Large scale stress redistribution around longwall panels in coal mines put rock masses in 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 coal seam in the Ruhr coal mining district in Germany have been subjected to conventional strength, creep and relaxation tests, respectively. From creep and relaxation tests estimates of time dependent strength properties are derived. It was found from creep tests that below a certain stress or strain level the samples showed negligible creep as evaluated by inspection of the axial, lateral and volumetric strain over time. This stress level in the order of 75 % of the failure stress was used to delineate the potential for delayed failure around the under excavation. Strain rates increase significantly above this stress level and will eventually lead to the short term failure of the rock. Numerical modeling was employed to delineate zones of states of stress around underground longwall coal panels at depth of approximately 1200 m which are prone to time dependent failure.
The knowledge about time dependent deformation in a mining environment is necessary to ensure safe and economic working conditions for miners. The temporal pattern of seismic events reflects the time dependent behavior of rock under excavation- induced stress changes. The need for understanding time dependent rock behavior as well as the need for a simple, mine-proof tool for estimating the time of stable rock conditions is evident. There exists a vast body of knowledge on time dependent behavior of salt rock as well as many theoretical approaches towards that problem (Cristescu, 1993) but little is known for hard rocks.
