Located in the idyllic Koenigs-winkel region between Fuessen, Pfronten and Schwangau in South Germany, the Forggensee lake stretches over an area of about 15.2 km2. It is Bavaria’s fifth largest lake and Germany’s largest water reservoir by surface area. The reservoir through which the River Lech flows generates electricity and provides flood protection from the water produced from melting snow in the Alps. Completed in the 1950s, the Rosshaupten barrage forms the northern dam of the lake and at the same time provides the inflow for the hydroelectric power plant, generating 45 MW of hydroelectric power from a drop height of 35.4 m.
To prepare the power plant and thus the flood protection for the future, work commenced in May 2018 to improve the dam’s watertightness. For this reason, the power plant operator Uniper Kraftwerke GmbH commissioned BAUER Spezialtiefbau GmbH, Schrobenhausen/Germany, to construct a diaphragm wall of 13,500 m2 to seal the dam (Figure 1). Special challenges for this project: the diaphragm wall can only be constructed from the 11 m wide, very narrow dam crest and is not located in the middle of the dam, but unfavorably offset on the side of the dam crest. In order to be able to carry out the order despite the difficult parameters, the construction of the diaphragm wall will be carried out with the help of an HDS-T turnable hose drum system from Bauer.
The 1 m wide and 70 m deep diaphragm wall is being constructed in two stages. First the upper 40 m of the dam body will be excavated using a DHG diaphragm wall grab on a BAUER MC 64 duty-cycle crane. The remaining 30 m below the actual dam fill will be removed using a cutter BC 40.2 on a MC 96 duty-cycle crane, because “the natural local bedrock consists of many and often very thin layers of clay stone, marl clay and sandstone.
Another special feature of the project is the guide wall for the execution of the diaphragm wall work. A Mixed-in-Place wall approximately 550 m wide and 8 m deep has already been installed this summer using cast-in-situ concrete casing to ensure optimal guidance of the machines during work. The work on the diaphragm wall by Bauer will also be carried out seven days a week, in day and night shifts, i. e. 24 h/d, and should be completed in spring 2019.