TBM performance assessment of an exploratory tunnel in hard rock

Tunnel projects with a length of several tens of kilometres, and generally consisting of double tubes, are increasingly based on the excavation of exploratory tunnels in the preliminary stage. This solution allows for the accurate assessment of times and cost of the main project due to the detailed evaluation of the rock mass characteristics as well as the rock mass attitude to be excavated by the method and the technique previously chosen for the main tunnel. The paper presents the assessment of TBM performance in the excavation of a 7 km exploratory tunnel in hard rock. The main parameters commonly used to evaluate the TBM performance were analysed and compared to the geological and operational variations occurred during the excavation. Overall performance were analysed by determination of the advance rate (AR) and the utilization factor (UF) obtained during the excavation. Actual performance (ROP and PR) were examined together with the main TBM excavation parameters. Rock mass boreability was investigated by field penetration index (FPI). Field specific energy (SE) and cutters spacing/penetration ratio were used to assess cutting efficiency and conditions.