Subsidence

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The ranging poles show the subsidence in the Emscherbruch in Herten (near the Ewald 1/2/7 colliery) over the past 25 years (1980: −7.8 m; 1985: −3.8 m; 1990: −1.3 m; 2000 : Shutdown)

A subsidence is a local subsidence of the earth's surface that is a result of mining . In simplified terms, the subsidence can be described as the upper layers of the earth sliding down when the cavities that have formed close up after mining. In the long term, the extent of the subsidence therefore mostly roughly corresponds to the subsurface volume extracted by mining. In special cases, however, mining can also lead to an expansion of certain rock layers. This can then lead to a rise in the ground . Mountain subsidence can affect structures and change landscapes.

Basics

Subsidence diagram

When mining an underground deposit , a correspondingly large cavity remains after the mineral resource has been extracted . If the hanging wall is not supported , the overburden will break into the open cavity after a short time and fill it. This process spreads to the surface of the earth, so that after a certain time a compact mountain body is present again. A distinction is made between regular and irregular mountain subsidence. Regular subsidence is gradual and even, irregular subsidence is sudden and uneven up to the break of day . Regular subsidence is typical of deep mining , irregular subsidence is caused by near-day mining. The extent of the mountain subsidence depends on various factors. The decisive factor for the depth of the depression is whether the cavity was filled with backfill or whether the work was carried out with backfill. The mining method used is decisive for the form of the subsidence . When using mountain offset, it is compressed to 30 percent of its volume due to the rock pressure. In the case of site construction and chamber construction , the mountain fortresses initially prevent the mountain from subsiding ; if these mountain fortresses are stolen , the hanging wall is destroyed. If local mining is used in deep mining, regular subsidence occurs, whereas near-day mining produces irregular subsidence. With longwall mining there are regular subsidence. The expansion and excavation work leads to irregular subsidence, the mountains are in motion for an indefinite period of time and there is a constant risk of cavities and the formation of new layers of rock. In the construction of the sinking plant , large cavities are created that close over time, causing irregular subsidence and also day breaks.

The lowering trough

Different reduction currents

Due to the regular subsidence of the mountain, a subsidence trough, also called a subsidence trough, forms on the surface of the day. This depression trough moves on the surface behind the mining direction . This leads to horizontal and vertical shifts and upsets on the surface of the terrain. This leads to strong stresses in particular at the edges of the sink trough. The size of the sinking trough is determined not only by the size of the excavation field but also by the break angle. This break angle is determined by the natural slope angle of the mountain layers. It is flatter in soft rock layers and steeper in solid rock layers. Due to the break angle, the sink trough is larger than the actual excavation field was. In addition to the type of mountain, the location of the mining limits also has a significant influence on the fracture angle. In underground lignite mining, a fracture angle of 72 gons is usually  established. In the hard rock of the Ruhr area, the angle of fracture, taking into account the depth of the strata, is between 75.6 and 91 gon. In the Ore Mountains, the existing gneiss layers are based on a rupture angle of 77.8 gon. The depth of the sinking trough depends on the height of the seams mined ; it is between 0.5 times the seam height in the case of excavation and 0.9 times in the case of fracture.

Effects

Technical expansion of the Seseke became necessary due to subsidence

Due to the different thicknesses of the deposits, the subsidence is not equally strong at every point. This leads to regionally different subsidence, the height differences of which are often several meters, resulting in a non-uniform land subsidence. This has a great influence on the natural gradient of the rivers and streams in the respective region. Due to the timing of the subsidence, there is a change in the distances between the groundwater levels and the receiving waters. Due to the subsidence of the mountains, around 75,000 hectares of polder areas were created in the Ruhr area alone. In order not to flood entire stretches of land, the subsidence makes it necessary to artificially keep the water level of the receiving waters high. This is done through embankments, the lower lying water has to be pumped into the receiving waters. The subsidence often destroys water-bearing layers so that the groundwater can run away downwards. The subsidence of the mountains can also cloud the water in deep wells. Due to the fissures of the seam-bearing carbon, due to the subsidence of the mountains, methane outgassing can occur on the surface. The subsidence affects the infrastructure and buildings in built-up areas. Simple, regular subsidence is usually not a problem. Mountain subsidence in the area of ​​the edge zones is problematic, particularly in distorted areas.

Schedule

The mountain subsidence usually subsided after a few years. In the Ruhr area, subsidence is to be expected after a period of six months to three years after the mining has been completed. In Polish mining, the duration of movement is a maximum of five years. As a rule, 75 percent of the total reduction is achieved after the first year and 90 percent after the second year. The situation is different for near-surface mining and salt mining. In near-surface mining, movement of the overburden is to be expected at any time without any time limit.As a rule, in near-surface mining, subsidence of a few decimetres occurs, but in extreme cases there can still be day breaks long after the end of the mining activity come. In salt mining, subsidence can be expected within a period of up to 200 years after the end of mining.

