## 4 Analysis of Physical Similarity Simulation Experiment Results
### Instability Characteristics of OR in Lower Coal Seam Mining
When the working face advanced to 121 m, it was 3 m ahead of the ICP, and the main roof experienced its fifth periodic weighting, as revealed in Figure 4c, The LKS fractured with a caving height of 15 m, and the support resistance reached 6897 kN. When the working face advanced to 135 m, the main roof experienced its sixth periodic weighting, as shown in Figure 5a. The support resistance reached 7885 kN. As displayed in Figure 5b, when the working face advanced to 150 m, it was 7 m past the ICP. The OR experienced shear failure, and the main roof experienced its seventh periodic weighting, with the support resistance reaching 10,934 kN, which made it difficult to move the support. The roof cantilever beam structure could not bear the concentrated stress of the ICP, causing shear failure of the OR along the working face coal wall. This formed a caving zone from the OR to the coal wall boundary. The working face continued to advance to 198 m, and the main roof experienced its tenth periodic weighting. The working face was mining out of the 2# failed TCP, with the support resistance reaching 9210 kN. The fissures in the OR developed to the surface, as shown in Figure 6a. When the working face advanced to 213 m, the main roof experienced its eleventh periodic weighting, with a support resistance of 7654 kN, as presented in Figure 6b. The IC caved, and the main roof failed behind the support, with the fissures connecting with the fracture line of the OR in the fourth mining strip. Figure 4: ICOR of the working face passing through the 1# failed TCP. (**a**) Working face advanced to 92 m; (**b**) working face advanced to 106 m; (**c**) working face advanced to 121 m.  
The mining pressure characteristics such as weighting interval, weighting intensity, support working resistance, and dynamic load factor during the simulated mining of the 3\({}^{-1}\) coal seam are summarized in Table 2. The dynamic load factor during the main roof weighting for mining under interval goal ranged from 1.07 to 1.74, and there was significant variability in the periodic weighting intensities of the main roof. When the working face was under the interval mining strip, the dynamic load factor was 1.07-1.17, with no significant pressure surge. When mining past the TCP, the dynamic load factor during main roof periodic weighting was 1.38-1.46, with increased mining pressure compared to the interval mining strip. When the working face was out of the TCP, the dynamic load factor reached 1.74, and the effective bearing area of the IRS for the TCP and OR structures gradually decreased. The IRS could not bear the concentrated load formed by the ICP and its roof double-arch bridge structure. This caused a penetrated shear fracture zone from the ICP boundary to the working face, accompanied by rotational instability of the double-arch bridge structure in the roof as well as cutting and caving of the ICP along the coal wall. This is the root cause of the increased support resistance in the working face of the lower coal seam. During actual mining at this stage, dynamic load-induced support crushing accidents are likely to occur. Therefore, it is necessary to strengthen mining pressure monitoring  
Figure 5: ICOR of the working face passing through the ICP [29]. **(a)** The working face advanced to 135 m; **(b)** the working face advanced to 150 m.  
Figure 6: ICOR of the working face passing through 2# failed TCP. **(a)** The working face advanced to 198 m; **(b)** the working face advanced to 213 m.  
and support quality management and to use single support reinforcement measures in a timely manner.