Published in the March 2011 Australian Longwall Magazine

Initially set up to supply coal to the nearby Eraring, Vales Point and Munmorah power stations, the mine is currently exporting 1.5 million tonnes per year of its thermal coal and this is set to increase significantly over the next few years.

The mine design ensures mining can continue with minimum impact to nearby residential and rural land users, by keeping subsidence to a minimum. The company is also set to make use of its abundant supplies of methane to generate its own power requirements.

Mandalong was developed from the old Cooranbong mine, which was built at the same time as the power stations, and were both NSW state assets. The mine came into Centennial’s hands when it acquiredthe state-owned Powercoal group in 2002.

Part of the package was the existing Cooranbong operation, which was in the last 12 months of its life because of lack of reserves, Mandalong mine manager John Tuner said.

“Between 2002 and 2005 we invested a lot of capital into Mandalong to take it from being a greenfield project into an operating longwall mine,” he said.

“Cooranbong was a 20-year-old continuous miner operation. Mandalong is a totally different operation – a modern thick-seam longwall operation.”

Mandalong commenced operation in January 2005 as per plan and exceeded initial production targets of 3.5Mtpa and is now running at a production rate of approximately 5Mpta.

It achieved 5.5Mtpa in the last financial year due to only having one longwall relocation in that year.

With the high retreat rate of the mine, it averages 1.5 relocations per year.

Mandalong, with its close geographic proximity, has been set up to provide the Eraring Energy and Delta Electricity markets by utilising underground conveyors all the way to the point of delivery with limited surface crushing.

With Eraring, all the coal goes underground through Cooranbong to the surface and is prepared at the Cooranbong coal plant for the customers.

The mine has in the order of 300 employees, plus specialised contractors. It operates three development units 24-7, to keep up.

“The West Wallarah seam gives a seven-metre-thick, good-quality, low-ash, low-sulphur steaming coal. Export infrastructure has been developed to access the export market. With production levels of about five million tonnes per annum we have production above the domestically contracted amount that we can put into the export market,” Turner said.

“In the last 12 months we have done work to access the export market. We have constructed a privately owned haul roadthat now connects Mandalong to our washery and rail loading facilities at Newstan.

“We’ve developed a link from our Cooranbong facilities to a haul road which allows us to truck coal to our Newstan site where we have a four million tonnes per annum washery which we use to prepare our coal, load it onto trains to export through the Port of Newcastle or Port Kembla.”

The export infrastructure at the Cooranbong surface is set up with truck loading bins. Those bins have a stockpile capacity of 100,000 tonnes.

Longwall

Mandalong is currently mining Longwall 10, which has a 150-metre-wide Joy longwall face that operates 24 hours a day, seven days a week, Turner said. The Joy longwall supports are operated between 3.6m and 4.8m in height and are rated at 1050t with 1.75m dual leg support. The Joy 7LS6 shearer has 2.5m drums.

“We cut to 4.8 for the majority of the block,” he said.

“The seam is up to seven metres thick in the centre of the blocks tends to thin out at either end of the block to 3.6 metres.

“We used to run 2.6-metre drums for the majority of the block and then stop and swap to 2.4-metre drums to make it easier to the reduced height. We then tried a set of 2.5-metre drums for the whole block, which was successful, so that’s what we operate with now.

The longwall face now runs at reasonably high levels of automation, Turner said.

“The first four blocks were slightly narrower that’s because the whole technique was still fairly innovative.”

“We applied civil engineering design principles to achieve what we’ve done. And we had to test that. We were always aiming to go to 150 metres-wide panels but the first four panels were 115 panels wide so we could verify the geotechnical assumptions with a live model.”

The steep seam zone or seam roll has no major faulting, but tectonic forces turned it into a very steep zone with grades of up to one-in-eight.

“Mandalong has successfully negotiated the steep seam zone, with a reduction in extraction height to 4.2 metres to increase face stability and control slabbing.

“At times in the seam roll we can have the tailgate about 15 metres higher than what the maingate is, over 150 metres – that is one-in-ten grade – but by carrying a very strong maingate lead we can get up through that zone.”

Volcanic dykes that intrude into the seam have caused the mine to shorten their blocks slightly. At one particularly instrusive dyke, Turner decided to pre-mine the dyke out, drive roadway alongside using drill-and-blast techniques to remove the dyke material and then mine back through the void.

“That dyke was 200 [megapascals] so we wouldn’t have been able to take the longwalls through,” he said.

“Some of the other dykes were cuttable using the longwall shearer.”

Mine design

Mandalong’s mine design has been set up to limit the surface subsidence impacts.

