# Uneven Stiffness Coal Seam: A New Structural Factor Prone to Coal Burst Based on Stiffness Theory
## 5 Further Discussion about USCS
### Artificial USCS and Stiffness Perspective  
Two extreme cases, Model 9 and Model 10, were simulated in the simple simulation of USCSs. Model 9 features the LSZs composed of ideal coal seam with an extremely low elastic modulus (EL = 0.1 Pa). Model 10 represents the scenario of a coal pillar and goat with a continuous roof, achieved by removing the LSZs within Model 9. Figure 10 portrays the calculation of Mises stress distribution and deformation for both models. As shown in Figure 10 and Table 2, the simulation results of the model 9 and model 10 demonstrate a high level of consistency. Substituting the goat in Model 10 with an ideal coal seam with a NSC close to 0 in Model 9 leads to negligible changes in stress distribution and deformation. This indicates that considering the goat as a coal seam with an extremely low NSC is justified. Consequently, the combination of the entity coal and goat can be seen as an uneven stiffness coal seam that, when coupled with the roof and floor, forms a distinctive USCS known as artificial USCS. The NSC of the excavated areas such as goats and roadways in the coal seam reduces to 0, creating artificial LSZ. Conversely, the areas that haven't been excavated, such as entity coal and coal pillars, become relatively stiffer HSZs due to the retained original NSC. The stiffness contrast between the goat and coal pillar is extremely significant, resulting in HSZ of artificial USCS bearing almost the entire overlying load. From a stiffness perspective, the process of mining, excavation, slotting, drilling, failure and fracture development in coal seams can be seen as a process of reducing stiffness and then adjusting related parameters of existing USCS or creating new USCS. In this context, entity coal, coal pillars, and affected areas of remnant pillars represent the HSZs of artificial USCS. Conversely, the goat, roadway, and affected areas of protective layer represent the artificial LSZs. Similarly, the HSZs of natural USCSs, such as thinning zones, bifurcating areas, and magmatic intrusion areas, can be regarded as coal pillars or isolated islands hidden in coal seam. Despite their apparent differences, both artificial and natural USCSs  
Figure 10: Comparison between Model 9 and Model 10 (stress unit: Pa): (**a**) Simulation results of Model 9; (**b**) Simulation results of artificial USCS (Model 10). pose greater risks of failure and instability compared to conventional coal seams. This perspective provides a novel approach to understanding various engineering phenomena associated with coal burst. By applying stiffness theory and the relevant findings of USCS, valuable insights can be obtained, further elucidating these phenomena. By mining the protective layers, the stress level exerted on the coal seam can be reduced, effectively mitigating the risk of coal burst. From a stiffness perspective, the combined presence of the protective layer, the protected coal seam, and the intermediate rock layers forms a composite coal seam with an uneven distribution of stiffness. The roof of the upper protective layer, the floor of the protected coal seam, and the composite coal seam, characterized by varying stiffness, collectively create an artificial USCS, as depicted in Figure 11. Drawing upon previous theoretical analysis and simulation results regarding USCSs, it is evident that the artificial LSZs, encompassing the affected areas of the protective layer, exhibit comparatively lower stress levels and a reduced risk of coal burst within the protected coal seam. In contrast, the artificial HSZs, including the affected areas surrounding the remnant coal pillar and the boundary of the protective layer, experience relatively higher stress levels and an increased risk of coal burst. The majority of technical measures implemented to prevent and control coal burst or relieve pressure can be viewed as strategies for adjusting key parameters of the USCSs existing in coal measure strata.