Hydraulic characterisation of longwall goaf
Professor Stephen Fityus
Longwall mining of coal in the Sydney Basin has left extensive areas of disturbed rock, known as goaf. These are effectively large underground volumes of rubblised rock with enhanced porosity and hydraulic conductivity. Historically, there has been little interest in the characteristics of goaf, so little is known. Recently, however, longwall goaf has been identified as a potential medium to host underground water storage, in particular, in the context of reservoirs for pumped-hydro applications. In 2021, researchers in the PRC for Geotechnical Science and Engineering at the University of Newcastle, in collaboration with Banpu Energy Australia and funded by ARENA and NSW Energy Co, undertook a program of testing to characterise the texture of longwall goaf and its mechanical and hydraulic behaviours when flooded whilst being subjected to overburden stress. Textural characterisation was carried out using both photographic analysis and borehole log analysis techniques, which yielded similar textural models. Testing included dry compression tests on one quarter scale specimens using a 550mm diameter oedometer ring, with subsequent measurement of collapse upon inundation, ongoing collapse during flooding-drainage cycles, and hydraulic conductivity measurement by falling head test. Reduced scale testing was also undertaken to establish a relationship the wetting-induced collapse of loaded goaf and the UCS of the goaf rock material. Because of the uncertainties in hydraulic conductivity measurement associated with downscaling of sample size, a large permeameter (2.4m high and 2.4m in diameter) was designed and constructed to undertake a series of constant head tests on full scale simulated goaf specimens. From the range of tests performed, data now exists to guide estimates of dry compression, wetting induced collapse, porosity and hydraulic conductivity in deep flooded goaf.
About the speaker
Professor Stephen Fityus University of Newcastle
Professor Stephen Fityus is a teacher and researcher in the fields of Geotechnical Engineering and Engineering Geology at the University of Newcastle. His research areas are reactive clay soils, soft soil estuaries, slope instability and the mechanics of granular soils and mine wastes. He has published more than 150 technical papers on a wide variety of topics and is an active member of the AGS.
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