The use of former opencast mines as lakes has a long tradition in the Rhenish lignite mining area, as does agricultural and forest rehabilitation. There are 57 lakes, that have been created from the final mine voids of a multitude of small-scale opencast mines, varying in size between 0.2 ha and 100 ha. Added to this are lakes that were planned specifically for reasons of landscape preservation in the context of a recultivation scheme (known as landscape lakes). All lakes are used today for a variety of different purposes, i.e. nature conservation, recreation and aquatic sports.
With the operation concentrated on a small number of large-scale opencast mines on the one hand and complete backfilling of opencast mines in the central part of the mining area with boxcut-masses from newer opencast mines (mainly Hambach) on the other hand, a concept was developed for the entire mining area which provides for the creation of three large opencast mine lakes in the active opencast mines of Inden, Garzweiler and Hambach. The planned lake sizes vary between 12 km2 and 40 km2 at depths of between 180 m and 330 m.
In the creation scheme for the opencast mine lake slopes with a general inclination of 1:5 (below the future wave braking zone) by using main mine equipment are planned. The area where the final water levels will later connected with the groundwater level is to be turned into a wave zone with a minimum width of 100 m and a compensation inclination of between 1:20 and 1:25. Since the lakes do not have a reservoir function and are not situated higher than the surrounding terrain either, the requirements of DIN 19700 (‘Dam plants’) are not relevant. Instead, geotechnical dimensioning of the slopes is carried out in compliance with the Guideline for Stability Analyses (RfS) issued by the Arnsberg regional government as NRW’s competent mining authority.
The filling scheme for the planned opencast mine lakes provides for one-time filling with external water from local streams and rivers. It is flanked by a purposeful continuation of opencast mine dewatering, preventing seepage pressures from the slopes in the direction to the lake.
The stability-related boundary conditions were already taken into account when the slope design was decided on, with seismic impacts also being considered in accordance with the requirements of the RfS. This was done by means of a quasi-static approach according to GOLDSCHEIDER that has been further developed and takes account of the acceleration effects of seismic impacts on the mass of the pore water as well.
A current field of investigation in line with an RfS requirement is the verification of the safety of lake slopes constructed by backfilling against possible liquefaction effects resulting from seismic impacts. A corresponding verification procedure is at present being developed in conjunction with Prof. Triantafyllidis at the Institute of Soil and Rock Mechanics (IBF) of the Karlsruhe Institute of Technology (KIT). It consists of a combination of field investigations, technical-centre and laboratory tests as well as numerical calculations. By comparing the dependency, derived from these tests, of the cyclic stress ratio with shear wave velocity and cone-penetration-test tip pressure with cyclic impacts determined by means of numerical calculations, we will in future be able to establish safety against liquefaction effects with sufficient reliability.
