Influence of Thickness of Weak Bedding Planes at Various Positions Within Pillar Height on Strength: a Numerical Modelling Study

Efficient extraction of coal from an underground mine is mainly done under the protection of different forms of coal pillars. It is observed that the coal seams and its host rock contain different geological discontinuities such as weak beds (bands), which affect the strength of pillars. The weak bed creates weak bedding planes. The available pillar strength formulae are based on the width-to-height ratio, depth of cover, and strength of intact coal, and they do not consider the effects of discontinuities. A numerical simulation study on the quarter symmetry model is carried out to assess the impact of weak beds with their positional variation and thickness. The strength of the pillar without discontinuities estimated through the simulation study was validated with those obtained by the indigenous empirical strength formula. A weak bed of 0.2 m, 0.3 m, 0.4 m, and 0.5 m thick was incorporated into the model within the pillar height at different positions and studied its impact on pillar strength. A simulation study revealed that the presence of a weak bedding plane at different positions in the pillar system reduces the strength of the pillar from 3.50% to 15%. However, it was found that the strength is reduced more in the case of a weak roof-pillar interface.