Frictional strength of discontinuities (joints and faults) is the key rock mass property for analyzing mining induced seismicity or rock slope stability. This paper reports about laboratory tests on discontinuities from coal bearing strata in the Ruhr area. Shear tests have been conducted on discontinuities in shale and sandstones. These were partly weathered and sometimes coated by coal. Friction angles range from 10° for coal-coated joints in sandstone to 40° clean joints in sandstone. The discontinuities were scanned with a 3D laser scanner for estimating the surface roughness. JRC was computed from scan profiles and compared to manual measurements. Implications for fault stability around coal mines are delineated.
It has been shown (Alber 2013) that the frictional strength of discontinuities (joints and faults) is the key rock mass property for analyzing mining induced seismicity. Up to now, however, frictional properties were only back-calculated from past events, i.e. by combining fault plane solutions with numerical modeling. Back-calculations from seismic events in the Saar coal mining district yielded friction angles as low as 9° (Alber & Fritschen 2011). For the in-situ stresses in the Ruhr mining district it was concluded that friction angles should be higher than 20°. As coal mining ceases in Germany mine working will be (partly) flooded and faults may be re-activated, leading to induced- seismicity caused by increasing fluid-pressure on discontinuities.
A first hint for such a case may be the 2 earthquakes of magnitudes ML 2.7 and 2.0, respectively, at the abandoned coal mine Ensdorf (Saar) on Sep. 15 and Oct. 10, 2014. It is not clear how the pore pressures at depth developed after mine-closure in 2008. Yet, the ongoing stress-release in form of seismicity makes a strong case for proper characterization of discontinuities.
Estimating the strength of discontinuities is mainly done through laboratory shear tests and/or index tests. In-situ shear tests are rarely executed so that large scale discontinuity properties stem from back-calculations. This paper summarizes results from laboratory shear tests on weathered and coal-coated rock joint in coal measure rocks from the Ruhr area and elaborates on the use of laser-scans for evaluating roughness.
