Structures and evolution characteristics of force chains in top coal and overlying strata under fully mechanized caving mining

The mechanism of force chain formation and its evolution laws in the non-continuous structure system of top coal and overlying strata are the important foundation for the calculation of support loading,the optimization of coal mining technology and the control of mine pressure. By photoelastic experiment of particles under biaxially loading and bilaterally flowing conditions,the distribution of force chains in top coal and overlying strata of fully mechanized caving mining is captured and force chain network structure is studied by topology. The study shows that the distribution of force chain in overlying strata and top coal is uniform in the initial state. After mining,the arched chain is formed near the drawing opening,and extends towards the front and upper part of the working face as the working face advances. Three main configurations of force chains in overburden strata and top-coal are identified,namely,chain,loop and chain-loop coupling respectively,which are distributed in front of the working face,undisturbed overlying strata and mining disturbed overlying strata. Besides,these configurations can be described by Betti number. Through the analysis of the distribution characteristics of the force chains in mining field,the paper reveals the forming mechanism of the supporting pressure in front of the working face and the mechanical essence of the loading action of the working face. Based on the statistical analysis of force chain strength,force chain azimuth and contact force frequency,the force chains in top coal and overlying strata display statistical features as follows:strong chain distribution of undisturbed ar- eas in the upper overburden is relatively uniform,whereas in the mining disturbed area strong force chains develop along the arch line;In front of the working face,the vertical force chain is the majority all the time.