WHEN longwall mining began at Kenmare in 1996 fault negotiation was always going to be the main challenge facing the mine’s operator South Blackwater Coal, a subsidiary of QCT Resources.
The coal resource was known to have significant faulting and at the outset the philosophy was to drive through faults of half-seam thickness or less, to critically appraise faults of half-to-full-seam thickness, and to avoid faults greater than full seam height.
In October last year there was perhaps more than a touch of concern as managers sat down to review fault management systems for operating the longwall. Traditional methods of fault negotiation were not working. Eleven days of production had just been lost after a “Graben-type” fault collapsed in the tailgate of longwall 1B, causing a complete shutdown of the operation. The shearer only narrowly escaped the falling roof material.
The 860m block contained a total of 13 faults, varying in displacement from 0.3-7m, and varying in orientation and type. Eleven of these were successfully negotiated, albeit with production delays. One fault was simply avoided and the longwall relocated. The fault that caused the complete cessation of production was encountered 60m from the take-off point of longwall 1B, with the inbye side displaced about 2.5m and the outbye side displaced about 0.8m. A distance of 10m separated the planes of the two faults and the strata in between was unconfined and broken.
The fault structure was known to continue through subsequent blocks and management urgently needed to mitigate the potential for a similar fall.
“We came up with an idea that maybe we could pre-inject faults with a grout system that would allow us to cut coal from underneath and get the chocks over to support the roof,” said underground production manager, Bryn Morrall.
It was “out of the box” thinking born of the experience of longwall 1B, but as Morrall remarked: “Necessity is the mother of invention.”
The idea was trialed in the tailgate of longwall 2. A fault seam was preconsolidated using inseam pro-ram drilling through to a point 6m above the coal seam. Holes were pumped with a microfine cementitious grout called Microcem 650.
“The rib side of the tailgate was first consolidated with PUR, for a depth of 8m, in an attempt to ensure that the gateroad did not collapse and to plug the edge of the zone,” Morrall said.
Into a total of 560m of drillholes, 5000kg of grout and 6770 litres of PUR was pumped. The trial proved a success, with relatively minor production losses resulting from the fault.
“The question still remained as to whether the preconsolidation gave the result,” Morrall said.
“A review of the trial was conducted with a cross-section of the workforce. Comparisons were made with similar structures in block 1A, where significant production losses incurred by strata recovery techniques led people to the view that preconsolidation was beneficial and would be included in the strategy.”
Preconsolidation has since been successfully implemented in a further two longwall blocks, confirming the methodology as a successful alternative to managing faults.
“We don’t try to glue it together and make it solid,” Morrall said. “All we try to do fundamentally is fill the gaps that are caused by the faults and rock movement so there’s no relative movement between particles and strata.”
He pointed out that though such a system had worked very successfully at Kenmare, there was no guarantee it would work anywhere else as conditions varied so radically from mine to mine. Despite the cost of about $300,000 to preconsolidate a fault, it is a cheaper option than a complete longwall shutdown.
“When mining of preconsolidated faulted zones is underway the longwall operates at a level of almost heightened awareness,” Morrall said. “Two maintenance shifts are brought on consecutively to make sure all the bells and whistles work on the wall and once in the zone production ramps up to 24 hours, seven days a week to get through the fault as quickly as possible.
“People are fully aware of the zone we enter. We have cross-sectional plans of every 5m of travel through the wall, so they know roughly what elevations they’re supposed to be at. Basically we don’t try to chase coal, we attempt to keep a straight line between the maingate and the tailgate. There can be a tendency if you are following the coal rather than the gateroads to lose horizon control on the wall, and obviously if you do that it is a lot more difficult to get back to right horizon.”
As is common with highly faulted panels, coal quality dilution does occur. Primary beneficiation at the site is performed by a coal scanner on the ramp conveyor belt which rejects coal with an ash content of greater than 30%. Kenmare utilises conventional methods of fault prediction such as surface borehole drilling at 500m spacings which is reduced to isolate any discontinuities. Seismic is used occasionally but the mine’s strata does not lend itself to very accurate prediction using seismic.
“Once we’re operating to the mine plan and we’re in the seam then we use long-hole in-seam drilling to drill out each block and that tells us where the faults are in each block,” Morrall said. “The continuous miner that drives the gateroads finds faults as well. That’s probably the best exploration tool we’ve got.”
Despite the nightmarish conditions last year Kenmare was able to meet budgeted production of 3.1 million tonnes for 1997-98. This year, with preconsolidation in place, Kenmare should be well placed to meet its budgeted 2.78mt.
