Published in the August 2007 American Longwall Magazine
Two researchers from the National Institute for Occupational Safety and Health, Tom Barczak and Steve Tadolini, told a recent industry conference that a jump in the size and capacity of roof support systems, especially longwall shields, may or may not be a step in the right direction for the industry.
"Rarely do we see a one-size-fits-all approach to roof support, except when it comes to longwall shields, where the new historical trend... is to simply purchase the highest capacity support you can buy," the two said. However, they said, doing so does not necessarily guarantee roof control failures will be averted.
It is the highest capacity tailgate supports that most typically fail, which tells us one significant thing - there is still information we do not completely understand when designing roof support systems.
While size and capacity are often intertwined, it is the latter that has caused debate in recent years. Control is the key word; specifically, whether the system is causing the ground to react for or against it when a problem arises - in short, we can change the way we look at support design by looking at ground reaction design.
"The reason this is so important is that they have opposite effects in terms of support design requirements. In a force-controlled load environment, the goal is to provide enough support capacity to offset the rock load and provide equilibrium of the rock mass," Barczak and Tadolini said.
"Conversely, in a displacement-controlled load environment, the ground movement is not influenced or completely controlled by the support. The goal is to maintain support stability and roof support capability as convergence inevitably continues."
With conventional support design, the two noted, the method is simpler, as supporting the full dead weight of detached rock masses has given way to a ground reaction design approach.
"Here, the goal is to match the support characteristics to the ground response, and not to try and overpower the ground forces with some massive support capability," Barczak and Tadolini said. The ground reaction concept, they added, produced a more accurate illustration for yields.
Design is another factor that should be examined, as at present the push for higher shield yields is driven by a desire to have higher setting pressures. However, the goal of design should continue to be matching support performance to ground response.
"Essentially, there is no need to have the shield do more work than the coal in the longwall panel ahead of the shields," Barczak and Tadolini said. "Therefore, from that perspective, there is a logical upper limit to shield capacity."
They urged the industry to remember that the larger the shield capacity is, the more load is transferred to those parts, which does not benefit ground control.
They said higher capacities also mean that the canopy and base structures must have a high stress transfer ability, and toe-legged toe loading is affected through higher toe pressures. Also, shield stiffness increases with capacity, and stiffer support will develop more loading than a softer support under the same load conditions.
"Obviously, a 'bigger-the-better' design philosophy provides insurance against conditions that are abnormal," they said. "However, history clearly shows us that all of the capacity is not needed all of the time, as many longwall panels have been successfully mined with no significant problems at far less capacity than is currently provided by modern longwall shields.
"The argument against using all of the capacity all the time is that it unnecessarily causes loading of the shield components, particularly the caving shield-lemniscate assembly and high toe loading in two-leg shields," they said.
Barczak and Tadolini suggested an alternate approach of developing a "smart loading technology" design that uses the capacity only when it is needed by lowering the set pressure; they also suggest the potential for convergence instrumentation to be installed as part of future shield design.
"Is bigger better? No, bigger is not always better, but is often required because of the support design. The ground reaction curve holds the key to optimizing support design," Barczak and Tadolini said.
NIOSH continues to look at the applications surrounding ground reaction design for roof support. In that, the industry can also play a part. "Thinking outside the box never hurt any industry," the two said. "Mining has made it a common practice."
