Published in Australia's Longwalls
Dartbrook is located near the Upper Hunter Valley town of Muswellbrook in New South Wales and is 75%-owned by Anglo Coal, in joint venture with Marubeni Thermal Coal (17.5%) and Ssang Yong Resources (7.5%).
For the year ended December 2003, Dartbrook produced a total of 4135 million tonnes of coal, run of mine. Dartbrook mines the thick Wynn seam at a depth of 250m and the original mine plan outlined nine longwall panels to the north of the main headings.
However, high levels of gas during mining activities have never allowed the mine to realise its full potential. With an in-situ seam gas content of 8-10 cubic metres per tonne and a specific emission of 40-50cu.m/t following mining, conditions at Dartbrook are never easy. Gas composition is 80% carbon dioxide and 20% methane.
By May 2002, during the mining of longwall seven the mine was in crisis. Ongoing problems with managing the high gas content and a heating disaster threatened the long-term viability of the mine. The mine was forced to take off the longwall equipment mid-panel and seal longwall seven. The adjacent block, longwall eight, was also skipped (it had similar gas levels to LW7) and the gear was installed in longwall nine.
Prior to the issues with longwall seven feasibility studies had been carried out on the Kayuga seam as the longwalls in the Wynn seam were approaching completion. It was essential to get the operation into a more favourable gas environment. To maintain longwall continuity two panels were developed in the yet unmined southern area of the operation.
Both panels had substantially lower gas content and as a result things began to improve dramatically for Dartbrook around June 2003. For the next three months the mine almost doubled its monthly output and broke all its own records. That month the mine produced 547,000t, followed by 486,000t in July and over 604,000t in August. Output dropped off in the lead-up to a change-out over September and October. Mining of longwall 11, the last of the two remaining panels in the Wynn seam is scheduled to be completed in early 2004. Mining will then move to the Kayuga seam.
Compared with the Wynn seam, Kayuga is a different animal, with some interesting geotechnical challenges.
Under depth of cover of 80-350m, the Kayuga and Mt Arthur seams are the target seams in the area. In the eastern half of the block the Kayuga seam is a discrete seam varying in thickness from 3-4.4m.
In the middle of the block the Mt Arthur seam coalesces with the top of the Kayuga seam resulting in a total coal section ranging in thickness between 5.4m and 10.2m.
In most of this area the working section will be in the Kayuga seam, with the Mt Arthur coal in the roof.
In the far west and south-west of the area, the lower Kayuga plies split away, leaving a thinner Kayuga section coalesced to the base of the Mt Arthur seam. In this area, the working section will step up into the Mt Arthur seam.
Based on the different seam thicknesses, the mine area is divided into five domains determined by the differing roof and floor horizons that will be used.
An interesting feature of the geotechnical design is the tapered pillar design that has been adopted with pillar width increasing with depth. Assessment by geotechnical consulting company Strata Engineering has assessed that the theoretically required chain pillar widths vary between 18.6m at a depth of 100m and 49.4m at a depth of 350m.
Over a typical block length of 4km this would entail ‘splaying’ the travel and belt roads of each block by around 0.36 degrees, such that an initial pillar width of 20m in the east (at a depth of around 100m) would translate to 45m at the western extremities of these blocks (at a depth of around 300m).
This allows for increasing depth of cover and has the benefit of leaving behind the minimum amount of coal possible as interpanel pillars, while maintaining the pillar stability at depth. Pillars are approximately 25m wide at the outbye eastern end of the panels and increase to 42m wide at the deeper western ends of the panels.
“The dimensions adopted will maintain pillar stability during longwall retreat and are expected to maintain tailgate stability while requiring moderate levels of secondary support,” said Tim Britten, geotechnical engineer at Dartbrook.
If the pillars were designed to cater for maximum depth a cut-through length of 38m would be required. Rationalising the layout reduces this to an average of 27.5m and saves over 2.9km of gateroad drivage alone.
Development of Kayuga is being undertaken in two gateroads at an extraction height of 3.2m. Contracting company Roche is running a super section using Joy 12CM30 continuous miners while the mine’s workforce is running two ABM20s (with Mach II upgrades).
