Landslides in the Sydney Basin - Geoscience Australia

Seismic Hazard in SydneyProceedings of the one day workshopSydneyAppin, Towerand West CliffCollieriesFigure 2. Shuttle radar image showing location of Appin, Tower and West Cliff Collieriessouthwest of Sydney. The north-south trending Lapstone Structural Complex is evident to thenorthwest and continues to the west of the area of mining. Complex NW trending normal faultsystems with throws of several tens of metres extend into the area from the coast. (Imagecompiled from http://jaeger.earthsci.unimelb.edu.au/Images/Landform/landform.html)Longwall mining extracts most of the coal seam. In response to the mining, the overlying stratafracture and displace (cave) into the void that is created. To a height of about 40 m above the seam,the strata are completely broken. For strata further from the seam, the displacements areprogressively more elastic and the beds retain their integrity. At the ground surface, the subsidenceis about 60% of the height of the extracted seam. For a 2 m thick Bulli seam, the subsidence isapproximately 1.2 m. There may be some tensile failure at the ground surface, related to stratabending over extraction edges, but the cracking associated with this has limited depth extent.To a certain extent, the movement of the overlying strata into the void left by the mining is mirroredin the underlying strata. Miners must therefore also manage movements and breakage in theunderlying strata . In the Appin area, one consequence of this is that methane gas contained inunderlying coal seams can make its way into the mine workings through fractures induced in thefloor strata.The caving associated with longwall coal mining also relaxes the strata in the vicinity of the mine.The extent of the relaxation is partly controlled by the nature of the strata but it is also related to theprevailing stress fields. In the case of the Appin area, the stress field is strongly deviatoric. At TowerColliery, the ratio of the horizontal to vertical pre-mining stress is in excess of 3:1 with apredominant northeast orientation for the major principal stress direction (Hebblewhite, et al., 2000).At Appin and West Cliff Collieries, the major principal stress has a more easterly orientation(MacGregor, 2003).110