Characterizing the Top Coal Cavability with Hard Stone Band(s): Insights from Laboratory Physical Modeling

One of the most challenging problems in longwall top coal caving is the top coal cavability. Stone bands, which are usually found in thick coal seams, have shown adverse effects on top coal fragmentation and cavability. Understanding the characteristics of the stone band caving process is a primary step in evaluating top coal cavability in such conditions. In this study, a series of laboratory tests was implemented to find an appropriate construction material that can truly display the whole caving process of top coal. On this basis, two large-scale physical models comprising sand, gypsum, and mica complying with relevant similarity rules were constructed to investigate top coal cavability with hard stone band(s) according to two real cases. Based on observations from physical models, it is concluded that hard stone band(s) embedded in top coal usually produce poor fragmentation and cavability of the top coal, which is also affected by the strength, thickness, and location of the stone band(s). According to its relation with the caved goaf and the top coal beneath it, the rock beam consisting of the stone band and the above top coal can produce three profiles before it caves, to which suitable fragmenting and caving characteristics were carefully discussed. Summarizing the stone band beam into different profiles enables the characterization of top coal cavability for various configurations of stone bands in top coal. The results also verified that the fragment size of the top coal can be used as an integrated parameter that reflects all potential factors that impact top coal recovery to evaluate its cavability.