In top coal caving mining, it is significant to reveal the distribution and evolution of mine pressure in the discrete overlying strata and coal seam. By means of photoelastic experiment of particles, force chains developed in discontinuous medium are recognized. Three basic configurations of force chains in top coal are elaborated, namely, chain, loop and coupled chain-loop. The intensity of force chains is quantified by strong force chain, sub-strong force chain and weak force chain. The configuration and intensity of force chains in relation to the packing fraction of coal and strata are investigated. From the viewpoint of force chains, the presented work shows that, due to the low packing fraction of top coals that causes the absence of sub-strong force chains in supporting of the strong force chains, the major components of the force chain network are those weak force chains in the top coal that results in a lower pressure in top coal above the supports. The study on the evolution of force chain network configuration and intensity helps a great deal in understanding of the meso-contact mechanism and the global mechanical behavior of the overburden strata in top coal caving mining.

1. Introduction

Fully mechanized top-coal caving mining method is widely used in extraction of uneven thick coal seam because of the advantages of high efficiency and good flexibility. It involves a lot of the key subjects including the mechanism and criteria of top-coal fracturing, and splitting, the laws of discrete top coal collapsing and flowing, the formation and transition of mining induced pressure with respect to time and space (Qian, 1981). Considering the physical status of top coal transiting from continuous to quasi-continuous and to discrete medium, it is improper and limited to describe the mechanical behavior of the top coal and overlying strata using continuous mechanics.

Force chain is a kind of proper mode of load transition in non-continuous and discrete media which can show the basic and inherent mechanical framework of the stability and movement states of non-continuous, discrete system under loading (Kruyt, 2016). By the photoelastic equipment, three basic configurations of the force chain structure are identified (Yang et al., 2018). These configurations can be quantitatively described by topology Betti numbers (Kramár et al., 2014). The evolution of force chain network configuration and intensity during the mining step are the focus of this paper. This study manifests the load transmission mechanism and the evolution laws of force chains in the rock strata during the top coal caving mining.

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