A flat-lying coal seam 3 m thick and 75 m below ground surface has been mined with 5.0m rooms and 7.0m square pillars, over the lower 2.2mof the seam. Determine the factor of safety of the pillars and assess the feasibility of stripping an extra 0.6 m of coal from the roof. The strength of the square pillars, of width wp and height h, is given by
S = 7.5h−0.66wp.0.6
where S is in MPa, and h and wp are in m.The unit weight of the overburden rock is 25 kN m−3.
