Scovazzo has been an itinerant consultant to Australian longwall mines for over ten years. As such he has been involved in a wide range of rock mechanics work, from roof support and pillar design, to providing expert advice in situations where equipment has been lost due to roof falls. He has compared US and Australian pillar practices and has been a rock mechanics trouble-shooter for longwalls with roof control problems.
In comparing roof control practices in the two countries, Scovazzo said people often miss some important points: “What an Australian and an American expects out of their gateroad system is different, and what the inspectorate expects is different again.”
One of the major and most obvious differences between US and Australian mines is the three or four-entry gateroad system in the former versus the two-entry system in the latter. Two-entries are very confining and are required to remain standing, Scovazzo said.
“If they fall it’s a problem, from a safety, ventilation, egress/digress and equipment stand point. With a three and four-entry system, if you lose an entry it’s not as big a problem as long as it still carries air.”
Compared with 3 or 4 entry systems, two-entry systems are typically designed with wider pillar sizes, which generate horizontal stress closer to the opening. This horizontal stress has to be handled with ‘stiff’ support systems, such as higher bolt density.
“This means you’re vulnerable at the end of the bolt for it separating from the roof so it has to be monitored because it cannot be observed. The only option then is to support above the break and the most popular application for that is cables in Australia,” Scovazzo said.
In contrast, gateroad systems in the US are designed with ‘yielding’ pillars – another concept differently understood on both sides of the Pacific.
“If we in the US are talking about a yielding pillar we’re talking about a typical 10m centre to centre pillar so it’s smaller than what you expect here,” he said.
In a yielding environment, the stress field is higher into the overburden such that the horizontal stress field is actually not being supported.
“Because it is more yielding, our support system is holding up rubbleized material more than it is trying to support the roof. When you see a coal mine roof in the US, even though it’s the identical rock type and identical strengths, it’s going to look a lot rattier than it’s going to look in Australia.”
Scovazzo said it was often surprising for an Australian operator to realise the roof in a US mine may be allowed to ‘relax’ by up to a third of a metre before first support systems go in, which helps by pushing the horizontal stresses further up into the overburden.
The most common remark from Australian operators is that US miners work in poorer conditions, he said. US operators on the other hand remark that Australian mines are so quiet. The word ‘eerie’ is used.
“The thing that will bother an American inspector is if he comes into an opening and he hears no sounds of adjustments and he sees a roof being held too stiffly he starts worrying he’s going to have a major collapse. He is used to a yielding environment and he’s cautious of the horizontal stress field.”
Scovazzo designs for both stiff and yielding environments and he says from a rock mechanics point of view, both systems work - neither is statistically more or less safe than the other. (While some Australian coal mines have dabbled with yielding environments he does not believe these experiments were as successful as they might have been.) But he does make the point that where the differences lie is from an operational/mining point of view, particularly with longwall development.
“Larger development pillars in Australia mean less efficient procedures, such as longer shuttle car change-out times. Also, the sheer quantity of support they put into the roof here means you have to sacrifice something, and that is development rates.”
The higher roadway development rates achieved in the US, compared with in Australian longwall mines, may be partly accounted for by the different configurations. US development method, place-changing, revolves around small pillars, which may also partly explain why the method has enjoyed mixed success in Australia.
Scovazzo also suggests that roof support systems for both places are becoming more similar, with a trend developing in the US to use cable bolts as the primary support system similar to what is happening in Australia. US mines have, however, adopted cable bolts differently.
“In Australia they tension them. In the US to tension them means to waste the strength of the bolt and what they mean by that is that the bolt will load up sooner or later and they want the ability of the cable to stretch.”
John T Boyd Company (Australia) are assisting Austral Coal with the specification and planning for a new longwall for the Tahmoor mine in NSW. Scovazzo is providing rock mechanics input to Tahmoor.
