Flood protection in a sinkhole area

The „Angry Seven“ is now safely sealed

The name of the small river to the west of Eisleben, Germany is „Böse Sieben”. The name literally means „Angry Seven”, which (accurately) suggests that the waterway does not always trickle along harmlessly. Floods and lateral seepage have often led to erosion. As a result, sinkholes have formed in the karstic subsoil. Thanks to a holistic geosynthetic concept, protective nonwovensgeogridsdrainage mats and bentonite mats now provide a secure and permanent stable seal/barrier.

Until the 19th century, the „Böse Sieben” was known as Willerbach („Wild Brook”), but this was just another name for the same phenomenon. Several small streams come down from the eastern foothills of the Harz Mountains and flow into the „Böse Sieben”. When it rains heavily, the water quickly gathers and forms a raging river.

The extremely karstic subsoil poses a major challenge. A gypsum horizon (main anhydrite) reaches up to 1-2m below ground level and easily leaches out. Water seepage from the stream causes sinkholes and slope failures. According to information from the Halle State Office for Geology and Mining, sinkholes of up to 3m in diameter are expected.

As early as the 1980s, 5m long steel half shells were installed and welded together for faster drainage and to bridge sinkholes. They formed a 492m long half pipe. But, when the capacity of the half-pipe was exceeded, water washed out and seeped into the sides of the steel half-shells. Also, the lateral water inflow from springs and water-bearing soil layers was not captured by the system. In addition, the ecological continuity with the smooth steel channel was not given.

Goal: Sealed river profile

In recent years, the development of sinkholes has increased. It created an acute need for action on the part of the Saxony-Anhalt State Office for Flood Protection and Water Management (LHW). The aim of the current construction measure, which will last throughout 2021, is to ensure the discharge of the Böse Sieben up to a design flood HQ100 within a sealed river profile. In the process, the steel half-shells will be replaced by more sustainable materials in their function as sealing elements. A fill of armour stone and the flushing in of bed substrate will restore the ecological continuity to the near-natural state above and below the 657m long extension section.

The only solution: geosynthetics

It was clear at an early stage that only a geosynthetic solution could optimally fulfil all requirements. Naue was therefore already involved in the planning process by the responsible planning office, Ingenieurgemeinschaft WTU GmbH. The decisive advantage of geosynthetics in this context is their lightweight construction. The extensive requirements with sealing, reinforcement and drainage could not have been realised with mineral materials within an economically justifiable framework. The structure would have been too high. In addition, the installation of bulk materials would have been very costly, if only because of the spatial situation. With geosynthetics, on the other hand, hundreds of square meters of materials are delivered to the construction site in rolls and on a minimum number of trucks (compared to the hundreds of trucks that would have been necessary to deliver mineral sealing elements). The geosynthetic solution is more ecological and considerably cheaper than it would be to remove, transport and install a conventional mineral seal.

The result of the planning was the use of four different geosynthetics:

  • Nonwovens as protection and separation layers
  • Geogrids for reinforcement
  • Drainage mats for the drainage of seepage water
  • Bentonite mats for sealing

The construction of the sinkhole with geosynthetics