Abstract

The Bedretto adit is a 5.2 km passageway which was excavated to aid in the construction of the Furka Base Tunnel in the Swiss Alps. The adit is largely unsupported and spalling slabs continue to fall from the exposed walls in the tunnel. On-site measurements were made using LiDAR scans to digitize the observed geometry of failure throughout the tunnel. Information from the scans were used to delineate the lateral extent and angle of failure at different sections of the tunnel.

An examination of the potential stress field was carried out using the observed damage geometry to calibrate the numerical models by continuously varying the applied horizontal stresses to represent tectonic shortening. The numerical geometry of failure was interpreted by following both empirical and numerical brittle damage depth determination methods. Past geological investigations of fractures, faulting and water inflows were added to the LiDAR database parameters. While these geological parameters were not directly implemented in the models, they were indirectly used to delineate different stress regimes along the tunnel alignment. The simulated inner excavation damage zones (EDZi), which represent the maximum extent of damage were compared with the field scans.

The model results were within the wide scatter of the observed geometry of failure. The best estimate of the local compressive tectonic stress was determined to be a magnitude of 20 MPa oriented N73W and dipping 15° to the horizontal plane. Further refinements of the models will continue to improve the calibration of the local stress field around the Bedretto adit.

1 Introduction
1.1 Brittle Failure in Hard Rocks

Brittle failure of rock has been the subject of ongoing research for the past few decades due to its relevance in the design of tunnels and surface excavations. In the case of deep-seated tunnels, the occurrence of brittle failure (usually referred to as spalling) is attributed to damage induced by the tunnel excavation and changes in the stress state around the tunnel, with the degree of damage usually decreasing radially away from the tunnel (Perras and Diederichs 2016). The failure process manifests as extensile cracks which produce rock slabs along the walls of the tunnel. As described by Diederichs (2007), rocks under stress may fail by spalling or strain bursting depending on the stress to strength ratio. Spalling may not necessarily be violent and may occur over a long period of time. Case history analysis of rock bursts by Lee et al. (2004) showed that there is a high potential for rock bursts in tunnels at depths greater than 400 m, especially if the tunnel was excavated by a tunnel boring machine.

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