Geomechanics and Ground Control · Ground Control · 2 marks · Numerical answer
A flat coal seam lies 250 m deep with vertical stress gradient 0.027 MPa/m. Square-pillar strength is 12.5 MPa. Find extraction ratio giving safety factor 1.5, to two decimals.
Overburden stress is 0.027×250=6.75 MPa. Safety factor 1.5 permits pillar stress 12.5/1.5=8.3333 MPa. Tributary-area stress isσv/(1−e), so 1−e=6.75/8.3333=0.81. Thus e=0.19. Extraction ratio is a fraction here, not 19 as a percentage entry.
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