In recent times there has been a number of incidents of varying magnitude and consequence resulting from seal failure or leakage, which potentially might have been averted through improved practice, and in particular through improved cooperation between mines and suppliers, Gallagher says.

 

“Putting the cost of seals into perspective, at some $A8000-12,000 per explosion rated seal, seal costs for a panel of some 25 cut-throughs in length would be some $A250,000 – far less than the costs of a longwall stoppage of around half a day,” Gallagher said.

 

While legislation in Queensland and New South Wales give some level of prescription for coal mine goaf seals – generally in relation to capacity to withstand overpressure events and resultant leakage limits – it stops short of actually specifying details on standards and methods, in accordance with risk-based management approach.

 

Gallagher says the onus of design and standards for seals has fallen on manufacturers, and that proper specification of mining environment by the coal mines is required to enable suppliers to design and recommend appropriate construction methods for seals. While testing of seals under controlled conditions in recognised test facilities provides some confidence in design, conditions are generally significantly different from those that exist in the field.

 

A previous ACARP study (C10014) completed in May 2002 examined this issue and concluded that field testing would be too expensive and limited by the approval of specific equipment required for completion of testing in the hazardous environment typically found in the vicinity of seals. The study concluded that lab testing in combination with modelling might be a suitable approach for assessment of seal design.

 

To provide some idea of the size of the market, assuming an average of 25 CT’s longwall retreat per year per mine for an Australian longwall population of some 24 mines, the potential annual market value is more than $A6 million.

 

There is no single solution available in regard to: site selection for a seal; consideration of the operational environment of the seal or whether a potential water head should be applied; control and state of the ground surrounding the seal. However, it is apparent that an overlap of input from professionals involved in ventilation, geotechnical, mining and operations is required to identify practical solutions, Gallagher says.

 

He says that both in the US and in Australian, there has been very limited development of guidelines as to how to identify defects in installations such as filling voids, systems to maintain the seal, and guidance on how to repair or identify the need to replace seals. Appropriate guidelines need to be developed at each site.

 

Gallagher has assembled a checklist of considerations for mine goaf seals, including approaches to mine and pillar design, geotechnical modelling and data collection, civil engineering design aspects, site evaluation, and practical options currently available.

 

“It appears that risk management and a full life cycle approach to seals has not been adopted to the same extent as for other aspects of operations. Further, because of the number of disciplines and personnel involved that may influence various factors affecting seal integrity, the opportunity for oversight or unclear allocation of responsibility is considerable,” Gallagher said.

 

He says the need for a customised design approach for each seal site should be applied in Australian coal mines, including a program whereby performance and triggers for rectification or maintenance action are introduced. The variability of conditions from seal site to seal site can be very significant (eg seam thickness, structure etc).

 

“Many mines rely on visual/audible inspection, use of smoke tubes and periodic bag sampling as the primary means of assessment of seal integrity.

 

“Other significant factors such as seal material properties, rib degradation, convergence, floor heave, effects of water on both structural integrity of the seal as well as the air tightness of the seal do not generally receive the same level of attention. Additionally, dynamic effects of horizontal stress and movements and their impacts on seal integrity do not appear to be specifically considered.

 

“It is apparent that while some research has in isolation examined issues such as overpressure resistance, leakage performance, seal materials, rib sealing, effects of longwall mining and assessment of seal construction and integrity, further research may be required to deliver answers to many issues to assist the industry and service providers develop and improve standards.”