Underground » Strata Control and Windblasts
Pillar design in underground coal mines has a long history, originating from pillar strength analyses in South Africa. Over time, modifications and developments of the early formulae have been introduced, notably the squat pillar formula and the Australian adaptation by UNSW. However, one aspect of pillar design that has not seen significant development over the same period is pillar load estimation. In mains, longwall gateroad development and bord and pillar panels, pillar load is almost exclusively based on the Tributary Area Theory (TAT). This theory assumes that the load on an individual pillar is equivalent to the weight of the full overburden immediately above it, plus an equal share - along with adjacent pillars - of the overburden above surrounding roadways.
For the TAT to hold, however, the panel width must exceed its depth. This is a key assumption, but rarely occurs in Australia's deep coal mining operations. This discrepancy is generally not an issue when designing typical gateroad chain pillars, as they are assigned a nominal rather than actual development Factor of Safety (FoS) based on the highly conservative TAT load estimate. These pillars will ultimately experience much higher loads during longwall retreat, making the initial FoS calculation less critical. It means it would not be appropriate to use the results of this project and the effective depth concept to determine the load on gateroad pillars. The abutment load concept is used for this purpose, with pillars designed based on the expected maximum loading, e.g. maingate loading, side abutment loading, tailgate loading or double goaf loading, all of which exceed the TAT load. Nevertheless, it is quite common for underground coal mines to excavate pillars that are significantly smaller than surrounding pillars and barriers due to operational constraints. This often occurs near the installation and salvage roads of longwall faces. Additionally, pillar size and shape may be altered due to pit design limitations or geological factors such as the presence of dykes or faults. Despite these variations, the main design method commonly applied in such cases (in Australia and worldwide) remains the assumption that pillars are loaded according to the TAT.
In practice, a pillar with a minimum centre distance of, say, 20m at a depth of 250m, and surrounded by much larger pillars in a two-road panel, will not be exposed to the full TAT load. The reason is that the stress on the surrounding larger pillars and barriers will limit the displacement of the smaller pillar, which in turn restricts the amount of stress that can accumulate on it. The difficulty that mines frequently have is that they have pillar design guidelines that stipulate minimum pillar FoS values, and the nominal FoS values of these small pillars often do not meet these criteria. Thus, there is a clear need to have a new design methodology that produces more realistic FoS values for such pillars of various sizes and shapes.
Another limitation that has been frequently raised by different industry experts is associated with the impact of high in situ stress (in particular horizontal stresses) on pillar stability assessments during the design stage, especially for non-horizontal seams. Current methods rely heavily on the depth of cover as a key driver for pillar design. The increasing depth of many underground operations leads to significantly higher in situ stresses. It would be highly beneficial to have a better appreciation of the role and effect of stress as an independent factor on pillar behaviour. Such an understanding is important, particularly to ensure the safety of roadways and the continuous productivity of the operation.
This project suggests two areas for improving pillar design and understanding pillar behaviour:
- A method to account for different load distributions across pillars, particularly in cases where a small pillar is surrounded by larger pillars or where the mains panel width is significantly less than the depth of cover.
- Investigates the effects of horizontal stress on coal mine pillar behaviour.
Through a carefully designed set of experiments backed by numerical simulations, the project's results suggest that the nominal pillar load of a small pillar estimated by TAT is significantly higher than the actual load at great depths or when the span is relatively small compared to the depth. This conclusion is based on large-scale physical modelling using similarity theory, which enables direct measurement of the actual load applied to “small pillars” in two-dimensional laboratory models, allowing for comparison between actual and nominal TAT loads. The findings have further been verified through numerical modelling in Rocscience2D (RS2), incorporating the effects of horizontal stress and roof stiffness.