TBM ready to start on Sarner Aa tunnel

Following its pre-assembly at the Klus works yard in Balsthal, the refurbished TBM was delivered to the site in parts in 29 truckloads. The parts were lowered into the 40 x 20m excavation pit by the second largest crane in Switzerland, the LR1750/2 – a process completed on 10 November.

On 2 December, the 900t TBM was operated for the first time with the spoil brought out in skips, given the conveyor belt was not yet assembled. Installation of the conveyor – which will operate at an angle of 12.7 degrees – is expected to be completed at the beginning of January 2021.

The 6.5m-diameter TBM was previously used on the Linth Limmern hydro plant in the Swiss canton of Glarus but has been refurbished extensively for the project by Marti and Marti Technik. The TBM is expected to start mining on January 11 when it will begin its journey through the mountain at an anticipated rate of 20m/day. The 6.5km bore is expected to be completed in June 2022 when the machine will reach the intake structure at Sachseln.

The Sarner Aa Flood Relief Project is designed to alleviate the severe flooding which occurs in the canton of Obwalden when the River Sarner Aa breaks its banks. As part of the project, Marti Tunnel is building the project’s main component – a 6.5km bypass tunnel between Sachseln and Lake Wichelsee. The outlet structure stands below Lake Wichelsee and will drain water directly into the Sarner Aa.

The project includes an 80m-deep shaft constructed by the raise-boring method. When complete, the tunnel is expected to drain water at a rate of 120m3/sec. The scheme is a total contractor project by ARGE Marti HWS comprising Marti Tunnel and Marti Luzern.