Some aspects for geotechnical investigations for TBM and NATM tunneling in rock

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 2 and 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.