## 3 Fracture Structure and Stability Analysis of Coal Pillar Overburden Rock
### Characterization of the Broken Structure of the Coal Pillar Overburden  
This study primarily focuses on the geological conditions beneath the soft coal seam. The movement law of the overburden rock during the process of rock breakage and the stress values on the coal pillars are analyzed using indoor tests [27; 28].  
As shown in Figure 3,The figure illustrates the failure characteristics of the overlying strata at varying mining distances. Due to the presence of a thin coal seam, only 1 m thick, above the roof, the roof's load-bearing capacity is compromised. When the working face advances to 34 m, the initial pressure is observed at the basic top. As the working face continues toward the coal pillar side, 11 cyclic breaks occur, averaging 17.3 m per step. The upper key layer experiences periodic breakages seven times, averaging 23.7 m per step. Consequently, the overburdened rock fractures and collapses, destroying its load-bearing structure and forming an \"articulated\" support configuration for the overlying rock. Based on calculations of the development height of the water-conducting fissure zone [29], once the overlying rock damage extends to 75 m, it is no longer influenced by the key layer, resulting in the formation of an \"articulated-articulated\" structure, with the breakage of two key layers. Throughout this process, significant deformation and deflection were observed on the coal pillar side of the 1513 return air lane at the site.  
Figure 3: \(Cont.\)An analysis of the structure and morphology of the overlying strata in a similar simulation experiment reveals that the direct roof, composed of interbedded coal and siltstone, collapses and settles neatly into the goat during the mining of the working face. As the working face advances continuously and reaches full extraction, the rock layers periodically fracture, resulting in a cyclic \"hinged\" structure in the lower bearing rock layer and similarly in the upper bearing rock layer. These two bearing rock layers combine to form a \"hinged-hinged\" structure above the coal pillar, exerting simultaneous stress on it, leading to stress concentration within the coal pillar.  
The data obtained from the stress meter placed on the floor of the physical similarity model indicate that the vertical stress peak occurs at the central axis of the coal pillar at a depth of 14 m, as shown in Figure 4. The coal pillar, with a width of 25 m, remains stable overall, exhibiting an M-shaped stress curve. The highest stress, reaching 12.9 MPa, is observed on the goat side. As the coal pillar width decreases, vertical stress increases rapidly, causing the stress curve to transition from an M-shape to a single peak. The vertical stress increases steadily as the width decreases from 25 m to 14 m. The transition from a double-peak stress distribution at 25 m to a single peak at 14 m results in a 29.1% increase in stress, reaching 18.4 MPa. Due to the weak nature of the five coal seams, a narrower coal pillar width corresponds to higher vertical stress.  
Hence, to ensure the operational integrity of the 1513 working face's return airway and maximize coal recovery, a thorough analysis of the coal pillar width is essential.  
Figure 3: Structural characteristics of overlying strata under different mining conditions. (**a**) Initial stage of excavation. (**b**) Breaking structure of low key stratum. (**c**) “Articulated-articulated” composite structure.