Geotechnical Stabilization Works at Knappensee

This contract involves essential construction services for the final stabilization of unsecured areas as part of geotechnical hazard prevention measures at Knappensee. The work includes: (1) Geotechnical stabilization of the "landslide basin," profiling of the eastern slope, land filling in area F, and restoration of the Knappensee circular path and the Koblenzer Graben; (2) Construction of a protective structure on the southern island in Knappensee to mitigate potential wave impacts on the E1 shoreline. There are two performance descriptions and execution plans for the two parts of the construction work, which are interdependent unless a specific order is mandated by geotechnical safety requirements. This is a comprehensive contract due to the technological dependencies mentioned in the performance descriptions. Since the establishment of the restricted area on May 1, 2014, hazard prevention measures have been implemented to secure the shorelines, with over half of the threatened areas already secured. A landslide event occurred on March 11, 2021, resulting in approximately 1 million cubic meters of material collapsing into Knappensee. The resulting landslide basin has a semicircular structure approximately 300 meters wide and 370 meters deep relative to the original shoreline. The eastern shoreline is directly affected by the landslide, with heights of approximately 20 meters on the slopes and 7 meters above water and 4 meters below water on the slope-parallel breaks. The entire eastern shoreline is impacted either directly or indirectly by the landslide event. A geotechnical expert has developed a concept for initial stabilization measures, including the installation of additional pore water pressure sensors and a barrier south of the settlement "An der Knappenhütte." The work to create a protective fill was completed by the end of July 2024. The northern area of the landslide basin is currently being secured. The first part of the work involves constructing a hidden dam and filling the landslide basin. The second part addresses the latent landslide risk on the islands in Knappensee and the adjacent small island, with a focus on protecting the publicly used shoreline E1 from wave impacts. The construction of the protective structure will commence only after the completion of the related geotechnical work on the eastern slope of Knappensee.