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9. Construction of Dendrobium ventilation shaft sets new safety and environment standards
Our Dendrobium underground coal mine is under development at Mt Kembla in the Illawarra region of New South Wales, Australia. As part of the ventilation system for the mine, the No 1 ventilation shaft was constructed. The shaft, 183 metres deep and 4.25 metres in diameter, was completed without one person entering the shaft during the construction period. The project has set new industry standards in safety performance and environmental care.
From a safety point of view, the technologically advanced method chosen for construction of the ventilation shaft proved to be superior to traditional approaches. There was only one classified injury, which occurred when a contractor's employee cut his hand while installing meshing.
Environmental performance on the project was also exceptional. There were seven distinct phases. Recognising that the location of the ventilation shaft lies within an environmentally sensitive water catchment area, the Company worked closely with the Sydney Water Catchment Authority and the former Department of Land and Water Conservation to implement measures to minimise the environmental impact of the project.
In the initial phase, a level area surrounding the shaft and a sedimentation pond of approximately 1200 cubic metres were constructed. The level area was used to erect and operate the drill rig, and the sedimentation pond was used to provide return water and remove drill cuttings from the vent shaft.
A shaft pad and collar was constructed to stabilise the top eight metres of the shaft and to provide a base for the vent air fan duct elbow. The drill rig was then assembled on the pad. It consisted of a 450-tonne lifting capacity derrick, with a winch powered by two 160-kilowatt variable speed controlled electric motors. The drill head of the rig consisted of six 55-kilowatt motors driving into gear reducers combined onto a final drive gear.
The shaft was excavated using a blind bore rotary drilling method. This method employed an assembly of drill weights attached to the drill head to maintain drilling pressure on the cutting face. The shaft was drilled approximately three metres beyond the floor of the mine tunnel to allow rebound shotcrete from the shaft lining to deposit.
The shotcrete lining to the shaft was applied by a tele remote rig, consisting of a rotating three-arm machine. The shaft was progressively dewatered and shotcrete finished to a final thickness of 50 to 100 mm, depending on strata type.
Water control was critical during the project. A series of holes was drilled and pre-grouted around the shaft area, forming a grout curtain to minimise loss of water to the surrounding substrata as the shaft was kept full of water during the drilling phase. The curtain also minimised the ingress of water after the shaft was emptied, minimising entry of water into the mine and assisting the integrity of shaft lining.
It wasn't all plain sailing. In the course of lining the shaft, a heavy localised ingress of water occurred. This prevented shotcrete adhering to the shaft strata. To overcome this, an air drill was attached to the tele rig, lowered into the shaft and remotely operated with vision from rig cameras. Polyurethane resin grout was applied, and this stopped the water ingress.
Once the shaft was lined to 177 metres, the shaft lining was filmed using on-board cameras as the tele rig was withdrawn. The drill rig was then demobilised and the site rehabilitated before the mine vent fan was installed.
The contractors' adherence to the Dendrobium Environmental Management Plan and procedures further assisted in minimising the risk of an environmental incident.
The ventilation fan construction will act as a benchmark for the industry in terms of environmental and safety performance.
