The modifications were needed to enable continued safe operation of the existing underground mine and ongoing employment for up to 80 open cut employees, while a major project application for a new open cut mine is being determined.

Higher than expected coal seam gas levels were encountered in the underground mine, requiring the extra gas wells to be built.

After three wells were installed at longwall 6A at the Pikes Gully seam as an interim measure, the company installed another 15 wells into the nearby longwalls 6B, 7A, 7B and 8.

“The development of the additional gas-drainage wells is an interim measure to enable continued safe operation of the mine in the short-term,” Ashton said in its environmental assessment. “A more substantial gas drainage network will be designed and implemented prior to requiring gas drainage in the Upper Liddell Seam.

“This will include consideration of gas capture and flaring or potential beneficial use, and will be the subject of a future modification to DA 309-11-2001-i. The gas wells developed into the PG Seam may form part of this future gas drainage network.”

Ashton is proposing to continue operations at the open cut mine by extracting coal through the Hebden Seam.

Approximately 100,000 tonnes of ROM coal will be extracted over three months with approximately 656,400t of overburden material to be hauled and dumped at the Arties Pit less than 400 m to the northwest, and within the Hebden Seam excavation area.

Ashton proposes using existing machinery at the northeast open cut (NEOC) to mine vertically down through the southeast corner of the existing Barrett Pit to access the Hebden Seam.

As with its existing approval, ROM coal would be transported to the coal-handling plant for processing. The Hebden Seam would be accessed through the current floor of the NEOC in the southern corner of the existing Barrett pit.

The current pit floor is at the Lower Barrett floor level and the eastern wall has been buttressed with spoil.

Survey monitoring data has showed a slowing of the eastern wall movement in response to placement of the spoil buttress.

The mine plan includes removal of approximately 12m to 13m of Hebden parting and coal, down dip of the spoil buttress. Hebden seam contours indicate the seam dips at 6.5° down towards the southwest within the proposed mine footprint and beneath the existing spoil buttress.

A minimum 10m wide bench will be left between the spoil buttress toe and crest of the Hebden pit.

Ramp access into the Hebden pit will be on the northwest side, and will abut in-pit spoil dumps placed over the Lower Barrett floor.

As the NEOC is approaching finalisation, the amount of overburden and ROM coal has decreased towards the end of 2010.

The continuation of coal extraction through recovery of the Hebden Seam would not increase this production rate.

Monthly production rates of 320,000bcm overburden and 35,000t ROM in the Hebden Seam are significantly lower in monthly production rate from NEOC operations.

Spectrum Acoustics has found that noise levels would comply with the criteria at all receivers for previous “low-level dumping” scenarios under adverse conditions and noise levels for the proposed Hebden seam recovery will be lower still.