With this in mind, Terry Medhurst, principal geotechnical consultant at AMC Consultants, suggests that a more detailed understanding of the interaction between the longwall support and strata is needed. This is of particular significance as companies increasingly mine thicker seam sections and particularly relevant to those operations considering the application of longwall top coal caving (LTCC) in Australian conditions.
Medhurst, who for years has investigated aspects of strata control design, has reached some conclusions that question accepted paradigms around roof support fitness-for-purpose.
Using an array of assessment tools, observational methods and anecdotal evidence, Medhurst contends that the interplay of several factors – some examples include support capacity, set pressure, coal seam strength and stiffness – have not always been adequately considered in roof support design.
In longwall operations, the Ground Response Curve (GRC) can be used to assess the load distribution between the four main support elements about the longwall face, namely the coal seam, roof strata, longwall supports and goaf.
The data for these factors comes from: monitored leg pressures; leg convergence/stiffness results; observations of face conditions, along with measurements of coal seam strength and stiffness; goaf geometry; and routine geotechnical data.
“(But critically) the matching of coal seam and longwall support stiffness is critical to successful longwall mining,” Medhurst said.
“A review of successful longwall support operating characteristics shows that support stiffness and set-to-yield operating range need to match tolerable levels of coal seam compression. Excessive levels of coal seam compression result in face spall, with a corresponding reduction in roof support performance.”
Medhurst said studies at several mines indicated that most modern longwall supports tended to compress between 5-7mm per 100 tonnes of applied load.
“It is noteworthy that many longwalls operate in seams 2-3m thick, with the difference between setting load and yielding load of the supports commonly 150-200 tonnes.”
A Bowen Basin coking coal seam might typically be able to withstand about three quarters of the abutment load when compared to a hard, steaming coal seam, he said.
The additional load needs to be carried by the roof supports or it may result in excessive roof convergence.
Increased cutting height is becoming a major issue as longwall mining is put into thick-seam environments.
Medhurst contends that much above 3.7m the factors affecting roof stability change.
This is because the tip-to-leg distance of two-leg supports is commonly about 3.7m, so at cutting heights above 3.7m the main support zone is higher than it is wide.
“In essence, at cutting heights greater than about 3.7m, the supports go past the ‘square’ and revert from a beam-type loading scenario to a column-type condition,” he said.
According to Medhurst, there is anecdotal evidence to indicate maintaining a critical minimum retreat rate can mitigate the effects of poor face stability.
“The extent of the damage to the coal seam in front of the face would typically be in proportion to the seam thickness, say 2-3m for a typical longwall. Therefore, to maintain relative competent ground ahead of the face, a minimum retreat rate in the order of 5m per day is warranted.”
Roof stability is also affected by geotechnical factors such as weak immediate roof, massive overburden strata and, of course, seam thickness.
A key issue for thick-seam longwall mining is the potential for the cave line moving over the support canopy, related to the fulcrum effect.
One effect includes increased potential for support rotation into the floor.
In LTCC, it is precisely this mechanism – using a four-leg design - that is used to facilitate caving over the canopies. However, it is believed the four-leg design is better suited to blocky coal, which has high shear strengths and is stiffer, like the Chinese seams where LTCC was developed.
“Weak Australian seams (coking coal) may be prone to premature caving,” Medhurst warned, which suggests that LTCC support capacity in the order of 900 tonnes or greater might be required in a typical Australian panel layout in deep (+300m) conditions,” he said.
There are other implications in Medhurst’s explorations. From a technology point of view, some additional features on roof supports may be worth developing.
Typically, when operating under weak immediate roof, operators often resort to turning off the positive set system, to maintain horizon control. This often exposes the longwall to increased roof convergence as a result of poor set pressures and hydraulic leakage, as well as poor canopy/roof contact.
Medhurst suggests reconfiguring the posi-set system to a dual leg pressure and convergence based system.
“In other words, once the supports are set against the roof, the posi-set system is activated to maintain the support within an allowable convergence limit.”
This technology jump would require appropriate sensor technology to measure convergence, either by a potentiometer system, tilt sensors or leg fluid flow sensors.
He said accurate measurement of leg convergence could have other benefits, particularly when the longwall was often operated in yield.
For longwall automation purposes, it could be linked to horizon control in the lower-advance set cycle. As face monitoring data is more commonly used to predict weighting cycles and support diagnostics, the leg convergence rate reflects the work done by any given support.
“The on-line measured work rate of a longwall support can provide a fundamental measure of its life cycle attributes as well as reflect load transfer effects such as heavy weighting.”
Article based on Medhurst’s paper, “Practical considerations in longwall support behaviour and ground response”, presented at the AusIMM-organised COAL2005 conference in Brisbane.