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mining methods
Similar mining methods are employed across Mt Owen and Ravensworth East mines. Mining is currently carried out by large excavators and haul trucks supported by ancillary equipment including water carts, dozers, graders, fuel and service carts, loaders and drills.
Mt Owen Mining Schematic
The sequence of mining involves clearing of vegetation and topsoil, drilling and blasting of overburden, followed by the excavation and haulage of overburden from a series of sequential mining blocks to the emplacement areas (see Figure left). The coal that is uncovered is then hauled to the Coal Handling and Preparation Plant (CHPP) for processing. The majority of the clean coal is conveyed to the product stockpile before being transported by rail for approximately 120 kilometres to the port of Newcastle for export. A small amount of coal from Ravensworth East Mine is supplied to local power stations for electricity generation via an overland conveyor system.
The clearing of vegetation at Mt Owen is a controlled activity within woodland areas due to the potential presence of threatened fauna in Ravensworth State Forest. Clearing is minimised to the smallest area possible ahead of mining and is generally limited to one strip width. Clearing of habitat trees is avoided during the breeding seasons for threatened fauna. Commercial timber, excluding habitat trees, in the mine path was removed as part of initial clearing within the State Forest, and the remaining vegetation is mulched for use in mine rehabilitation.
Following the removal of vegetation, topsoil is stripped and either placed in topsoil stockpiles or placed directly on reshaped overburden for rehabilitation purposes.
Once the surface area is cleared of vegetation and topsoil, the overburden requires drilling and blasting to enable it to be removed by excavator. Notionally all overburden is required to be drilled and blasted. However, the top layer of overburden is generally a weathered material that does not require blasting. This layer is of variable thickness, typically between 10 and 15 metres thick. Once the weathered material is removed, holes are drilled into the overburden using a blast hole drill rig.
Due to the number and orientation of the coal seams, most blasts are ‘through-seam’ blasts, utilising the ‘Aquilla’ drill hole monitoring system. This method uses blast holes that extend through the coal seam down to a nominated drill depth. This requires inert “stemming” material to be placed in sequence with the explosive in each hole, so that adequate fragmentation of the overburden and inter-burden material is achieved with minimal disturbance to the coal seams. It is therefore necessary to undertake detailed design of each blast conducted as the sequence and amount of explosives varies both between blasts and between holes within the same blast. This dedication to blast design ensures that not only is blast efficiency maximised but dust, fume generation, vibration and overpressure is addressed on a per blast basis.
Blasting is currently restricted to the hours of 9.00 am to 5.00 pm Eastern Standard Time (EST) and 9.00 am to 6.00 pm daylight saving time (DST) Monday to Saturday. Blasting is restricted during adverse meteorological conditions to minimize potential impacts on private neighbouring residents.
Large excavator loading a
rear dump truck with overburden
Exposed coal is mined by excavators into rear dump trucks for transportation, via internal haul roads to the Run of Mine (ROM) coal receival point adjacent to the CHPP. As the Mt Owen Complex is designed to operate on a ‘just-in-time’ basis, the large majority of the ROM coal is directly fed to the CHPP without stockpiling.