15
Collapse area modelled
The area coloured green was the surface expression of the
collapse modelled, also with a curved outline, confirming the
role of the overburden in controlling the shape of the collapse.
Pillar Deformation Behaviour used in the model
Indicative rockmass softening curve demonstrating the plastic
strain transition points 1 and 2. This is how pillars behave as
they are loaded and slowly deteriorate by scaling, until they
reach the transition zone, prior to collapse.
Subsidence from start to end of weakening stage, at surface
measurement station N02
Above:
Conclusion
The simulations demonstrate that the FE simulation framework
can capture rapid pillar collapse and surface subsidence by
simulating pillar deterioration which leads to “snap-through”
of the overburden. This is an important result, because this
mechanism is believed to be responsible for rapid subsidence
events in the Witbank Coalfield.
Recommendation
A cautious approach to re-evaluating pillar strength must
be followed, given the potentially serious consequences of a
collapse in 2 Seam, when mining the 4 Seam above.
Remote sensing techniques, via surface boreholes, would
be appropriate to determine the condition of the 2 Seam
pillars and by evaluating the geological cores, determine the
overburden characteristics above the old workings.
