Boundary Spatial Morphology of Top-Coal Limit Equilibrium Zone in Fully Mechanized Top-Coal Caving Slope in Steeply Dipping Coal Seam  
[PERSON]  
1 School of Energy and Mining Engineering, Xi'an University of Science and Technology, Xi'an 710054, China; [EMAIL_ADDRESS] (D.  
have concentrated on two primary aspects: the top-coal fragmentation mechanism and the top-coal caving law. Subsequently, the authors of [3,4] have conducted a series of specialized research endeavors. The top-coal state evolves from a macroscopic continuous medium to a fractured medium and further to a granular medium, representing a gradual and progressive developmental process [5, 6, 7, 8]. [PERSON] et al. [9,10] revealed the complete process curve of top-coal movement, indicating that horizontal displacement prevails in the area ahead of the longwall face. As the top coal approaches the longwall face, vertical displacement becomes the dominant factor. [PERSON] et al. [11] revealed the development characteristics of the top-coal fracture field. [PERSON] et al. [12] established the progressive damage model of top coal under the evolution path of mining-induced stress in the fully mechanized caving face.  
The cavability of the top coal stands as a pivotal factor that determines the success of longwall top-coal caving. [PERSON] et al. [13,14] put forward the proposal to utilize the damage parameter as a comprehensive index of top-coal cavability. [PERSON] et al. [15,16] systematically studied the law of top-coal caving in fully mechanized top-coal caving and established a BBR research system. [PERSON] et al. [17] analyzed the shape of the coal-rock interface and the influence of the support and moving support on it. [PERSON] et al. [18] studied the mechanism of top-coal caving by considering the overburden pressure. The shape of the caving body, the coal-rock interface, and the top-coal recovery rate under different overburden pressure values are analyzed. [PERSON] et al. [19] studied the dynamic evolution of a top-coal crack under the multi-cycle action of the support. The stability of a fully mechanized top-coal caving face can be indirectly judged by the support load [20].  
The research on a fully mechanized caving face in SDCS mainly focuses on special problems under specific conditions. [PERSON] et al. [21] systematically studied the influence of top coal on the stability of support under unbalanced load and constraint conditions. [PERSON] et al. [22] revealed the variation in characteristics within the scope of SDCS; as the dip angle increases, the top-coal recovery rate gradually decreases. [PERSON] et al. [23] revealed the damage deterioration mechanism and the degree of top coal under different external loads along the dip direction of the working face. [PERSON] et al. [24] analyzed and obtained the zonal characteristics of surrounding rock deformation and failure as well as support stability in the fully mechanized caving face with variable dip angles in SDCS.  
In summary, experts and scholars have conducted systematic research on the top- coal fragmentation mechanism and caving law. However, there is a scarcity of research on the three-dimensional morphology of the top-coal failure in SDCS in order to ensure the safety of the working face slope, reveal the three-dimensional boundary shape of the top-coal limit equilibrium zone (TLEZ), and further quantify the three-dimensional boundary of the inclined coal seam. Based on the above scientific issues, a systematic study has been carried out. The purpose is to provide feasible opinions for safe and efficient mining in mines.