Break­through at Lake Sarnen
Break­through at Lake Sarnen

Break­through at Lake Sarnen

In the past, the Sarner Aa Valley in Ob­walden has in­creasingly been the victim of severe weather events. There­fore, to protect against further flooding, the Marti Group is building a flood relief gallery on behalf of the canton of Obwalden. Now, with the break­through of the tunnel boring machine at Lake Sarnen, driving operations of the 6.5-km-long tunnel has been completed.

March 15, 2023, shortly after nine o’clock. The invited guests have gathered in the finish tube at the in­take structure on Lake Sarnen. This 90-m-long gallery was ex­cavated by blasting is located below the water level, but a sheet pile wall con­struction safely seals the structure. A few minutes later, the tunnel boring machine (TBM) starts rumbling. Only about one meter of rock se­parates the machine from the finish tube. Just under 20 minutes later, it is done – the machine breaks through the working face and the rotating drill head comes to a halt. Only a few moments later, the man­hole in the drill head is opened and the first miner squeezes through the narrow opening. In his arms is the figure of St. Barbara, the patron saint of tunneling crew. To great cheers, he hands over the St. Barbara figure to his colleague waiting on the other side, as is tradition. One by one, the rest of the team follows, climbing with beaming faces through the man­hole to the waiting crowd. Relief and pride spread. Because the team has done a great job with the tunnel boring machine in the last 27 months. And that is being duly celebrated today.

The Sarner Aa Flood Relief Gallery East

This new flood relief gallery runs from Lake Sarnen to Alpnach, where it ends below the Wichel­see lake. The gallery is about 6.5 km long and will significantly in­crease (about 100 m³/s) the dis­charge capacity of Lake Sarnen in the future, thus re­gulating the water flow through the Sarner Aa River. This is be­cause the Sarner Aa Valley, located in the canton of Obwalden be­tween Lake Sarner and the Alpnacher­see lake, has been hit by severe weather events several times. During the great Alpine flood of 2005, for ex­ample, the Sarner Aa flooded large areas of land here, including the village of Sarnen. 

Everyone can rely on each other here. Every­one wants to build this tunnel; every­one has the same goal. What a great team!
Dominic Stadlin,
Construction site manager, Marti Tunnel AG

Challenging Driving Operations

After the construction of the launch pit in Alpnach, below the Wichel­see dam, Marti used blasting to build the launch tube of around 190 m. This was followed by the assembly of the tunnel boring machine (TBM) in autumn 2020 and in January 2021, the team started the TBM driving opera­tions. The path to­wards Lake Sarnen held some un­expected challenges in store. Al­ready on the first few meters, the Schratten­kalk turned out to be much harder over long distances than originally anti­cipated. Instead of the ex­pected 90 to 120 mega­pascals (MPa), the TBM had to cope with up to 270 MPa. In addition, there were count­less fracture zones, which caused severe damage, especially to the roller chisels on the drill head. From tunnel meters 1,359 to 2,012, the miners also en­countered aquiferous karst systems. Constantly over 400 liters per second entered the tunnel. In case of heavy rain­fall even up to more than 650 l/s. The in­coming mountain water could be drained off via pump sumps in the clean water pipe­line and re­introduced into the Sarner Aa at the Alpnach portal. The rest of the way, the miners continually struggled with the difficult geo­logy and the con­sequent­ially in­consistent rock, which often had to be secured with re­inforced nets, longer anchors and additional steel arches. This further delayed the TBM’s driving operation speed. “Our team is doing a really tough job here,” says Dominic Stadlin, construction site manager at ARGE HWS Marti. This makes the co­operation in the long-standing and well-rehearsed team all the more important. Thank for your commitment and good fortune for future projects!

the stakeholders include