The stage two mining plans cover 26 longwall blocks targeting the bottom section of the 4.25-10m thick Hoskissons Seam.
Each panel will be 305m wide and extracted to an average height of 4.2m, while the face height will be 3.5m high at the Main Gate and Tail Gate ends, matching the 3.5m high gate entries.
Cover depths are expected to range between 160-180m over the eastern area of the mine site and extend up to 380m over the western ridge area.
The company proposes to leave a single row of 3.5m high chain pillars between the extracted longwall blocks with the widths ranging from 24.6m to 37.6m at the larger depths.
Upon approval, Narrabri will ramp up its capacity from a 2.5 million tonnes per annum continuous miner operation to an 8Mtpa longwall mine for a life of 30 years.
Subsidence impacts are harder to determine compared to other longwall projects as there is no knowledge of how the Gunnedah ground responds to underground mining.
“As a consequence, it has been necessary to make predictions using proven empirical modelling techniques developed in other coalfields with similar geological conditions,” consultancy R.W. Corkery said in its Environmental Assessment for the project.
“This approach has been used to produce total and differential subsidence predictions across the mine site.”
Consultancy Ditton Geotechnical Services said the subsidence predictions were made using an empirical model coming from a 2003 ACARP research project on the effect of massive sandstone and conglomerate channels on subsidence development above longwalls in the Newcastle Coalfield.
This ACARP data was fed into Surface Deformation Prediction System software, with maximum subsidence expected to range between 2.17-2.44m for the panels that do not span a basalt sill formation known as the Garrawilla Volcanics.
In areas where the volcanic formation is present, maximum subsidence is expected to be between 0.79-2.44m.
Subsidence above the chain pillars is expected to vary between 0.12-1.32m.
R.W. Cookery also said subsurface aquifers within 110-180m above the longwall panels might be impacted by direct hydraulic connection to the workings.
Potential damage to farm dams and creek flows has been foreshadowed, but Whitehaven will also have monitoring and response plans in place before mining.
The gas content of the seam provides another challenge, as it ranges from 3.5-7.5 cubic metres per tonne.
While the gas composition is expected to vary considerably, for the EA it was assumed to be an average of 90% carbon dioxide and 10% methane.
Using Surface to In-Seam drilling, Whitehaven plans to reduce gas content to less than 5 cubic metres per tonne to manage the risks of outbursts and rib emission prior to panel development.
Once gate road development advances ahead of the longwall operations, predrainage is expected to be performed through conventional underground in-seam drilling.
About two-thirds of the land above the underground project is mainly used for livestock grazing along with some cereal crop farming. The western third consists of native woodlands in the Jacks Creek and Pilliga East State Forests.
The Narrabri Coal Joint Venture owns about 32% of the land above the project.
At its annual general meeting last year, Whitehaven flagged the potential to perform longwall top coal caving at the mine. This could lift capacity to 9.5Mtpa and the company ordered a Bucyrus longwall capable of being retrofitted for LTCC mining.
Whitehaven expects the earliest LTCC could start would be around late 2012, during a change from longwall block 2 to block 3.
Coal quality and washability analysis also identified the potential to produce up to 45% of pulverised coal injection coal with a 7.5% ash content at the thermal coal operation at the time.
The shift to produce PCI coal would be through using a two-stage dense-medium cyclone to the jig plant under stage two development.
The JV behind Narrabri consists of Whitehaven (77.5%), Upper Horn Investments (7.5%), J-Power (7.5%), Daewoo (5%) and Kores (2.5%).
