Seismicity at the surface

How can mining induced seismicity on the surface occur?

Deep underground mining removes layers of rock, leaving a void. The weight of the overlying layers may deform the cavities, may also cause a collapse, producing seismic waves. In some cases, the seismic waves reactivate existing faults causing minor ground movements.

To what extent can ground motion take place at individual sites?  "Are collapses" of many meters common?

An accurate location prediction of ground motion is not possible at the current state of planning. Mining takes place at great depths of 1000 meters which can lead to long-span hollows that have a maximum extraction of one to two meters in the center and run out to the edges. Separatory extraction funnels (pinging)are excluded. Extraction hoppers (pinging) only occur immediately below the ground surface where voids are created and then collapse unexpectedly. 

Sensitive surfaces are considered separately in the procedure. There are a variety of operational possibilities which influence ground motion. These include, amongst others, geometry degradation, mountain hard dimensioning, and the displacement.

Will there be subsidences?

Yes, because of the large rock pressures arising from the degradation depth of 1,000 meters underground. Operational measures can reduce subsidience, but not prevent it entirely.

Will individual houses or roads in Spremberg decline or cave in, as has been observed recently in other parts of Germany?

No, the towns, villages, and roads will not be impacted. Extractions take place over a large area, therefore rough edges will not occur.

Will KSL use the backfill method for the entire mining area? Why is, in this case, the impact on the environment not excluded?

There are various offset methods. In parts of the mining area dry offset will take place in order to control ground movements. Darren rock will be filled in the underground voids. Offset with material supply from the surface will be examined as a method for other parts of the mining area, regarding the technical and economic feasibility. The offset method reduces the risk of soil movement, but cannot completely prevent it. Due to the high convergences at great depths a complete offset with material supply from above ground is not possible.

How is it ensured that unexpected disasters do not occur, during degradation and that the extractions do not exceed certain sizes?

There are significant and reliable prediction methods that are used and they give advance information on ground motions.

The underground mining work is regularly measured and monitored. At the ground surface a variety of observation methods are used. This ensures that no unexpected changes occur. In addition, in the deposit, only one seam is mined, unlike mines in other mining districts which can help prevent unexpected large ground movements.

What impact will land subsidence have on agriculture and forestry?

In areas near groundwater subsidence can occur through extraction. This can either be consciously allowed which can possibly result in an increase in ecological diversity.  Or, especially in agricultural areas and roadways a variety of measures can prevent the subsidence.

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KSL