Time dependent properties of sandstones and their effects on mine stability

Abstract. Large scale stress redistribution around longwall panels in coal mines puts rock masses in the vicinity of the underground excavations close to failure. While immediate failure is reflected for example by instantaneous seismic events, there is also a delayed response of the rock mass as noted from decaying seismicity during non-operating times. Sandstone samples from the hanging wall of a coal seam in the Ruhr coal mining district in Germany have been subjected to conventional strength, creep and relaxation tests. From creep and relaxation tests estimates of time dependent strength properties are derived. Numerical modeling was employed to delineate zones of states of stress around underground excavations which are prone to time dependent failure.