Doctor of Philosophy
Department of Geology
Stone, Ian J., Geological assessment of coal mine roof conditions, southern Sydney basin, Doctor of Philosophy thesis, Department of Geology, University of Wollongong, 1990. http://ro.uow.edu.au/theses/1394
Roof conditions in coal mines of the southern Sydney Basin, Australia, are typically poor due to the effect of a relatively high horizontal in situ stress field (horizontal to vertical stress ratio (ơ 1ơ 3) is approximately 2/1). The Permian Bulli Coal seam is mined from a portion of the basin which has undergone relatively little tectonic deformation apart from faulting and regional warping. The ơ 1 orientation acting on mine roadways is determined using the mining induced shear fractures and is comparable to results from in situ overcore measurements. The angle between ơ 1 and the mine roadway (0sr) together with the ơ 1 magnitude and the ơ 1/ơ 2 ratio defines the range of roof conditions to be expected in a mine roadway. A twelve point scale of stress induced roof failure has been developed to rank the severity of stress acting across the mine roadway. For a given stress field the roof conditions expected over the range of Osr is defined by a Roof Failure curve. A number of Roof Failure Curves are able to define roof conditions for a varying stress field. The distribution of Roof Failure Curves represents the relative changes in stress field intensity across the mapped area. Mapping techniques are able to show the variability of both relative stress field intensity and lateral stress field orientation. Successful management of the in situ stress effects in mine development drivages and around longwall blocks is crucial to the economic viability of coal mines in the southern Sydney Basin. To further understand the origin of the in situ stress field a technique was developed to measure strain anisotropy in the coal maceral vitrinite. Vitrinite mean maximum reflectance (Ŕo max), using oil immersion, was measured from oriented sections normal to bedding and results indicate that the vitrinite in medium volatile bituminous coals has biaxial optical properties. The range of true maximum reflectance (Ro max) of vitrinite in the study was between 1.04% and 1.48% reflectance. The orientation of the R max can be determined from the long axis of the elliptical Calculated Bedding Plane Section of the Indicating Surface (CBPSIS). The biaxial nature of the vitrinite is thought to result from asymmetrical growth of its molecular structure and to be related to stress fields which developed simultaneously with coalification. The R o max direction is formed normal to lateral compressive forces. The CBPSIS figure from the bituminous coals studied is not usually elliptical but commonly takes the shape of two superimposed ellipses and, therefore, has two R o max peak directions, indicating overprinting of successive stress field orientations. Results from studies of Ro max orientations indicate that they show palaeostress patterns around faulted areas (including small faults with <10m>displacement) consistent with fault formation, and are also able to show variation of regional palaeostress events. The Ro max directions and in situ stress directions were determined from study areas in different coal mines. Five palaeostress directions are identified, two of which were recognised in all of the study areas. The same two palaeostress directions are coincident with the two in situ lateral stress field directions (NNW and ENE) recognised within the study area. The ENE stress field is coincident with a palaeostress active during sedimentation, prior to uplift. The NNW stress field orientation has been linked with a major wrench faulting episode in the area, following the cessation of sedimentation, but during coalification. Vitrinite reflectance data is able to record stress field events which have been_imprinted and locked into strata around the Bulli Coal seam, indicating that the in situ stress field acting in the southern Sydney Basin has a strong residual component. The study shows that the use of vitrinite as a tectonic fabric indicator is viable in weakly deformed terrain and is able to provide information concerning palaeostress regimes.