Preliminary study of strength of coal composites

Large scale floor convergence and sudden failure of pillars in room-and pillar underground mining have been reported in various countries over many years. Failure of these mines has been attributed, amongst other causes, to the stiffness criterion used by the mining operators and most importantly mine design practices, that tend to reduce the width-to-height ratio i.e. slender pillars in pursuit of greater coal recovery yields, in order to maximize coal recovery in the seam. It is recorded that the room-and-pillar method can leave behind about 40% of the total coal available for mining. The current study seeks to determine the appropriate width-height ratio for coal composite and stabilized coal pillars. Pure coal was mixed with granular soil and stabilized with cement to form cylindrical composite columns which were crushed to determine their unconfined compressive strength at different width-to-height ratios. The strength increased with the increase in W/H of the columns. Increase in coal percentage in a composite reduces the strength. The stabilized coal pillars mobilized less strength than the composite pillars. Based on limited data, relationships amongst column strength W/H and mix ratios were formulated and are proposed for the estimation of strength of mine support structures. More data is however required to formulate a general regression equation