Published in June 2006 Australian Longwall Magazine

At the time, Gallagher, who works at IMC Consultants’ Brisbane office, raised concerns over the performance of underground coal mine seals and outlined a number of issues related to their design, installation and ongoing maintenance.

Discussions about the use of seals in underground mines have now moved centre stage in the United States following the Sago mine disaster in January, during which a high-density foam block wall failed.

After the event, the Pittsburgh Post-Gazette reported the foam block wall that failed in the Sago mine explosion did not fully match designs that passed earlier tests. Last year, concrete-and-fiber blocks were used to seal 10 openings to an abandoned section of the mine; where the methane is believed to have ignited, blowing out all 10 walls.

The Gazette reported the seals that were approved for use at Sago had been originally developed for rapid deployment during emergencies, not as long-term, permanent seal structures.

The installation process of the seals was described as intricate and whether Sago’s seals were assembled according to specifications is one area investigators are looking at.

These early findings highlight the critical importance of seals being fit for purpose and considered in terms of their full life cycle. But it is precisely these things that are in general missing in both Australian and US approaches to seal installation, according to Gallagher.

The onus for the design and standards of seals has fallen on manufacturers, he said, which means proper specification of the mining environment becomes critical to enable suppliers to provide appropriate construction methods for seals.

He said the design and selection of seals needed frank and open communication between the individual mine and seal suppliers regarding expected duty, capacities of the seal type and where available, a review of historical performance data.

Information for duty specification of seals comes from a range of inputs, including the mining environment and individual mine design – pillar and roadway dimensions, etc.

However, this information is typically spread across a number of technical and operational disciplines, with collaboration among all of them needed to make the best decision.

“In addition, each cut-through may have differing needs and one product may not be suitable for all cut-throughs,” he warned.

Over and above this, Gallagher called for mines and suppliers to consider the full life cycle of seals, which includes ongoing maintenance, as well as emphasizing the critical importance of following strict installation guidelines.

And how many mining operations can truly say they take such an engineered approach to seal installation?

There are three general categories of seals: thin wall, thick wall and plug. Each of these has different requirements regarding perimeter anchorage and different failure modes.

One gap in understanding seal behavior is the lack of a database of actual seal performance and the impacts of roof and rib support. Seal performance is generally not monitored in a quantitative way. Again, this poses challenges for mines that are seeking more information.

Seal certification by statutory authorities is problematic, too.

The expense of seal testing for a relatively small market necessitates limiting these tests in number and scope with test results being extrapolated from test facility to mine site.

But of course there are a host of differences between a test facility and a mine such as differences in roadway dimensions; differences in the behavior of roof and floor; and the effects of time on seals in mines, for example.

Furthermore, Gallagher said current practice for seal repair sometimes means arbitrary decisions are made based on visual/audible observations of the seal’s condition, use of smoke tubes and periodic bag sampling.

He said both in the US and in Australia, 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.

But during the course of its life, a seal is subjected to a large number of changing variables. These include ability to withstand loading and convergence; and the effects of water and air pressures.

Sometimes seal designs are modified without undergoing new retesting and certification.

Gallagher warned more issues would arise in future, given increases in multiple seam mining and moves to thick seam mining which required more effective airtightness.

He suggested seal designers consider incorporating a means for repair in the design, like a pumpable cavity seal or injection rods incorporated in the seal itself.

“The need for a customized design approach for each seal site is proposed in order to take into account the many and variable factors that may influence a site so that improved seal performance reliability and predictability can be developed,” he said.