Lake Thun repeatedly broke its banks when there was heavy rainfall or melting snow. The outflow channels were incapable of handling the huge volumes of water and, as a result, the region had been hit by severe flooding - most recently in 1999 and 2005. To prevent any recurrence, a relief tunnel was to be built. Below the water table throughout its length, the alignment of the 15-meter deep tunnel run from the shipping canal along the railroad line, crossing below the station forecourt and discharging into the River Aare downstream of the Thun power station. The relief tunnel would run only eight meters below underpasses. An additional challenge were the extremely restricted space in both the start and end shafts. The end shaft also had a heavily graded bottom.
From August 2007 to April 2008, the Herrenknecht Mixshield was tunnelling below the city of Thun. Having already been put through its paces in Cologne, the S-398 drove the 1,169-meter-long tunnel against the later direction of flow, i.e. upwards from the River Aare to the shipping channel. The Mixshield technology in the hands of a most competent jobsite team reliably prevented that, for example, the railway operation was affected by the tunnel construction underground. The tunnel was lined with reinforced concrete segments, the joints of which were sealed with neoprene tape. The inflow and outflow structures were constructed after TBM disassembly.