The thesis focuses on the study of the performance of TBMs in highly jointed and faulted rocks. Despite the great production rates recorded in favourable ground conditions, the TBM advance can be significantly slowed down in limiting geological situations such as extremely fractured and fault zones. After an introduction about the major issues resulting from tunnelling in bad ground conditions, a brief review of some TBM performance prediction models is reported and the most common parameters adopted for evaluating the TBM performance are identified. The geotechnical characterisation of fault zones is very difficult due to their heterogeneous nature (i.e. weak and strong rock components). This particular aspect is highlighted through a literature review about the geological/geotechnical description of fault rocks. In order to investigate possible relationships between difficult rock mass conditions and TBM performance, data of several tunnel projects are collected in a database by including information from the field, laboratory tests and literature. Preliminary analyses are carried out in order to identify correlations between TBM performance (i.e. penetration and advance rate) and rock mass parameters (e.g. rock strength, fracturing degree, etc.). Although some trends are identified, the scattered results confirm the difficulties in predicting the machine performance in complex geological environments. In order to obtain a more complete geotechnical description of disturbed zones, a classification system for highly fractured rock masses and fault zones is developed. Four “fault zone” classes are identified by considering the fracturing and the weathering degree of the rock mass. Furthermore, in order to analyse the response to mechanical excavation of the identified classes, a set of numerical simulations are run with the aim to investigate the TBM performance at the cutter-scale. The data recorded in the TBM-performance database are then analysed according to the new classification system. For each “fault zone” class, a reduction rate for selected TBM parameters is defined with respect to the tunnelling performance observed in good ground conditions. The results of these analyses are of great relevance as they allow to successfully quantifying the effect of altered rock mass conditions on the TBM behaviour. The results obtained in the previous steps are used to carry out probabilistic analyses of the tunnel construction time and costs by means of the Decision Aids for Tunnelling (DAT). The DAT are a software package that evaluates the influence of uncertainties related to both geotechnical conditions and excavation process on the final tunnel construction time and costs. In this framework, the TBM advance rate reductions previously defined for each “fault zone” class are input in the simulations. A reliable estimation of the effect of degrading geological conditions (i.e. faulted and highly fractured/crushed rocks) on the tunnel construction process is obtained. The results of this research provide useful insights regarding the TBM performance reduction in bad grounds, with respect to the “ordinary” tunnelling conditions. The study highlights the need to better characterise, from a geomechanical point of view, the highly fractured and faulted rocks. This can be done by considering the parameters representative of the degraded state of the rock mass, instead of those commonly used for estimating the TBM performance in good rocks.