Construction work on the Thalwil flood relief tunnel is progressing rapidly. After the Tunnel boring machine (TBM) breakthrough in Thalwil was celebrated in November 2024, all work on the mining tunnel has now been completed. The excavated tunnel has an internal diameter of 6.60 metres. After the TBM had installed its 1,098 segment rings over a length of 1,976.4 metres, the 12-metre-long transition block to the stilling chamber in Thalwil had to be constructed from in-situ concrete. In this block, the profile changes from a circular profile to a U-profile with a flat base so that the maximum water volume of 330 m3/h flows hydraulically favourably into the stilling chamber during gravity flow. Even in the tunnel, which has a gradient of 3.3%, the water is slowed down by ribbed segments, thus destroying energy. In this 300-year flood, the water nevertheless reaches a speed of 50 km/h. It only takes about 2:30 minutes to travel from Sihltal to Thalwil.
Prior to the TBM breakthrough, the 12 m long tunnel section of the transition block was excavated conventionally by blasting under a settlement-minimising pipe umbrella. This successfully prevented possible settlement of the Zurich–Chur railway line, which is only 10 m above the tunnel. The tunnel block was reinforced and constructed using in-situ concrete. The geometry of the profile change was modelled with a very elaborate wooden formwork.
