## 5 Discussion  
Through various research methods, the boundary spatial morphology of the TLEZ boundary is revealed, and the analytical characterization of the TLEZ boundary is achieved. The mining-induced stress on top coal along the dip of the longwall face has obvious regionalization characteristics that make the top coal also show regionalization characteristics in the process of gradual deterioration, resulting in the \"asymmetric distribution\" of the TLEZ boundary along the dip. In the fully mechanized top-coal caving face with SDCS, the mining-induced stress acting on the top coal along the dip of the longwall face exhibits distinct zonal characteristics. This causes the top coal to also display zonal characteristics during its gradual deterioration process, thereby leading to the \"asymmetric distribution\" of the TLEZ boundary along the inclined direction. The boundary spatial morphology of the TLEZ takes on an \"asymmetric arc-shaped ribbon-like curved surface\". This paper reveals, for the first time, the spatial configuration of the TLEZ boundary in a fully mechanized top-coal caving face with a large dip angle. Previous researchers [2] took the damage angle as the breakthrough point to determine the spacing between the  
Figure 19: Comparative analysis of the boundary spatial morphology of the TLEZ. TLEZ boundary and the longwall face based on the displacement of the top coal in different sections. By making a comparison with previous studies, this paper quantitatively characterized the top-coal limit equilibrium boundary by analyzing the stress state of the top coal in SDCS and integrating the principal stress data of the top coal. By comparing the upper boundary of the TLEZ determined through damage characterization, numerical simulation, and theoretical analysis, as shown in Figure 20, the patterns of the three curves are generally consistent, and all exhibit asymmetry. However, there are certain discrepancies in the lower part of the longwall face. At the 0 m position of the longwall face, the distances from the longwall face obtained through damage characterization, numerical simulation, and theoretical analysis are 1.24 m, \(-\)0.5 m, and \(-\)0.85 m, respectively. Because the gob in the lower part of the longwall face is compactly filled, the displacement characteristics of the top coal in front of the longwall face are weaker than those of the top coal in other areas of the longwall face. This limits the continuation of research on the TLEZ boundary from the perspective of damage. Finally, the rationality and effectiveness of the TLEZ boundary are further verified by combining the frequency distribution law of top-coal leakage in the working face with data measured in the field. Building upon the previous two-dimensional single boundary, this paper conducts a further study on the three-dimensional spatial morphology of the TLEZ boundary. The determination of the spatial configuration of the boundary enables workers and researchers to have a clearer understanding of the failure process and extent of the top coal. Additionally, it allows for a more scientific judgment regarding the issue of guiding the top-coal caving rate. The revelation of the boundary spatial morphology of the TLEZ provides a clear direction for the stability control of the \"support-surrounding rock\" system in SDCS. Moreover, it enables the formulation of a reasonable coal caving scheme based on the distance relationship between the top-coal boundary and the longwall face in different regions. Building on the regional distribution characteristics of support stability under the influence of the boundary spatial morphology of the TLEZ, the following caving principle for the fully mechanized top-coal caving face in SDCS is proposed:  
1. Coal caving should be conducted from top to bottom along the dip of the longwall face. 2. Along the inclined direction of the working face, the amount of coal discharged varies in different areas.