Implementation of reliability-based thresholds to excavation of shotcrete-supported rock tunnels

Modern tunnelling in rock relies heavily on information from monitoring and observations during construction as a means to reduce the considerable uncertainty that originates from a lack of knowledge about the ground conditions. By formally integrating the monitoring information into the structural safety evaluation, a comprehensive risk-based design framework can be achieved. The establishment of relevant thresholds against unacceptable structural behaviour is a key aspect of this work. This paper presents an extensive application example based on real-case data, showing how a reliability-based threshold can be established to ensure the serviceability of a shotcrete lining in a rock tunnel in a challenging geological setting. The authors address and discuss practical approaches to consider the model uncertainty related to the confinement loss caused by the tunnel front advance, as well as the transformation uncertainty related to the use of indirect monitoring information. The established threshold is discussed in the context of the observational method.
QC 20221125