By Romy Udanga
Stowing flowable fill made from on-site material of sand and crushed rock mixed with a cement-like bonding agent has proven effective in reducing the risk of operating over and through old underground workings and voids.
After a 15-month stowing test at the Millerton mining area, Stockton Alliance has this month moved in a 115-tonne, purpose-built void-stowing machine nicknamed "Big Yellow" to take the stowing job to operation level.
Trials to work out the best way to reduce the risk of operating over and through underground workings and voids in the mines have shown that stowing – filling the voids with a concrete-like material – provides the greatest risk reduction compared with collapse blasting which is generally effective but can leave residual risks and requires careful monitoring. It is also better than remote bulldozing, which is only partially effective.
Development manager Dave Stone says their trials have shown that stowing mitigates the risks of void.
"Once the material sets, all activity after that can proceed with greater confidence and efficiency. The method would also improve coal recovery and is likely to reduce the fire risk. Whilst this method will not entirely eliminate the risk, because of the difficulty in finding all voids so that they can be filled, it will provide a more effective risk mitigation process."
Big Yellow mixes water, crushed rock and crusher sand with cement and kiln dust – a by-product from Holcim’s cement process – to produce a material that is placed through boreholes.
The engine, hydraulics and track assembly of the 17-metre track-mounted machine are from a 100-tonne Komatsu digger. The body is made from 40mm, 32mm and 25mm steel plates, which needed specialist welding machinery to do double V full penetration welds and ultrasonic inspection.
Buller Industrial, Big Yellow’s builder, invested in three Cigweld 500i multi process machines; 3.6 tonnes of Verticor flux cored wires, Verticor XP 2.0mm, and 1.6mm and 1.2mm Verticor E71-T1 vacuum packed products; premium comet torches; stainless steel extensions; and heavy duty LPG heating equipment for the job.
Machines were coupled to two Bugo automated tracking systems to carry the Tweco supra XT mig guns on an extremely high continuous welding—at times running long periods on 430A 33V.
To get the job completed they also operated older Cigweld mig welders.
Designed by Graham Ashby of Hokitika, Big Yellow was built in Westport by Buller Industrial, a Jim Rae Farm Engineering company.
Stone says having a local fabricator and welding expertise was a real advantage, allowing a high-degree of interaction between the Alliance, the builder and the designers.
"Welding played a significant part in the project, Cigweld through Weldworx in Christchurch did not only act as suppliers.
"Ken Durbin’s welding expertise was very valuable. He trained operators, supervised the welding and kept our staff performance consistent – this being the base to a very successful smooth running project."