Ground elevation

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In mining damage science, the elevation of the surface that is emphasized by mining activities is referred to as ground elevation , also terrain elevation , or simply elevation . This elevation of the ground can, depending on the local situation, lead to mountain damage . Particularly problematic are highlighting differences in the range of above ground outgoing Unstetigkeitszonen , especially if these areas are farmed.

First findings and conclusions

At the beginning of the 20th century, a few centimeters high ground elevation can be observed at the edge of several subsidence troughs. At the time it was thought that this uplift was an elastic bulge of a bent sandstone layer or a tilting movement of mountain blocks. In 1940 the phenomenon of soil uplift due to the rise in mine water in the area of ​​the Wittener Mulde was described. In the years 2001 to 2005 several papers were published in which the problem of the uplift caused by the rise in pit water in the Erkelenz mountain area was pointed out. In 2007, the mining authority of the State of North Rhine-Westphalia commissioned an expert opinion on possible changes to mine water levels in the Ruhr area. This report was intended to examine the possible effects of the rise in pit water on the various protected assets. The appraisal recommended the creation of a cadastre, with which knowledge about discontinuities in the individual mine fields should be recorded centrally.

Elevation by rise in pit water

If the dewatering is switched off after a mine has been shut down, depending on the local conditions, the mine water will rise to a greater or lesser extent . As a result, the mine workings are gradually flooded. The remaining weather will gradually be displaced by the water from the mine workings. The weight of the water column that now arises then loads what is lying on the ground and causes the open pits below to sink. At the same time, the water pressure also affects the hanging wall . Due to the water pressure acting upwards, the excavation cavities that still exist are widened somewhat and the as yet uncompacted backfill is relieved. The rising pit water initially affects the groundwater balance in the overburden . The rising pit water can also have a negative effect on the surface of the day. The rock layers above are slightly compressed by the differential pressure. If the upper rock horizons consist of alternating layers of clay and loose sand , the shape and location of these are influenced by the pit water. By entering the mine water washed by water sand grains are now distended . The clay layer below the sand layer is relieved and swells up. The reverse process can then occur as with a mountain subsidence . The rising pit water ultimately leads to a rise in the ground. These ground elevations can be continuous or discontinuous. The height of the ground elevation varies depending on the local conditions. It can be assumed that the uplift is around 2–3 percent of the previous subsidence. Thus their value is between a few centimeters to a few decimeters.

Elevation through geothermal drilling

With deep geothermal wells it can happen that the well is led through an anhydrite layer. If the water now rises from a drilled water reservoir underneath, it also comes into contact with the anhydrite. Permanent contact with water triggers a chemical process in the anhydrite, which leads to mineral remodeling and mineral regeneration and the anhydrite is converted into gypsum . This leads to an increase in volume. In theory, this is around 17 percent in all directions, making a total of around 61 percent. The swelling of the rock leads to an increase in pressure, the so-called swelling pressure. If the swelling pressure is higher than the imposing rock pressure and if there are no rock layers above the swelling rock layer that can compensate for the increase in volume, the terrain surface will rise. Since the water uptake by the anhydrite takes place only gradually, the swelling and thus the uplift takes place over a longer period of time. How strong this uplift is depends not only on the thickness of the anhydrite layer but also on the thickness of the overlying mountain layers. Thin layers of alternating layers and layers of marl with a finely divided anhydrite content of around five percent or more are particularly susceptible to these transformation processes . But certain clay minerals also tend to swell strongly.

Examples of consequential damages

The first damage to buildings caused by uplifts appeared in 2000. After the closure of the Sophia-Jacoba colliery in 1987, damage to several buildings was noticed in several locations in the cities of Hückelhoven and Wasserberg over a distance of nine kilometers. Strong cracks were noticed in the old town hall in particular. However, this building damage could not be assigned to mining- related subsidence. Investigations showed that this damage is attributable to the uplift caused by the flooding of the mine workings by Sophia Jacoba.

In 2007, in Staufen im Breisgau, cracks appeared in the old town hall shortly after a deep geothermal well . This damage can be attributed to an uplift that arose as a result of a drilling for the use of geothermal energy for the town hall.

Individual evidence

  1. ^ A b Peter Rosner, Michael Heitfeld, Volker Spreckels, Peter Vosen: Effects of terrain elevations in the course of the rise in mine water in the Ruhr area . In: 14th Altbergbaukolloqium, Gelsenkirchen 2014, online article (accessed on November 20, 2015).
  2. ↑ a b c Volker Baglikow: Rise in mine water in hard coal areas - effects on the surface . In: Ring Deutscher Bergingenieure eV (Hrsg.): Mining. Makossa Druck und Medien GmbH, January 2012, Gelsenkirchen 2012, pp. 16–21.
  3. ↑ a b c d e f g Axel Preuße, Jörg Krämer, Anton Sroka: Technical assessment of the consequential loads of hard coal mining . In: Ring Deutscher Bergingenieure eV (Hrsg.): Mining. Makossa Druck und Medien GmbH, December 2007, Gelsenkirchen 207, pp. 540-544.
  4. ↑ a b c Helmut Kratzsch: Bergschadenskunde . Springer Verlag Berlin / Heidelberg / New York, Berlin 1974, pp. 150–151.
  5. ↑ a b Peter Rosner: The rise in pit water in the Aachen and South Limburg coalfields - a hydrogeological-mining analysis of the interdependencies . Dissertation at the RWTH Aachen University, Aachen 2011, pp. 162–167.
  6. ↑ Federal Ministry of Economics and Technology (BMWI) (ed.): Mining in the Federal Republic of Germany in 2011 . 63rd year, Berlin 2012, pp. 42–45.
  7. ↑ a b c Werner Grigo, Michael Heitfeld, Peter Rosner, Andreas Welz: A concept for monitoring the effects of the rise in mine water in the Ruhr area . In: 7th Altbergbau-Kolloquium, Freiberg 2007, VGE Verlag GmbH, Essen 2007, pp. 250-252.
  8. ↑ a b c d e f AD-HOC working group geology / geothermal energy: Geothermal projects in the federal states . Technical report of the federal / state working group of the geological services, pp. 10–12.
  9. ↑ a b Heinrich Otto Buja: Handbook of drilling technology, shallow, deep, geothermal and horizontal drilling . 2nd edition, Books on Demand, Norderstedt 2012, ISBN 978-3-7357-3409-9 , pp. 535-545.
  10. ↑ a b c d e Helmut Prinz, Roland Strauss: Engineering Geology . 5th edited and expanded edition, Spektrum Akademischer Verlag, Heidelberg 2011, ISBN 978-3-8274-2472-3 , pp. 65–66.