There are many assumptions about roadway support that need to be re-thought and re-tested, according to Russell Frith from the University of New South Wales.

He delivered a presentation on why dead-load suspension design for roadway roof support is fundamentally flawed within a pro-active strata management system.

“Risk-based roadway roof-support design is now a critical part of the Australian coal industry,” he said.

“Safe and efficient mining demands that roof support be tailored to the prevailing geotechnical conditions and legislation in both NSW and Queensland is clear in requiring that formalised roof support design be undertaken.”

The assumption of dead-load suspension of an otherwise unstable roof has been used in roadway roof support design for many years.

However, longevity does not necessarily translate into either best or even reasonable practice in the current industry, particularly when pro-active strata management systems are now routinely used and reinforcing-support design methods are available.

Frith discussed why the assumption of dead-load suspension is fundamentally incorrect in almost all instances when pro-active or reinforcing roof support is being applied, and how it can easily result in misleading and potentially under-designed roof support systems in various circumstances.

He summarised the commonly used design methodology of balancing the installed axial capacity of long tendons with the assumed “weight” of a future roof fall and identified several fundamental flaws.

Accepting that under certain circumstances the roof of a roadway will require to be “suspended”, Frith delivered suggestions for how a robust support system can be developed, designed and applied.

He also provided a review of the common use of suspension design for roadway roof support and highlighted a number of critical technical and risk-based issues that are not currently being given due consideration in the design process.

Of most concern is that ignoring these issues can result in the design being far more optimistic than should otherwise be the case.

“The reason that many suspension designs are proving to be effective, thus also potentially convincing the designer that their design was appropriate, is that long tendon support is being applied pro-actively to the roof when it is not in a failed state and so in reality is acting to reinforce rather than suspend the roof,” he said.

“All underground coal mines rely on reinforcement mechanisms in order to provide for efficient and economic roof control and few would argue this point.

“Furthermore, reliance on suspension roof support designed to a factor of safety appropriate to the exposure of persons to a design failure would in fact render the industry as uneconomic within a very short period.

“Therefore, it is not only advisable that strata control engineers dispense with suspension design for roof support – other than for very specific situations – it is in fact obligatory if the design is to mirror both the manner in which roof support almost always acts to stabilise the roof and also its application within a pro-active strata management process.”

Bronya Wiklund from the University of Queensland spoke about the development of a cavity prediction model for longwall mining.

Advancements in technology over the past decade in data collection and computer modelling systems, have created opportunities to develop and improve the current methods of predicting roof stability issues in longwall mining operations.

The ability to accurately predict roof instabilities and cavity developments has great benefits for the coal industry, she said.

A research project, conducted on the evaluation of cavity developments at Moranbah North Mine, aimed to determine the cause of roof instabilities and cavity developments that have plagued the mine site in the past.

The knowledge gained from prior literature emphasised features of the coal seam and surrounding strata that contribute to roof instabilities and the development of cavities at the longwall face.

Using these features a prediction model was developed, which aimed at accurately highlighting areas prone to instability within the roof and the development of cavities.

“Early prediction will allow for appropriate actions to be taken to avoid such events, removing the potential for harm to personnel and loss of production,” she said.

“A case study of Moranbah North Mine, investigating the causes of roof stability issues, concentrating on the development of roof cavities in the longwall face is presented.

Wiklund said results from an investigation of the effect of particular geological factors on the occurrence of such instability events were recorded.

“From the investigation a stability index was developed from geological data collected from boreholes on site. A hazard map was developed, using the index, to indicate areas in the roof where failures and cavities were most likely to occur.

“Although some correlations were found between the index and geological factors, the results were not entirely satisfactory as some important factors had not been included in the prediction model, which is still being improved.

“Longwall mining is the most common method of underground coal mining used in Australia today, with the method becoming ever more prevalent and adaptable to coal seams that were previously too difficult to mine.”

As a result, determining the cause of stability issues that lead to compromised safety of employees and lost production time was a high priority to the coal industry, she said.