Force chains in top coal caving mining

Abstract

During the top coal caving mining, rock mass becomes fractured, and the physical status of top coal and overlying strata in the vicinity of the working face changes from continuous to discontinuous medium. It is considerably difficult to explain in theory the transition and action of mining-induced pressure in discrete top coal and fractured overlying strata. Force chain is a proper modality to describe loading transition in discontinuous and discrete media. By creating the photoelastic experiment apparatus of granular materials, trajectories of force chains in discrete top coal and overlying strata were observable by means of digital image processing technology. The characteristics and evolution of force chain network were elaborated on the basis of the theory of topology. Three configurations of force chains in overlying strata and top-coal were identified, namely, force chain, force loop and coupled force chain-loop, which were described in terms of Betti numbers of topological geometry. The study shows that there was a complicated force chain network composed by initial force in fractured strata. Top coal caving mining destroyed the equilibrium of the initial force chain network. Force chains developed and grew in the overlying strata in an arch-like form over the mining area, which transmitted the overlying loads to the front of the coal seam with advancing the working face. After mining the intensity of force chain dropped promptly inside of the goaf instead. According to the intensity of force, force chains were graded into strong, sub-strong and weak force chains. The strong force chains determined the load bearing capacity of the network, while the stability of the force chain network relied on the existence of weak force chains. The loading mechanism of top coal on the supports clarifies that low-pressure phenomena in top caving mining face were caused by the loss of laterally developed weak force chains, resulting in the reduction of strong force chains. Besides, the statistical features, such as force chain intensity, the force chain azimuth and contact force in the top coal and overlying strata, were manifested during the mining process. The study will help understanding of the mechanism of force chain formation and evolution in the fractured overlying strata and cracked top coal due to top coal caving mining.