Photoelastic experimental study on the force chain structure and evolution in top coal and overlaying strata under fully mechanized top coal caving mining

In top coal caving mining, the state of top coal and overburden strata changes from continuous media to discontinuous and loose aggregate media. It is difficult to theoretically describe the transition and action of mining induced pressure in discrete top coal and fractured overlaying strata. Based on the photoelastic experiment principle, the network structure and the evolution characteristics of force chains in discrete top coal and discontinuous overlaying key strata are investigated during fully mechanized top coal caving mining by employing photoelastic test equipment that allows biaxial loading and bilateral particle flowing. The study shows that overburden load in discontinuous strata displays a complicated network composed by weak and strong force chains. Top coal caving mining destroys the equilibrium of the initial force chain network structure, and a composed beam-arch force chain structure forms in top coal and overlaying strata, where the overburden load is transformed into the front coal seam in the form of strong force chains. With mining face advancing and top coal caving, the beam-arch force chain network in overlaying strata develops, and a larger force chain arch structure forms. Bending, breaking and instable movement of the key strata give rise to an inverse moving of the force chain arching foot and a compaction process of the strong force chain network. The distribution density and intensity of force chains evidently increase, resulting in strong pressure phenomena taking place in the mining face. Under the biaxial loading condition, the effect of the beam-arch force chain structure becomes much evident in the earth. The entire structure of the force chain arch displays perfectly, the structure of the strong force chain network becomes denser, and due to breaking and instability of the key stratum, the pressure phenomenon of the force chain arch in front of the mining face and coal seam manifests much pronounced.