Another factor inhibiting shotcrete use for primary support was the flat roof shape preferred in coal mines. Wood said for shotcrete to be an efficient roof support an arched roof shape was preferred.

Contrasting with the primary support outlook, Wood said there were many potential benefits from using shotcrete as secondary support.

“In any situation where shotfiring is carried out to break through the machine-mined roof, shotcrete for ground support would generally be superior to conventional bolts and mesh,” he said.

“This would be the case for ventilation overcasts, belt transfer points and drive heads, where some mines have shotcrete but there is not yet a general acceptance of the benefits. Applications of shotcrete would generally be quicker than conventional support, the results would lead to an active load-bearing membrane being installed as opposed to the passive nature of a mesh support, and shotcrete would seal off the rock mass to prevent any scatting or deterioration with time.”

Enhancements to surface smoothness would also lead to improvements in ventilation efficiency.

Wood said adverse effects of major faults and fault zones on roof and rib behaviour could also be alleviated with shotcrete. Development advance rates could be slashed to 5-10m a shift when crews encountered a fault zone. The area affected by the fault could also produce long-term disruptions to development if weakened material continued to unravel from the rib or roof. This was often accompanied by increased water flows.

“An application of shotcrete to seal the rock mass, perhaps in association with consolidation grouting to reduce the permeability of the rib and roof, would be highly beneficial in maintaining a safe and productive heading,” Wood said.

It appeared likely, however, that the greatest gains in productivity in underground coal mines would come from the use of shotcrete in construction.

Wood said considering shotcrete as a means of conveying and placing concrete effectively meant any application of concrete underground could potentially be carried out using shotcrete. “Recent experience indicates there are many different designs for the construction and operation of ventilation stoppings and overcasts,” he said.

“Many of the designs call for a large number of components including pre-fabricated steel sheets, Hebel brick walls, thin bed adhesives, bedding compounds, mesh sheets and expanded metal panels. A simple metal and mesh frame could be sprayed with just one material in a shotcrete construction to provide the required structural components — two or five psi over pressure — as well as the necessary sealing effect.”

The use of mobile, self-contained underground agitators and shotcrete spray equipment could also assist in placing concrete without formwork for installation foundations, roadways and sumps, and for enabling concrete materials to be delivered and pumped to locations up to 250m away from the equipment.

“This could prove invaluable in re-establishing a longwall after a roof collapse,” Wood said.

He said major improvements in ventilation efficiency, and cost, could also come about through greater use of shotcrete.

“We have not been able to establish the exact design criteria for ventilation, but have empirically noted the ventilation losses associated with the design and construction of some overcasts in some mines,” he said.

“The cross-sectional area of the opening affects both flow velocity and flow volume. A constant cross-section will allow constant flow and conversely changes in cross-section will lead to variability in flow.

“Where overcasts are constructed the height of the heading is reduced and the flow velocity increases. There is a lower pressure above the overcast and a higher pressure in the belt road or gate road below, and these pressure differentials lead to leakage.”

The profile of the shotfired overcast excavation was often also rough, which produced further inefficiencies as the airflow was stalled close to the excavation boundary.

Wood said careful redesign of the shape of the overcast to make good use of thin shell membrane strength, as well as the use of shotcrete to improve the shape of the excavation boundary, could produce improved efficiencies in airflow of up to 20%.

If the cross-sectional area was more uniform, achieved by minimising the over-excavation required to construct the overcast and modifying the roof shape of the overcast itself, and the excavated roof was less ragged, a result of shotcreting the excavated surface immediately after shotfiring, ventilation requirements could be met without adding fan capacity, Wood said.