried out as required. The tunnel driving according to the NATM is however time-consuming in Certain cases.
Thus, the application of a TBM is often an economic alternative. Using this method, the excavation diameter, the thickness of the lining and the type of the machine have to be selected in an early phase of the project. This Includes an assessment of the stability of the working face. Thus, a TBM heading is far more sensitive to a change of the predicted ground conditions. This will be illustrated by means of two practical examples (chapter 2and 3). For the second example the problems with machine tunneling could have been avoided by selecting a different type of machine.
Also with regards to the stability of the temporary face, the selection of an adequate machine is decisive (chapter 4). It results that the knowledge of the rock conditions must be far more reliable if it is planned to select a TBM-heading.
A profound exploration of the ground conditions Within the design phase is decisive for the technical and economical success of a tunnel construction. Since below certain depths the overburden stresses can not be carried by the support and lining respectively, parts of the load must be carried by the arch which develops In the rock around the tunnel. For a reliable assessment of this arch action and the load resulting for the sup- Port, the mechanical properties of the rock mass and adequate stability analyses are decisive. Also for the selection of the equipment for excavation the mechanical properties, especially the intact rock strength and Its mineral composition, are important.
Thus, the rock conditions combined with stability analyses are of central importance for selecting a suitable and economic support and construction method. Common construction methods are the New Austrian Tunneling Method (NATM) and the construction With a tunnel boring machine (TBM).
Applying the NATM one can choose between a full face or sequential excavation. The latter can e.g. consist of an advancing vault heading with or without a temporary shotcrete supported invert, which is followed by the excavation of bench and invert of the total cross section. Besides the mode of heading the tender documents for a NATM driven tunnel should also describe the amount of support, such as the thickness of the shotcrete as well as the amount of anchoring and steel arches and its variation along the alignment of a tunnel.