Terrain assessment

In order to assess subsidence and subsequent subsidence forecasts, it is necessary to map the relevant terrain. First, a pre-evaluation is carried out using existing geological and hydrological maps. Topographical historical maps are also useful. With the help of these maps, geologists can already gain initial knowledge of whether the terrain has already been mined and what the rock formations of the terrain are like. Another way to assess the terrain is to evaluate aerial photographs. Using the different aerial images, comparisons between old and current images reveal changes in the relief of the terrain surface. In mining areas, the mine-siding cracks are used for assessment. Other procedures include checking the hydrological address and checking the biological changes in the terrain. Plants often react quite differently to changes in water conditions. The examination of the streets and paths and structures also allows an assessment. All knowledge gained is mapped and compared with one another.

Large-scale ground movements have been recorded in the Ruhr area since the 1980s, using aerophotogrammetric measurements. The terrain surface is initially divided into several large examination rooms, each examination area is then mapped by a point field. The changes in elevation of the terrain are determined by calculating the difference between the individual measuring points. Since subsidence is not only caused by mining activities, other non-mining subsidence is determined and excluded accordingly. The knowledge gained is evaluated with a special computer program.

Mountain subsidence forecasts

On the basis of the knowledge gained, mountain subsidence is forecast using mechanical and empirical models. In the case of regular subsidence, there are findings that can be derived from long-term observations. Assuming that the resulting cavity is not filled, the ratio of the maximum expected subsidence to the mined seam thickness is formed and defined as the subsidence factor. For the Ruhr area, the British and French coalfields and for most of the Russian coalfields, a subsidence factor k of 0.9 applies . In Great Britain, subsidence measurements were carried out in deep mining fields. Based on these measurements, empirical subsidence diagrams were developed with the aid of which the maximum depth of the subsidence trough that can be expected for the respective areas can be predicted. In addition, these diagrams can be used to show the dimensions and shapes of the sink trough. With the help of further diagrams, the expected compression and expansion of the earth's surface can also be estimated. Empirical subsidence diagrams can also be created for the mining districts of other countries.

literature

  • Helmut Kratzsch: mining damage. 5th updated and revised edition. Papierflieger Verlag, Clausthal-Zellerfeld 2008, ISBN 3-00-001661-9

Individual evidence

  1. ↑ Energy and Water Lexicon . Online ( Memento of October 17, 2013 in the Internet Archive ) DEW21 network;
  2. ↑ Lothar Scheidat: How does a mountain sink . In: Perspective on site; Bergwerk Ost, Online ( Memento from October 22, 2013 in the Internet Archive ) (PDF; 286 kB) Deutsche Steinkohle AG; (accessed August 12, 2013).
  3. ↑ a b c d Dieter D. Genske: Engineering geology basics and application. Springer Verlag, Berlin / Heidelberg 2006, ISBN 978-3-540-25756-1 .
  4. ^ A b Gustav Köhler: Textbook of mining history. 6th edition. Published by Wilhelm Engelmann, Leipzig 1903.
  5. ↑ a b c Fritz Heise, Fritz Herbst: Textbook of mining science with special consideration of hard coal mining. First volume. Published by Julius Springer, Berlin 1908.
  6. ↑ a b Günter Meier: To determine areas of influence caused by old mining. In: 9th Altbergbaukolloquium, Leoben 2009, online (PDF; 549 kB) (accessed May 10, 2011).
  7. ↑ Diethard E. Meyer: Geofactor human - interventions and consequences through the use of geopotential. Online (PDF; 641 kB) (accessed May 10, 2011).
  8. ^ City of Hamm (ed.): Something new in the west; Development concept for the Hammer West. April 2009, online  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. City of Hamm; (accessed April 30, 2015).@1@ 2Template: Toter Link / www.hamm.de  
  9. ↑ Dietmar Schulz: Ruhr mining and water, tailings and groundwater. Accessed online (PDF; 249 kB) May 10, 2011.
  10. ↑ Klaus Joachim Soiné: Manual water master. Useful information for the operation of water supply systems . DVGW, 4th edition. Oldenbourg, 1998, ISBN 3-486-26392-7 .
  11. ↑ Chamber of Architects of North Rhine-Westphalia (ed.): Is the building site safe? The old mining situation in NRW. Online ( Memento from May 25, 2010 in the Internet Archive ) (PDF), (accessed May 10, 2011).
  12. ^ A b Carl Hellmut Fritzsche: Textbook of mining science. Second volume, 10th edition. Springer Verlag, Berlin / Göttingen / Heidelberg 1962
  13. ↑ Werner Grigo, Michael Heitfeld, Peter Rosner, Andreas Welz: A concept for monitoring the effects of the rise in pit water in the Ruhr area. Online (PDF; 1.2 MB) Altbergbau-Kolloquium, Freiberg 2007, (accessed May 10, 2011).
  14. ↑ Andreas Streerath, Rainer Roosmann: Analysis and modeling of large-scale mining-related subsidence from photogrammetric observations. Online (PDF; 672 kB) (accessed May 13, 2011).