Tunnelling in a landslide – Technical challenges and monitoring

The Riedberg tunnel, part of the A9 highway in Switzerland, traverses an active landslide slope with movement rates of several centimetres to decimeters per year. This paper discusses the investigations and construction measures implemented to manage the slope’s behaviour. Long-term monitoring revealed that movement rates vary along the tunnel, heavily influenced by water inflow, especially heavy rainfall. Based on these findings, the tunnel's inner lining and design were optimised for flexibility and durability. Continuous monitoring of the slope and the tunnel during construction and the future operation tracks slope movements and tunnel stresses, guiding further measures to dewater the slope and enhance stability. Given the correlation between slope movements and water infiltration, further measures are also planned to dewater the slope, aiming to reduce movement rates and enhance the stability of the slope and tunnel.