Bord-and-pillar pillar distress at a goaf edge under a seasonal stream
A bord-and-pillar coal mine operates at 320 m depth in a flat seam 2.6 m thick. Development uses 28 m square pillars with 5.0 m galleries. The immediate roof is 6 m of massive sandstone, overlain by 45 m of laminated shale. In the last month, three pillars along the goaf edge have shown vertical slabbing and corner spalling, and a roof extensometer records 40 mm of sag over a 10 m span. Average pillar stress by the tributary-area method is 11.6 MPa. Back-analysis of stable pillars in the same seam gives a pillar strength of 17 MPa. The goaf-edge abutment stress concentration is estimated as 1.7 times the average. The mine plan proposes depillaring 60 m of pillars under a seasonal stream. Production targets require at least 70% extraction in the next year. Management wants to know whether the observed damage is a local roof issue or a pillar-stability warning.
1. Calculate the average and goaf-edge pillar stress and the factor of safety. Is the observed spalling a local roof issue or a pillar-stability warning?
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Use the tributary-area check first. With a unit weight of about 26 kN/m3, the vertical overburden stress is 26 x 320 / 1000 = 8.32 MPa. The tributary-area factor is (28 + 5)^2 / 28^2 = 1.389, so average pillar stress is 8.32 x 1.389 = 11.56 MPa, consistent with the stated 11.6 MPa.
At the goaf edge, apply the concentration factor: 1.7 x 11.6 = 19.7 MPa. The factor of safety is 17 / 19.7 = 0.86. The edge stress exceeds the back-analysed pillar strength by about 16%, so the spalling is not merely a local roof issue; it is a pillar-stability warning. The 40 mm roof sag reinforces that the roof is responding to pillar compression and abutment load.
2. Should the mine proceed with depillaring 60 m of pillars under the seasonal stream? Recommend a layout and sequencing change.
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Do not depillar under the stream under the present geometry. The goaf-edge factor of safety is already below 1.0, and depillaring will increase abutment loading and transfer stress to the remaining pillars and the stream bed. A small widening of pillars is not sufficient: a 35 m pillar with 5 m galleries gives a tributary factor of (40/35)^2 = 1.306, average stress 10.87 MPa, edge stress 18.5 MPa and factor of safety 0.92, still below unity.
Leave a solid barrier pillar at least 45-50 m wide on each side of the stream, and do not extract pillars within 60 m of the stream. Mine any future depillaring panels away from the stream, with a sequence that retreats away from the barrier rather than toward it. If the extraction target cannot be met, reduce the extraction ratio in the buffer zone rather than increase it, and consider stowing or partial extraction only after a trial panel proves stable.
3. What monitoring and data would you require before any future depillaring?
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Require pillar convergence stations, borehole stress cells, roof extensometers and seismic monitoring in the trial panel. Map joints and roof falls, record pillar spalling dimensions and timing, and install piezometers to check whether the stream is losing water into the workings. Back-analyse any trial extraction to update the pillar strength and abutment concentration factor. A trigger-action-response plan should define warning levels for convergence rate, stress change and water loss, with a stop-work threshold. Do not authorise full depillaring on the basis of average stress alone; the goaf-edge stress and observed damage must control the decision.