The contractor is pressing ahead with its bid to spread the word about shotcreting benefits to the coal industry.

Re-evaluation of the role shotcrete plays in underground coal mining could be one result of extensive trialing of a new shotcreting system at Moranbah North in central Queensland.

The testwork, involving Boral Shotcrete and its purpose-built shotcreting rig (Australia’s Mining Monthly, October 1998), has temporarily shifted from German Creek to Shell Coal’s new $500 million underground mine. Australia’s Mining Monthly understands one of the reasons for the move was union opposition to the presence of a contractor on site at the Southern colliery at German Creek.

Both Southern and Moranbah North are Shell-operated mines, however, the latter is working under a more progressive work practice and management regime as a greenfields site.

Despite the union pressure at Southern, a trial program did get underway at the mine late in 1998. This followed the receipt of approvals for Boral’s two self-contained shotcreting modules (application and mixing units) and induction of the Boral Shotcrete crew.

Some underground spraying was carried out to demonstrate the capabilities of the equipment and crew, including application of a few cubic metres of shotcrete to a coal rib, and repairs to a ventilation overcast wall damaged by underground equipment.

Commenting on the stint at Southern, but not all of the reasons Boral Shotcrete shifted its equipment to Moranbah North, the company’s marketing engineer David Wood said trials were also conducted to evaluate the IS shotcrete equipment when pumping wet-mix materials up an inclined pipe. He said 220m of 75mm ID pipe were laid out up a roadway and connected to a 150mm outlet from a swing tube pump on the shotcrete rig. A coarse aggregate mix containing 20mm-stone and a 120mm “slump” was successfully pumped up the pipe at a rate of about 15 cubic metres an hour.

“A further 20m length of 50mm pipe was added to the 220m already laid and a 10mm shotcrete mix with a 90mm slump was successfully pumped at approximately 10 cubic metres per hour,” Wood said.

“The final measured length of pipe was 249.5m.

“The slump values used would be equivalent to standard mixes used underground and indicate the applicability of using Boral’s shotcrete equipment as a means of delivering and placing concrete underground in coal mines.”

In the short time spent at Southern, other shotcrete applications were also identified. These included:

— Reinforcement of stoppings and overcasts to reduce leakage and increase available air in the longwall panels.

— Construction of ventilation overcasts in main headings.

— Secondary ground support over belts to prevent lockouts being tripped.

— Repair of water leaks under explosion proof seals.

— Reinforcement of goaf seals.

— Construction of a section of the main travel road.

Boral Shotcrete is a leading supplier of shotcreting products and services to Australian underground metalliferous mines. The company believes the country’s underground coal industry could become a more substantial market for shotcreting services.

“Shotcrete has been used in underground coal mines on a sporadic basis over the past 10-20 years,” Wood said.

“It has never been considered part of the main excavation and support cycle and has usually been placed as secondary strata control or for construction purposes.”

Examples of shotcrete use for secondary strata control included roof and rib support in stonework excavations for ventilation overcasts, transfer points and driveheads, general ground control in “sensitive” areas such as crib rooms, electrical substations and belt installations, and roof and rib support in areas affected by major faults cutting through a coal seam.

In the construction area shotcrete had been used as a rib seal to control spontaneous combustion, to build sumps and ventilation stoppings, and in general foundation work.

“Anecdotal information indicates the majority of the shotcrete placed has used the dry mix method, and concerns have commonly been raised about the downstream air quality due to dust,” Wood said.

“Although modern developments in dry mix technology have gone a long way in controlling the generation of dust at the discharge into the shotcrete pot, through the dry mix gun and at the nozzle, there is no doubt that the wet mix method is far preferred.

“Not only is the potential for dust generation reduced to almost nil, the rebound associated with the wet mix method is usually about 10% compared with more than 25% with dry mix shotcreting.”

Boral Shotcrete sees significant opportunities to expand shotcrete use in the secondary support/strata control and construction areas, however, Wood said its role in primary support and strata control would remain limited.

“In contrast to the underground metalliferous mining industry where shotcrete has been shown to improve productivity in the drill, blast, muck and support cycle, shotcrete cannot be integrated into the continuous miner/support cycle without causing a considerable reduction in productivity,” he said.

“It would appear that roof and rib support in development are not a major issue in most underground coal mines where pattern support with resin bonded rock bolts and metal straps is installed on a routine basis.”