Rock mechanical investigations of seismic events in a deep longwall coal mine

Seismic events, i.e., fractures in rock that radiate detectable seismic waves, are a worldwide phenomena associated with mining and reservoir exploitation. Longwall mining in German coal mines often causes seismic events, which can be felt at surface where they can lead to a considerable annoyance of residents and sometimes even lead to minor damages of buildings and infrastructure. Strong seismic events are in a large part responsible for a more and more decreasing community acceptance of mining. The mines therefore have to investigate the local conditions, which cause the generation of seismic events, to find ways of avoiding strong mining induced seismic events. This paper describes the research on mining-induced seismicity around a specific active longwall at 1100 m depth under several previously mined longwall panels. Seismic events are recorded by a local network featuring 15 surface and six subsurface stations. Location and fault plane solutions show the occurrence of at least three different event types, which are characterized by distinct modes of failure. Underground core drilling and an extensive rock testing program are used to build up a rock mechanical model featuring a multilayer strata for use with numerical analyses. The numerical model is able to explain the different failure mechanisms as fault activation, remnant pillar punching and beam failure. The use of rock mass classification, rock mass strength criteria and in situ stresses together with 2D and 3D numerical analyses facilitates the evaluation of the results from seismology approaches, and makes future analysis of longwall-related seismic events possible. (C) 2008 Elsevier Ltd. All rights reserved.