Design methodology for tunnels

Tunnel design frequently is based on crude empirical methods. Increasingly, owners request the application of engineering standards for the support design. Among those standards the EUROCODE 7 is a prominent ex-ample. The partial safety factor concept, as proposed by Eurocode 7, appears to be applicable only in limited cases for tunnel design. From the options listed in the Eurocode, only the observational method seems to be applicable for tunnels in rock masses with medium to high overburden. The fact that in most cases the rock mass is both the source of the loading and a load bearing element is completely unaddressed in the Eurocode. On the other hand, the Guidelines for the geotechnical design of underground structures, published by the Austrian Society for Geomechanics represent a sound and practically tested design methodology. However, the issue of proofing that the chosen excavation and support approach fulfills the requirements is still unre-solved, respectively no reasonable guidelines exist. A working group has been established to develop a design strategy addressing the issues described above. The working group meanwhile has made considerable pro-gress. First results are presented.