“The mine design criteria is that it does not damage any surface buildings beyond the point where they are safe, serviceable, and repairable, which means we don’t have to acquire anyone and it doesn’t trigger any acquisitions,” Turner said.

“Basically we mine in accordance with our approvals and we don’t want to increase the flood impacts in low lying areas. The valley is already flood prone – it is a low-lying valley. It’s got a creek that runs through the centre of it, but we want to make sure that our operation doesn’t significantly impact on the flood characteristics.

“We take a fair bit of coal out while leaving very little damage basically.”

The mine’s normal panel width is only 150m wide, and the extracted void is 160m, compared to industry averages in Australia of typically between 250m and 400m for panel widths.

“The shorter panel widths helps us control subsidence but also changes the economics of the operation,” Turner said. “It puts a lot more stress on the development side as well. We set ourselves a target to expand to five million tonnes per annum by 2009 and we achieved that.

“The original approval was for 250-metre-wide faces but we decided, weighing up all the business risks including surface impacts, to run at 150-metre-wide blocks. It’s all about subsidence management – we use narrow blocks and particular geology that’s specific to this area to achieve that in which there is a massive conglomerate overburden.

“Our face will take up to 4.8 metres of extraction, where seam thickness allows.

“We leave big pillars. Industry-standard pillars are maybe 30 metres wide, ours are 46 metres wide. Our interpanel pillars are bigger. So what happens underground is we extract the coal, and create a caving zone and above that we get the fracture zone similar to other operations.

But at Mandalong a massive conglomerate beam spans across the excavation. It is strong – in the order of 40 metres thick.

“Normally a caving zone would extend further, but in our case caving zone is stopped at the conglomerate beam," Turner said.

"This beam then deflects due to the overburden load which in turn creates low levels of subsidence. It bends and it sits down on the caved waste to a certain degree, but most of the weight is taken by the pillars.

“We’ve got big 46-metre-wide pillars. So the load is accepted by those pillars. It doesn’t break fracture or anything it just bends a little bit. So what that allows us to do is we take our 4.8 metres and limit our subsidence to 0.5 metre to one metre.

“Typically if you take five-metre seam out you create three metre or above of subsidence which brings significant cracking, impact on waterflows, and damage to the overburden permeability, and an impact on groundwater. But with what we do there is limited damage to the surface and only minor cracking. We don’t impact groundwater tables, and it’s a lot lower impact form of mining.”

Development

Mandalong has also introduced improvements to the mine’s Sandvik ABM 25 miners. It has three ABM25 continuous miners and one ABM20 continuous miner in action to drive out roadways. Access is via a one-in-eight materials drift.

The uneven shape of the floor being produced by the continuous miners at Mandalong was a significant safety and operational issue at the mine.

The Autocut project successfully automated the cutting cycle of the ABM 25s through “closing the loop” – a process of monitoring and continuous refinement of the Autocut system.

The mine found Autocut helped reduce the number of slip-trip-fall injuries due to the improved floor horizon.

It also improved the quality of the travelling roads for worker transport and shuttle car movements, and improved the workplace for belt moves.

One of the most significant advantages of the Autocut system is the consistent improvement in productivity.

Since installing Autocut in two ABM 25s, Mandalong has twice broken weekly development records and in February last year smashed its monthly development record.

Gas drainage

The mine is being placed on a more sustainable footing and is ensuring it is safe to operate by an extensive gas drainage system.

“Our in situ gas contents are not high by industry standards – only about six cubic metres per tonne but our coal is very permeable and we found during some early mining experience here as we mined developed roadways we were subject to gas inflow so we basically developed what is now very mature gas drainage system," Turner said.

"Every four pillars we drill out six holes to de-gas the block.

“Currently we vent all our drainage gas to the atmosphere but we are about to conclude agreements with AGL to develop some onsite facilities to initially flare our methane.

"We are working towards a full facility to where we will be able to use gas engines to generate electricity with our methane and also a facility to take the methane out of the ventilation air and utilise that as well.

“We can exhaust about 500 litres per second of pure methane. So it’s good for gas-powered generation.”

Expansion

Going forward mining is planned for the southern area of the existing mining lease.

“Mandalong South is an extension to our existing Mandalong mine," Turner said.

"It is currently an exploration project which we will progress to a full environmental assessment.

“Community consultation is a very important part of our business. Our mining consent has a multitude of conditions of how we deal with the property owners around the mine.

“We need to give two years notice before we mine under a property and during this time a subsidence plan is developed for each property."

That subsidence plan predicts how much subsidence may affect each property well in advance of mining activity.