Illawarra Coal sustainable development general manager Wendy Tyrrell said the company was faced with a dilemma when considering how best to proceed with rehabilitation work at the New South Wales site.
“The unique thing about the Corrimal site is its location in proximity to the Cataract Dam, that it featured a single, deep shaft, which was sunk in 1927, and we had to figure out the best – and safest – way to fill it,” Tyrrell said.
When the 340m shaft was excavated the material was dumped right next to it, so BHP had to either put that material back into the shaft or bring in new material to fill it.
“We identified a potentially high risk in hauling material from elsewhere because of the road transport necessary, so it made sense to use the original material,” Tyrrell said.
“Also, extreme care had to be taken in the filling process as the shaft had potential to emit methane and we developed procedures to ensure gas ignition could not occur as a result of sparking as the fill was placed down the shaft.
“But then again, looking at safety, the process of bulldozing matter directly into a hole could result in the loss of the machinery and life, so we had to look at alternative methods of doing the actual work.”
The company carried out flora and fauna studies and heritage impact studies before beginning work.
As well, the site entrance was on a busy main road with limited sight lines, so a traffic management control plan had to be prepared and put into place as well.
The Illawarra Coal team designed a hopper with weighted gates which were padlocked on weekends to prevent unauthorised access. As well, a grille was placed over the top of the funnel and a fence erected around the work site.
An excavator was then parked on a mound a short distance from the shaft and outside the fenced area. It was there to pick up material and place it in the shaft, so there was no danger arising from working next to the shaft.
The original excavated material wasn’t all to go into the shaft, however. Tyrrell said that to ensure complete rehabilitation, the shaft was initially lined with stone dust – to snuff out sparks during filling.
About 1000 tonnes of boiler bottom ash from Newcastle was then placed into the bottom of the shaft. This was covered with cement to provide a gas seal. Then the original excavated material was used, with a small quantity of water for dust suppression and as an aid in consolidation.
Finally a layer of sand was placed, to preserve the existing shaft structure at the surface and ensure all heritage concerns were addressed. The shaft was then capped with a concrete slab.
The first shaft has been sealed and two other shafts are out to tender. Tyrrell said the same method would be used to fill them.
The next step at Corrimal will be the planting of seedlings.
Monitoring of the shaft fill will be undertaken through specially designed cast-in penetrations in the concrete cap, to measure any fill settlement that may occur.
