Analysis and estimation of gripper TBM performances in highly fractured and faulted rocks

This paper focuses on performance analysis of Tunnel Boring Machine (in particular gripper TBMs) in highly jointed rock masses and fault zones. In order to investigate possible relationships between these difficult excavation conditions and TBM performance, the data of several tunnel projects have been collected in a specific database (TBM-performance database). Despite the difficulties in gaining complete TBM data sets and detailed geological information, this database compiles data from the field, laboratory tests and literature. Preliminary analyses have been carried out in order to find possible correlations between TBM performance parameters (penetration and advance rates) and rock mass characteristics, such as rock strength and fracturing degree, generally used in common TBM performance prediction models. Although some trends could be identified, the scattered results confirm the difficulties in predicting the machine performance in complex geological environments starting from characterising indexes proper to good rocks. Then, a classification system for highly fractured rock masses and fault zones has been developed, based on representative parameters, providing a more complete geomechanical description of disturbed zones. Four “fault zone” classes have been identified. For each class a reduction rate of the selected TBM-performance parameters (i.e. cutterhead rotation speed, penetration rate and daily advance rate) has been evaluated with respect to the tunnelling performances recorded in good ground conditions. The results of these analyses allow effectively quantifying the effects of degrading rock masses on the TBM performances.