Authors: Turnbull, R; Foley, G; Missen, J

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Turnbull, R, Foley, G & Missen, J 2020, 'Outcomes of an aquifer assessment on the M1B aquifer ahead of Loy Yang Mine and considerations for future dewatering/depressurisation', in PM Dight (ed.), Slope Stability 2020: Proceedings of the 2020 International Symposium on Slope Stability in Open Pit Mining and Civil Engineering, Australian Centre for Geomechanics, Perth, pp. 1319-1328,

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AGL Loy Yang Mine is a brown coal (lignite) mine owned and operated by AGL to supply coal to its adjoining Loy Yang A power station and the neighbouring Alinta Loy Yang B power station for electricity generation. The open cut mine is located south of Traralgon, in the Latrobe Valley region of Gippsland, in eastern Victoria. Brown coal is mined using bucket wheel excavators and conveyors, from thick coal seams interbedded with sedimentary silt/clay layers. To date. groundwater extraction has occurred from confined aquifers of the Morwell and Traralgon Formations beneath the mine to mitigate floor heave by maintaining groundwater pressures below defined trigger levels as no significant aquifers have been exposed in the batters requiring dewatering. Geological modelling indicates that a sand interseam within the upper Morwell Formation, identified as the M1B aquifer, significantly thickens ahead of the mine to become a high yielding, regionally extensive aquifer of up to 20 m thickness. Future mining operations, which have been planned until the mine’s forecasted closure date of 2048, will expose this aquifer at various thicknesses within the mine’s batters and floor from 2024 onwards, with potential for significant seepage. The presence of the aquifer therefore presents a number of challenges to future mining operations, particularly in relation to depressurisation, dewatering and slope stability. In particular, recent proposed changes in the rate and direction of mine expansion have expedited the need to investigate operational impacts posed by this aquifer. This paper details the findings of recent pumping test investigations undertaken on the M1B aquifer which have shown that the aquifer is regionally confined, has a high transmissivity and is hydraulically well-connected across the proposed future mine footprint. The groundwater chemistry is of low salinity, with slightly acidic pH and temperature around 40°C, and various management options including surface water discharge are being assessed. The target aquifer’s remoteness from the current mine footprint and associated dewatering infrastructure also presents logistical challenges regarding electricity supply and discharge options. The pumping test results are being used to refine the existing mine groundwater model to confirm the location and number of future M1B aquifer dewatering pump bores, with the ultimate purpose of reducing groundwater seepage and aquifer pressures to reduce risk of batter instability. In the longer term, the geological and hydrogeological modelling will assist in the detailed design of the permanent batters, mine grades and placement of overburden within the mine taking into consideration mine closure and rehabilitation options.

Keywords: Loy Yang, brown coal, aquifer, depressurisation, pumping test

Duffield, GM 2008, AQTESOLV Aquifer Test Analysis Short Course, November 2008.
GHD Group Pty Ltd 2015, Geology of Loy Yang Mine, August 2015, AGL Loy Yang, Traralgon.
Government of Victoria 2018, State Environment Protection Policy (Waters).

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