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Three water tunnels were excavated through mostly hard metamorphic rocks in northern Italy by tunnel boring machines. A survey of 14 km of tunnel yielded over 700 sets of data featuring rock mass characteristics and TBM performance. The empirical relationships between rock mass rating and advance rate clearly showed that TBM performance reached a maximum in the rock mass rating (RMR) range 40-70, while penetration was slower in both very poor and very good rock masses. As different rocks give different penetrations for the same RMR, the use of Bieniawski's classification for predictive purposes is only possible if the RMR index used is normalised with reference to the basic factors affecting TBM tunnelling. Poor agreement was found between actual penetrations and those predicted by models, highlighting the difficulties involved in TBM performance prediction., Three water tunnels were excavated through mostly hard metamorphic rocks in northern Italy by tunnel boring machines. A survey of 14 km of tunnel yielded over 700 sets of data featuring rock mass characteristics and TBM performance. The empirical relationships between rock mass rating and advance rate clearly showed that TBM performance reached a maximum in the rock mass rating (RMR) range 40-70, while penetration was slower in both very poor and very good rock masses. As different rocks give different penetrations for the same RMR, the use of Bieniawski's classification for predictive purposes is only possible if the RMR index used is normalised with reference to the basic factors affecting TBM tunnelling. Poor agreement was found between actual penetrations and those predicted by models, highlighting the difficulties involved in TBM performance prediction.