The Kestrel coal mine extension project, first approved Rio Tinto Coal in late-2007, will revolutionise the Bowen Basin mining sector by the time of completion in 2012.
The extension, worth around $1.2 billion, will expand the mine’s capacity from 4.2 million tonnes per annum to 5.7 million tonnes per annum and increase the lifespan by 20 years.
The mine, located 51 km north-east of Emerald, will boast a new 375 km longwall, automated stockpiling systems, upgraded infrastructure and sustainable water usage technology.
A host of contractors and service providers are currently onsite working at various aspects of the projects, including AECOM Technology and Ausenco.
One of these contractors, Queensland-based Australian Shaft Drilling (ASD), has been commissioned to drill a ventilation shaft for the new underground operations.
To complete this task, the company has been using a drilling technique which has become increasingly popular in recent years; blind shaft drilling.
According to ASD director Dean Furness, the blind shaft drilling technique means that the shaft is completely constructed from the surface.
“For the raise drilling technique, an initial pilot hole is drilled from the surface and then the shaft is reamed out from the bottom up, which requires underground access,” he told Australian Mining.
“However, with blind shaft drilling, the shaft is completely drilled and then lined with steel and concrete from the surface.
“This means the workers do not need to go down the hole, which therefore makes the whole operation much safer.
“The bigger mining companies tend to put a high value on safety; Rio Tinto specifically says safety is the most important aspect for any contractor onsite.”
According to Furness, the technique has also become more popular because it can be carried out independently from other mining operations.
“Three of the last four projects this company has worked on have been for Greenfield mine developments, which tells me it is becoming the chosen method for new sites,” he said.
“These operations just want to get on with their work, whether it is building new infrastructure or mining the existing coal.
“When we are blind shaft drilling, we do not need anything from the mine.”
Because the work is carried out from the surface, the shaft can actually be completed before the underground workings.
“This means that once the underground workings intersect with the shaft, there will be a ready-made air supply in place,” Furness said.
“This allows the miners to bring in machinery to start developing headings at a much faster rate than they would if they were operating on limited ventilation
Rio Tinto requested a 230 m deep shaft with a five metre diameter and a life expectation of 30 years.
ASD began setting up their site as the preliminary design and earthworks stages were being conducted.
“We installed the required infrastructure, including the foundations for the drill rig, and set up a nine metre presink around the planned location of the shaft,” Furness said.
“We then brought the other facilities needed for the task, such as compressors, workshops and buildings.”
The company planned to drill the shaft with a six metre diameter head, which would allow enough room for the installation of the lining structures.
Once the lining was in place, the shaft would have a diameter of 5.5 m.
According to Furness, the geology of the Bowen Basin has also made blind shaft drilling an attractive option.
“The tertiary strata in the region are often very weathered and unstable, so shaft construction previously involved a few headaches,” he said.
“The tertiary strata at the Kestrel site consist of very weak clay, so securing it has proved quite challenging.”
Furness said the weaker strata stretched to depth of 75 m, meaning the drill rig’s mud drilling quality control system had to support the strata to prevent a collapse.
“Because the hole has such a large diameter, you have to make sure your drilling mud system holds up for the duration of the drilling,” he said.
“It would be catastrophic if a hole that size caved in.”
“But thanks to our system, we have proved that typical Bowen Basin strata can be supported, despite their weaknesses.”
Since the project began earlier this year, the company has drilled at a rate of up to 43 m per week to a depth of 160 m.
“The remainder of the shaft is now pretty straightforward; the rock is just sandstone,” Furness said.
“The last 70 m should be drilled within the next three weeks.”
According to Furness, ASD has refined its lining system to ensure the lining materials are installed quickly.
“Once the drilling is stopped and the fluid systems are shutdown, you only have a certain amount of time to line the shaft, because it can become unstable,” he said.
“The shorter the exposure times the better, so we survey the shaft to make sure the profile is correct and install the lining in as quickly as possible”
In awarding the contract, Rio Tinto stipulated the project must comply with their safety regulations and procedures.
“In order to receive the contract, we had to provide systems that would ensure that we were operating with the best practices and safest methodologies” Furness said.
“In fact, we actually had to redesign some of our equipment.
“One of Rio’s golden rules for employees near heavy machinery is that they are not allowed to work near or under a suspended load.
“So we had to design some new features into our equipment and methodology to change that.”
According to Furness, the stringent safety regimens have actually proved quite beneficial to ASD.
“Because there are so many different companies working on different projects, Rio has put a big emphasis on teamwork to ensure everything is safe,” he said.
“But this has been to our advantage, because we have been able to operate with their full safety resources at our disposal.”
Furness expects blind shaft drilling will only get more popular in the coming years, as more and more operations see the benefits of the technique.
“The company has been around since 2004 and we have been very busy ever since,” he said.
“I can only see us getting busier, because these methods can get ventilation shafts up and running in a hurry, without any fuss.”