Assessing Long-Term Excavation Damage in Switzerland's Bedretto Adit: A Field Investigation in the Northern Section

The Bedretto adit, in the Alps of Switzerland, was excavated in 1972-1982 as an auxiliary adit for the main Furka Base Tunnel. The adit is 5218 m long, being excavated mostly in the Rotondo Granite with drill and blast methods to create a horseshoe cross-section. Much of the tunnel has remained unlined and such long observation periods are seldom available since tunnels typically are lined shortly after construction and mines are often closed or backfilled. Previous research in 2004 characterized the depth of failure (DoF) around the adit and found on average a depth of 1.1 +/- 0.03 m. A site investigation was carried out in 2017, aimed to measure the DoF through 3D light detection and ranging scanned sections again. The new 3D scans covered a longer section of the tunnel with each scan, up to 6 m in length, compared to the previous laser scans with a single circumferential pass. The results were compared with the data from 2004 and with empirical and numerical prediction curves for Excavation Damage Zone depths. The findings indicate that between 2004 and 2017, the measured DoF generally extended, on average, by 0.1 m, implying that failure has progressed with time. Furthermore, the analysis showed that the volume of failure from 2004 extended up to 1.9 m3/m, and this volume increased to 3.2 m3/m in 2017 on average. The VoF was used as a means to overcome the challenge of comparing past to present scan results, since the exact location of the 2004 measurements were not known. Significant variations in the measured DoF at sections of the adit near faults and below a glacier indicate that these external factors can significantly influence the rock mass behaviour. When comparing the DoF to empirical excavation damage zone prediction intervals, the level of damage at the Bedretto adit is comparable with the outer excavation damage zone. This is a region of isolated micro-cracking, and it conceptually suggests that the continued spalling in the Bedretto adit is the results of crack propagation from initial excavation induced damage that leads to interaction even after 41 years since excavation works ceased.