Brittle Rock Failure in the Steg Lateral Adit of the Loetschberg Base Tunnel

During the crossing of brittle rock formations at the Loetschberg base tunnel, failure phenomena have been observed both at the tunnel face and at the walls. A detailed analysis has been undertaken to explain these behaviours, based on the recent developments of Canadian research on brittle failure mechanisms. At the tunnel walls, a very good agreement is found between the calculated and observed damage and between two prediction methods, i.e. a semi-empirical failure criterion and elastic calculations with the 'brittle Hoek-Brown parameters.' Near the face, due to the 3D nature of the stress conditions, some limitations of these approaches have been highlighted, and the growth of wall failure has been analysed. This research allowed a better understanding of the brittle rock mass behaviour at the Loetschberg base tunnel and showed that brittle failure processes dominate the behaviour of deep, highly stressed excavations in massive to moderately jointed rock. It also illustrates where improvements to the adopted approaches are required.