Published in March 2005 Australian Longwall Magazine

The Centennial Coal owned Mandalong mine, near Newcastle in New South Wales, started mining operations in January 2005 after meeting all the development consent conditions laid out by the State Government. One of the more stringent conditions restricted surface subsidence.

The consent conditions allowed “unrestricted” mine planning if surface movements did not exceed predefined values: tilt had to be less than 7mm per metre and compressive strain had to be less than about 3-4mm/m. According to mine geotechnical consultant Ross Seedsman of Seedsman Geotechnics, this translated into a maximum vertical subsidence of about 500mm.

Applying an additional factor of safety and reducing planned subsidence to 250mm, Seedsman designed Mandalong’s mine plan for subcritical extraction using Voussoir Beam concepts. Voussoir refers to ancient Roman bridges and aqueducts, which used a series of blocks to form an arch. A series of blocks has no tensile strength, so the ability of a Roman arch to span required the generation of compressive stresses – the same principal can be applied to thick jointed rock beams.

According to the theory, if a longwall panel is overlain by a thick enough conglomerate beam, this beam can span across the longwall panel with an associated reduction in vertical subsidence. In other words, surface subsidence is not directly a result of sub-surface caving. Subcritical subsidence, as it is called, can develop above narrow longwall panels. The geotechnical challenge is to be able to identify the beams and maximise the panel width.

Mandalong was an ideal candidate for the application of Voussoir Beam concepts as its strata consists of a 40m thick conglomerate beam some 90m above the seam: another 90m above that is the surface.

The end result is that the face is a relatively short 125m. Seedsman said earlier work performed at the nearby Wyee Mine (where mining took place under Lake Macquarie) demonstrated that caving to the surface did not occur and deformations were consistent with the Mandalong requirements.

“Matthew Fellowes, the technical services manager at Mandalong, had been at Wyee at the time and knew the details of that mining. This experience showed you could have an economic longwall under a conglomerate and still be sub-critical,” Seedsman said.

A key aspect of sub-critical subsidence design is to form strong pillars. Mandalong’s are 41m rib-to-rib, and designed for long-term and high levels of stability and acceptable deformation.

“An integral part of the pillar design was the need to account for the infamous Awaba Tuff,” he said.

“The Awaba Tuff has been associated in the past with some unacceptable subsidence events, but extensive work at Cooranbong had identified the mechanisms that were acting and this could be translated to Mandalong.”

Taking into account the desired factor of safety, and to demonstrate the approach to regulators and the community, the first four longwall panels were narrowed from 250m to 125m, with expected subsidence projected at a maximum of 250mm. An increase in panel width should be possible once the approach is demonstrated. At the moment, the expectation is that 170m wide panels will be possible within the constraints of the consent.

The coal itself is very strong but that does not mean rib conditions are good. Roadway development is sometimes onerous, particularly when affected by the joint sets in the coal. The joint direction runs in a north-westerly direction, 10-15 degrees off the direction of the gateroad. A secondary joint is set at right angles to the primary set sub-parallel to the face.

When joints are closely spaced, drilling and, in particular, inserting the rib bolts becomes difficult as the coal can buckle at the face and the buckled slabs collapse onto the drill holes.

The joints in the coal have also been identified as a potential hazard on the longwall face, particularly when mining height goes up to 4.8m. Face sprags are installed on all the supports.