# Uneven Stiffness Coal Seam: A New Structural Factor Prone to Coal Burst Based on Stiffness Theory
## 5 Further Discussion about USCS  
### Connection between USCS and Existing Mechanism Research Results about Coal Burst  
Based on the cusp-type catastrophe theory, [PERSON]. [16] highlighted that the occurrence of coal bursts is contingent upon the roof experiencing a sudden jump subsidence characterized by an adequate amplitude. The roof above the damaged coal mass is expected to sink, but the presence of surrounding undamaged coal mass provides support and inhibits further subsidence. In Figure 9, it can be observed that roof in HSZ of USCS exhibits an upper convex bending prior to the failure of the coal mass, and the subsidence of the roof after failure (\(\Delta H_{2}\)) is considerably greater than that of the ESBS (\(\Delta H_{1}\)). Based on [PERSON]'s viewpoint, it can be inferred that coal mass fails in HSZ of USCS is more likely to result in coal burst than ESCS. Figure 9: Roof subsidence after coal mass failure of ESCS and USCS. (**a**) Roof subsidence after coal mass failure of even stiffness coal seam structure; (**b**) Roof subsidence after coal mass failure of uneven stiffness coal seam structure. [PERSON] once said: \"rock bursts often occur due to the violent failure of columnar rock units that are relatively isolated\" [10]. These \"relatively isolated columnar rock units\" mentioned by [PERSON], similar to the \"mine pillars\" in [PERSON]'s description of stiffness theory [12], were previously understood by most researchers as coal pillar. However, these terms can also encompass columnar coal units with high NSC surrounded by coal mass with lower NSC. The HSZ of the USCS can be likened to a concealed coal pillar embedded within the solid coal, which is enclosed by LSZ. The contrasting stiffness levels result in disparate deformations and create the appearance of isolation in mechanical behavior for the HSZ amidst the LSZ. Similar to load-bearing column, the HSZ plays a crucial role in supporting the roof within a broader range than its own cross-sectional area. Once the coal mass of HSZ fails, it triggers significant stress adjustments and configuration changes within the coal mass-surrounding rock system, which may cause the significant sudden subsidence of the roof proposed by [PERSON]. [16], and ultimately leading to coal bursts. From an energy perspective, when coal mass of the ESBS fails, the energy released by the surrounding rock is the elastic deformation energy; Under the same conditions, when the coal mass in HSZ of USCS fails, the surrounding rock not only releases the same amplitude of elastic deformation energy, but also additionally releases the bending deformation energy of the roof and floors. Therefore, the failure of coal in HSZ of USCS is more likely to meet energy criterion, and according to energy theory, it also has a higher risk of coal burst. Several scholars have emphasized the significant role of accumulating and releasing bending deformation energy in promoting coal burst incidents [3; 4]. Nevertheless, it is challenging for the roof of ESCS to undergo bending. Consequently, a sufficient span of hanging roof is required in the goat to amass an ample amount of bending deformation energy, often necessitating roof breakage to facilitate its release. In the main roadway or excavation tunnels, the hanging roof span is typically below 10 m. Although a thick and rigid roof can accumulate a substantial amount of bending deformation energy, It is unlikely to break under a 10-m span. Conversely, a soft or thin roof can fracture but fails to accumulate enough bending deformation energy to cause a coal burst. Therefore, while the assertion that bending deformation is released through roof fracture, leading to coal bursts, can account for coal bursts near the mining face, goat pillars, and similar areas, it struggles to explain coal bursts occurring in the main or excavation roadways located far from the working face and goat.