Latrobe Valley Regional REHABILITATION STRATEGY Regional Geotechnical Study

REHABILITATING THE LATROBE VALLEY’S COAL MINES The Latrobe Valley’s three brown coal mines – Hazelwood, Yallourn and Loy Yang – have fuelled most of Victoria’s electricity generation since 1924. Hazelwood closed in 2017 and Yallourn and Loy Yang are planned to close by 2032 and 2048 respectively. Each of the Latrobe Valley brown coal mine operators are required to develop a plan to rehabilitate the mine sites in order to achieve a safe, stable and sustainable landscape once mining activity ceases.

BACKGROUND ACHIEVING A SAFE AND STABLE A reopened inquiry into a coal fire that burned LANDFORM FOR THE LONG TERM for 45 days at Hazelwood in 2014 found that Latrobe Valley Regional Geotechnical Study using water to create ‘pit lakes’ in the areas where coal has been mined is likely to be the Active engineering controls such most viable way to achieve safe and stable as groundwater pumps, surface water rehabilitation of the mines. However, the Inquiry diversions and extensive monitoring recognised that significant knowledge gaps systems are required to maintain safe existed in relation to the feasibility of this operating conditions at the Latrobe Valley rehabilitation option and recommended coal mines. further investigations be carried out. Coal mining has created land movement These investigations – geotechnical, water since it began in the Latrobe Valley. and land use planning studies – have This has ranged from small, continuous been undertaken as part of the Victorian movement of mine walls (known as batters) Government’s preparation of the Latrobe Valley to several major batter failures extending Regional Rehabilitation Strategy (LVRRS). beyond the mine crest. Exposed coal has caught fire a number of times, causing impacts The studies, which were carried out by technical to communities and the mines, most notably specialists from 2017 to 2019, considered the during the 2014 Hazelwood Mine Fire. regional benefits and risks associated with stabilising the mine pits (or voids) by creating As part of the Latrobe Valley Regional full or partial pit lakes. Rehabilitation Strategy, the Victorian Government commissioned a Regional The Strategy provides information on the: Geotechnical Study. Specialists investigated • regional risks that need to be considered the stability and fire risks associated with and addressed in rehabilitation plans the coal mine voids, whether these risks could be mitigated by filling the voids with • feasibility of using water if required for safe, water to a level that achieves stability stable and sustainable rehabilitation and (referred to as the pit lake rehabilitation • possible future land uses for the option) and whether filling the voids rehabilitated sites in a regional context. with water could result in adverse ground movement impacts within and around the mines. BLUE ROCK RESERVOIR

MOONDARRA RESERVOIR

LAKE NARRACAN LAT ROBE RIVER YALLOURN NORTH

NEWBOROUGH MOE

YALLOURN MINE R

YALLOURN E V TRARALGON M1 I R L L E

R O

MORWELL LOY YANG MINE

HAZELWOOD MINE

K E E

HAZELWOOD O

G PONDAGE L A R A R T

LEGEND CHURCHILL Mining licence boundary Current mine pit Lakes and Rivers Towns Forest M1 Freeway

The Latrobe Valley with mine voids and mining licence boundaries identified

KEY FINDINGS STABLE FLOORS AND BATTERS Mining induced ground movements The Regional Geotechnical Study found that: of significance to rehabilitation are • The pit lake rehabilitation option can identified to be: achieve a safe, stable and sustainable • Block sliding landform through a largely passive control by: • Sinkhole formation – Providing a counterweight to upward • Floor heave pressures from aquifers below each • Subsidence mine, thereby preventing instability caused by ‘floor heave’ and Each of these movement types can occur eliminating the current need to pump separately or together depending on the large quantities of groundwater conditions prevailing in the mine. While block (around 30 GL/year in total, across sliding typically results in rapid movements all three mines) from the aquifers after onset, sinkhole formation, floor heave to maintain stability. and subsidence are all longer time processes that occur over weeks to decades. – Increasing lateral pressure to stabilise batters, thereby reducing Passive controls that avoid the need for the current need to actively ongoing action in managing risks from control batter movement through these ground movements and coal ignition groundwater drainage and surface are identified as the preferred option for water management. rehabilitation; this is achieved through landform design. Passive design elements • Extended fill times arising from include the use of sediments and water limited water availability, or not filling to stabilise mine floors and batters, and to a level that provides the required covering coal (e.g. with soil) to prevent coal counterweight for lateral pressures, ignition by external sources. present stability challenges that would need to be actively managed. • Minimisation of seepage inflow through • Coal coverage above the water line the management of surface waters and can be achieved by providing a suitable coal cover materials, in addition to coal soil-vegetation cover system that is groundwater pressure control through resistant to erosion. As no soil cover drainage boreholes, may be required system will be completely resistant to to maintain low ground movement risks. erosion a long-term cover maintenance Controls will be required during the filling plan is likely to be required. period and potentially on an ongoing • Extended fill times arising from limited basis for any coal batters extending water availability present stability above the final water level. challenges and fire risks (due to the • Areas outside of the pit requiring extended period of exposed coal) that ongoing management and/or planning would need to be actively managed, controls would be reduced compared to compared to a shorter fill time. current requirements if water levels are • If final water levels vary, a zone at or above the minimum required to of exposed coal may occur between achieve counterweight to future vertical the water level and the soil-vegetation and horizontal groundwater pressures. cover at or above the maximum water • Ground movements will occur in response level. Assessment will be needed of the to filling a mine with water, due to a ‘lake fire risks arising from the exposed coal loading’ effect. These are not expected to and a suitable fire risk management have adverse impacts but will need to be plan developed. monitored to ensure that if any impacts arise to existing and future infrastructure, CONSIDERATIONS FOR they are addressed as part of the mine rehabilitation process. REHABILITATION PLANNING The study highlights that water can provide GROUND SUBSIDENCE an effective resource to support the long- AND REBOUND term rehabilitation of the mine voids that reduces the need for ongoing active The Regional Geotechnical Study found that: management of ground movement and fire risk. The study also shows that there are • Groundwater extraction for mine stability issues that remain unresolved at the present has led to gradual land subsidence time in relation to water level fluctuations, across the region. spontaneous combustion, seismic risk, • If groundwater extraction for mine lake loading and ground surface rebound. stability control can be stopped Further explanations are provided in the following mine rehabilitation, land Latrobe Valley Regional Rehabilitation surface rebound is expected. The Strategy Regional Geotechnical Study magnitude of the rebound is expected Synopsis Report and LVRRS. to be less than the subsidence that has occurred and it is anticipated that this rebound would occur gradually over CONTACT US many decades and relatively evenly For further information, you can find a copy across the region. This is an area that of the Latrobe Valley Regional Rehabilitation requires further consideration as part Strategy and related factsheets online of rehabilitation planning. at earthresources.vic.gov.au/lvrrs or by contacting the LVRRS Project Team MANAGING FIRE RISK via email at [email protected] or on (03) 5184 2000. The Regional Geotechnical Study found that: • Coal fire risk is best managed by covering exposed coal. • If water is provided for ground stability control it would cover large areas of exposed coal. As such, active controls on fire risks would be significantly reduced for the final rehabilitated landform if the water levels required to passively control ground movement are achieved. DJPR 12625 10.19 DJPR 12625