Background: Micro-seismic events in the Asse overburden

Between 19 and 27 January 2011, a microseismic incident accumulation occurred in the overburden in front of mining districts 3 and 4 in a depth of ca. 500 metres. The number of incidents increased further until 31 January.

The staff and the operational safety are not at risk. Similar occurrences were also observed in the past.

As a result of the high excavation ratio and the so far insufficient backfilling of many cavities in the southern flank of the Asse mine, the salt rock in the Asse mine is currently deforming by up to ca. 15 cm per year. These salt rock deformations also damages the rock layers outside the salt structure, the so-called overburden. Built-up tensions discharge in microseismic incidents, and cracks in the rock generate as a consequence.

The now observed incidents are located in the rock layer of the upper new red sandstone, some incidents were also detected in the Röt anhydrite. In the area where the incidents have occurred there is a cross fault.

This development confirms the results from the rock-mechanical state analysis and the prognoses of the BfS and the commissioned experts. The fact that these damaging processes continue to exist and are progressing shows the need to stabilise the Asse II mine. Therefore, the BfS has already started to carry out comprehensive backfilling measures, the so-called backfilling of roof clefts.

The concentration of these fracture processes at a cross fault increases, however, also the possibility that layers that are permeable to water get contact to the mine. This development confirms the need to implement precautionary measures against an enhanced inflow of groundwater and the establishment of emergency preparedness as quickly as possible.