# Uneven Stiffness Coal Seam: A New Structural Factor Prone to Coal Burst Based on Stiffness Theory
## 6 New Explanation of Engineering Phenomena Based on USCS  
### New Explanation of the Time-Delayed of Partial Coal Bursts  
Certain coal burst accidents exhibit an unconventional characteristic known as the time-delayed property [35]. These coal bursts occur after a certain lag time following excavation disturbance. For instance, during the \"11.3\" coal burst incident at the 2121221 working face of Qianqiu Coal Mine, Yima in 2011, the location of the coal burst lagged behind the heading face of the excavation roadway by a distance ranging from 65 to 425 m [33]. Similarly, in 2017, the \"2.3\" coal burst transpired on the main roadway of Gaojiabao Coal Mine in Shaanxi during the mine's shutdown period for the Spring Festival [35]. It is possible that the coal burst resulted from mining disturbance prior to the shutdown period and only manifested during the Spring Festival shutdown period. Excavation and unloading operations induce stress redistribution in the coal and rock mass around roadways, leading to localized fracture aeras known as loose rings or ring-shape broken rock zone, which consist of annular zones of broken rock distributed around the roadway [36]. The unloading effects of a developing loose ring can cause damage to the outer layer of coal and rock mass, gradually expanding the loose ring layer by layer, until the stable equilibrium is restored. Although the fractures resulting from the developing loose ring possess some elements of spontaneous and time-delayed characteristics, their weak burst intensity renders them incapable of causing coal bursts. Therefore, relying solely on the presence of a developing loose ring is insufficient to explain the time-delayed property of coal bursts. It is necessary to consider other factors, such as USCS, in conjunction with the loose ring. The preceding analysis of USCS reveals that, similar to the effect of increasing static or dynamic loads, an increase in the value of \(E_{\text{R}}H_{\text{R}}K_{\text{H}}S_{\text{L}}/K_{\text{L}}S_{\text{H}}\) or in NSC gradient between HSZ and LSZ can improve \(\sigma_{\text{VH}}\) and enable the coal mass in HSZ to reach the failure criterion. From a stiffness standpoint, the development of loose ring can reduce the NSC of the coal seam. When this process occurs in LSZ of USCS, the reduction in \(K_{\text{L}}\) can lead to a higher stress concentration and a lower LMS in HSZ, which promotes the satisfaction of failure criterion and instability criterion. In the case of ESCS coal seams, minor disturbances like roadway expansion or loose ring development have limited impact on the stress field and hence are unlikely to cause coal bursts, since the load transfer caused by such disturbances is dispersed, as depicted in Figure 12a. In contrast, minor disturbances occur in LSZ of USCS may have significant effects on the stress field due to the pressure concentration function of USCS, which allows the loads from various parts of LSZ to converge congruously towards the HSZ, as shown in Figure 12b. Although the load transfer generated by a developing loose ring around a unit length of roadway is slight, a considerably lengthy roadway with a growing loose ring around it, or a close proximity between the HSZ and the growing loose ring, would result in a substantial increase in \(\sigma_{\text{VH}}\), even leading to the failure criterion being met. Furthermore, compared to the occurrence of failure in coal seams of ESCS under similar conditions, failure of HSZ in USCS is more likely to result in coal bursts due to the stiffness reduction function of USCS. In 2013, the \"8.5\" coal burst in the upper roadway of the 3302 excavation working face at Shandong Xingcun Coal Mine. The location of the coal burst was approximately 200 m away from the excavation heading face, surpassing the disturbance range of the excavation. Since 3302 working face was the first mining face, there was no other sources of disturbance in its vicinity.