Mechanized tunnelling 40 degrees uphill – an unusual mission for a tunnel boring machine. A Herrenknecht Gripper TBM with a diameter of 5,200 mm is safely equipped to excavate this steep route for the construction of two pressure tunnels for the new pump storage power plant in Limmern, in the Swiss canton of Glarus. The 40 degree climb through the mountain for two 1,030 meter long tunnels is based on an innovative safety concept in the design of the TBM. On October 14, 2011 the construction site team celebrated the first breakthrough to the target cavern of the future service chamber at 2,300 meters above sea level. The top weekly perfomances were more than 130 meters.

Limmern, Switzerland / Schwanau, Germany, October 15, 2011. The Muttsee, Tierfehd and Linthal‎ (Canton of Glarus, Eastern Switzerland) hydroelectric power plants of Kraftwerke Linth-Limmern AG currently generate a total of 480 MW of clean energy. As part of the »Linthal2015« project, the new Limmern pump-fed power station will raise the capacity by 1,000 MW. Construction of the new power plant is being carried out by the consortium ARGE Kraftwerk Limmern under the leadership of contractor Marti Tunnelbau AG. The core infrastructure of the major project is the excavation of two pressure tunnels, each 1,030 m long. On October 14, 2011 the Herrenknecht Gripper TBM (diameter 5,200 mm) successfully completed the mechanized excavation of the first tunnel.

The two pressure tunnels are bored from a cavern equipped with a pump turbine at 1,700 meters above sea level with a route gradient of 40° to the 600 meter higher service chamber on the shores of the Muttsee lake. The tunnel route runs predominantly through Quintner limestone with an overburdon of up to 565 m. The rock strengths are up to 120 MPa. A route gradient of 40° (84.7%) is a particularly unique challenge for both the machine technology and the project team.

In September 2009 Marti Tunnelbau AG ordered a specially designed tunnel boring machine (Gripper TBM) from Herrenknecht for this mission. It has a boring diameter of 5,200 mm and cutterhead power of 2,205 kW. During tunnelling, with two gripper plates a Gripper TBM braces itself sideways against the rock of the tunnel already excavated. Hydraulic cylinders press the rotating cutterhead against the tunnel face, which is crushed by the disc cutters. After completion of a stroke the gripper plates are tightened and braced again. The enormous gradient associated with the Limmern project requires an extremely reliable safety concept that absolutely makes certain the TBM cannot slip back when the grippers are moved. Previously special gripper TBMs were used here with a single anti-reverse lock.

The construction company Marti Tunnelbau AG and Herrenknecht AG have gone a step further. They have developed a double anti-reverse lock with full backup redundancy of the available bracing levels for the 130 meter long and 800 tonne TBM. This significantly increases safety for man, machine and structure: in all operating states (advance, standstill or regripping) there are always at least two of three locking systems independently – and thus absolutely safely – braced against the mountain. A slipping back of the machine can be reliably prevented. The anti-reversing locking systems work mechanically on the basis of a self-locking toggle lever (automatic mechanical wedging). This means that even in the case of a failure in the energy supply and the hydraulic systems the necessary bracing of the machine against the mountain is assured.

The tunnel boring machine was fully assembled and tested in the Herrenknecht plant. On June 22, 2010, nine months after the contract was awarded, the workshop testing of the Herrenknecht Gripper TBM S-575 with the engineers of Marti Tunnelbau AG was completed in Schwanau. When transporting the machine to the site, a very unique challenge was waiting on the last section. The machine components had to be transported by funicular from Tierfehd (about 800 m above sea level) to an altitude of 1,800 m above sea level. When the first design drawings were prepared and later when the machine was dismantled in the plant, it had to be ensured that none of the components exceeded the maximum permissible weight of 37 tonnes.

After the beginning of tunnelling by the Herrenknecht Gripper in November 2010 and top weekly performances of more than 130 meters, the construction site team finally celebrated the first breakthrough to the future service chamber at 2,300 meters above sea level on October 14, 2011. Excavation of the second tunnel is due to start in March 2012. The power plant is to be commissioned in the year 2015.