An evaluation of longwall face stability in thick coal seams through a basic understanding of shield–strata interaction

Face falls of longwall are the most complex ground control problem in mining thick coal seams in China. In this study, the mechanism of face failure is assessed from the perspective of shield–strata interaction. The paper develops a better understanding of fundamental mechanism of face failure and shield–strata interaction through a theoretical coal-support deflection model and a physical modelling study, with a consideration of shield stiffness and basic operating principles of longwall shields. The theoretical model is adapted based on the Winkler model of an elastic foundation beam, and the physical model simulates the actual performance of the shield in the field by including both the initial and cyclic loading stages. The vertical displacement of the roof and the real-time horizontal displacement of the face are recorded by a Vernier caliper and a wireless displacement monitoring system, respectively. The results show that: (i) shield stiffness has a positive relationship with the load capacity. Increasing shield stiffness/capacity enhances the ground control in the open face area, but this effect becomes negligible at higher stiffness. (ii) The physical face maintains its stability and only shows slight structural failure during the shield initial loading stage. Roof deflection and face displacement increase slowly with the progressive roof loading. (iii) During the cyclic loadings, roof vertical displacement increases in a step function upon each shield yielding; face horizontal displacement increases rapidly and face fall occurs at the final yielding event of the shield. (iv) Most face failures and ground movement are observed during shield-yielding events.