Threatened two years ago with closure because of windblast conditions, the Moonee longwall mine is today at the cutting edge of an extraordinary array of technologies that enable the accurate prediction of windblasts and ensure the ongoing safety of employees.
Moonee is believed to be the only mine in the world using real-time microseismic warning systems, and the only coal mine using hydraulic fracturing to generate goaf falls on demand. The mine also runs one of the world’s most highly automated longwall faces.
Moonee is located 100km north of Sydney in the Newcastle coalfield of New South Wales and is operated by Wallarah Coal Joint Venture, 80% owned by Billiton subsidiary Coal Operations Australia and 20% owned by Nissho Iwai. Billiton announced plans last month to sell its interests in the joint venture.
Mining is focused on the Great Northern Seam with a 100m wide longwall face under 40m of massive conglomerate structure. The goaf hangs up for up to 300m and when it caves en masse, windblast measuring speeds of 123m per second is generated.
Moonee had its first windblast event in January 1998, 200m into the first block, two months after longwalling began. No-one was present but some face equipment was damaged and some items were hurled 30m outbye. That same month, the fifth goaf fall in the panel injured six crew.
Following this incident a more extensive windblast management plan was developed as well as the introduction of real-time microseismic monitoring to use escalating microseismic activity as a windblast predictor. This system consists of an array of geophones installed in the maingate and tailgate roadways near the face. Each geophone transmits a signal to the surface monitoring computers, manned 24 hours a day by geologists. Three types of alarms are recognised — trend, magnitude and frequency alarms.
With this system in place, the rest of panel 1 and all of panel 2 were injury free. In 90% of cases the microseismic system gave sufficient warning of impending roof falls that could cause windblast. In April 1999, it became apparent that the windblast management plan and the microseismic system were not 100% reliable. The goaf suddenly collapsed with no warning, either audible or microseismic, and a worker was injured after being thrown 3m against a support. The helmet he was wearing and the phone piece he was holding were blown more than 40m through the adjacent cut-through. The Mines Department Inspector responded by closing down the mine.
The mine’s future was in the balance, as well as several hundred jobs. Ken Mills of Strata Control Technology and Rob Jeffery of CSIRO Petroleum suggested trialing hydraulic fracturing, used by the petroleum industry. Hydrofraccing is achieved when high pressure water is injected into a borehole to initiate a fracture around the borehole. An unsuccessful trial was attempted from the surface into an old goaf area.
The first underground trial saw an inclined shallow projecting hole drilled from the maingate which brought down 70,000t of rock. The next hole was an uphole drilled from the middle of the longwall face, bringing down 100,000t. The number of holes per fracture was refined to two holes, with approximately 47m of goaf standing when the fracture was initiated.
Prior to the introduction of hydrofraccing, goaf hanging up before a fall could be as much as 31,000 square metres — roughly the size of six football ovals, according to Col Macdonald, Moonee’s projects co-ordinator.
Hydrofraccing is now a routine part of the production cycle at Moonee. As soon as the longwall has retreated 20m from the previously fallen goaf, three 10.8m-long, 57mm-diameter holes are drilled into the roof across the face. Mining then continues for a further 40m before hydrofraccing is initiated.
Where more than 20m — or 2000sq.m — of goaf remains standing the face operates in “red” zone. This is when there exists the threat of a windblast capable of generating significant velocities of greater than 20mps. The windblast zone of influence is physically demarcated by mesh barriers, 650m from the face, where the wind velocity has been shown to attenuate to 20mps in Moonee’s two heading development.
When the longwall is in red zone, operating procedures are governed by detailed risk assessment and written procedures for all tasks. During red zone operations, those working on the face wear full face helmets, leather jackets, knee guards and elbow guards. All workers are attached to rock climbing lifelines located on the face and in the maingate conveyor road. Safe havens or refuge areas are provided on every second longwall support and at several places at the maingate area.
Only three operators are allowed on the face and the longwall operates in full automatic mode, using DBT’s PM4 electrohydraulic system.
Mick Webster, DBT manager automation, said: “The successful interface with the Long-Airdox shearer via a serial tracking link allows the shield advance and AFC bankpush operation to run virtually unattended.”
The ability to “dial-a-fall” as Peter Hayes, general manager of Wallarah Coal calls it, has enabled the mine to regain the level of productivity required to remain financially viable. “These processes together have significantly reduced the risk of injury from windblast. Without this process it is unlikely the mine would have survived,” he said.
Apart from its obvious technical challenges Moonee is also interesting from a management and people point of view. On reopening, the mine turned its back on the traditional work practices and negotiated a no-demarcation agreement with the CFMEU. It allowed for a small, multi-skilled workforce where tasks were allocated on skills rather than on trade/non-trade background or stream. The end result, according to mine manager Ross Campbell, is a highly skilled and disciplined team with a “can do” attitude. To continue to run efficiently, despite the obstacles, has been Moonee’s greatest achievement.
Originally published in the March 2001 edition of Australia's Longwalls.