Tunnel breakthrough in the Sihl Valley: precision and teamwork to safeguard against flooding in the region
A large part of Zurich’s city centre is situated on the alluvial cone of the Sihl river. Were the Sihl to break its banks, massive flood damage could result. The city narrowly escaped disaster in 2005, prompting the Zurich cantonal government to commission the construction of a drainage relief tunnel. Its purpose in the event of high water is to divert the excess from the Sihl directly into Lake Zurich and thus protect the city centre from flooding.
A milestone for flood protection in the canton of Zurich
The tunnel, two kilometres long and over six metres in diameter, has capability to drain up to 330,000 litres of water per second into Lake Zurich. This would save lives and prevent billions in economic losses.
After seven months of intensive work, the construction consortium led by Marti was able to celebrate the tunnel breakthrough. Advancing by more than 20 metres per day on average, the team has rapidly created a precise, ready-for-use tunnel tube that from 2026 will protect the lower Sihl valley and the city of Zurich against the threat of high water peaks.
A masterpiece of modern tunnelling technology
The drainage relief tunnel breakthrough marks a significant milestone in this challenging construction project. Since August, a tunnel boring machine (TBM) with a drilling diameter of 7.45 metres and specially designed for the geological conditions has been progressing tirelessly along the stretch of around 2 kilometres. Thanks to state-of-the-art technology, including a high-performance cutter head and a drive system with over 3,800 kW of power, it did an exact and efficient job of handling the freshwater molasse of the Sihl region.
For this project, the Marti Group worked with various Group companies including Marti Tunnel, Marti Zurich, Marti Solothurn and Marti Technik, plus other specialists. At times, there were over 50 Marti employees working directly on the construction site.
“The breakthrough is a milestone for flood protection and proof of the outstanding teamwork of everyone involved.”
Challenges surmounted
Construction of the drainage relief tunnel was not only technically demanding, but also required a tremendous amount of logistical coordination. From delivery and setting up of the TBM to driving the tunnel, each stage demanded precise planning and flawless coordination among the teams involved. The result is a tunnel tube that meets the highest quality standards and was created in the shortest possible time while conserving resources.
Work is now focusing on the inlet and outlet structures, which ensure that the tunnel can best fulfil its flood protection function.