# Analysis of Characteristics of Roof Fall Collapse of Coal Mine in Qinghai Province, China
## 2 Materials and Methods  
### Overview of Collapse Event  
On 14 August 2021, the roof fall collapse of a coal mine occurred in Haibei Prefecture of Qinghai Province, China (see Figure 1a). Figure 1b depicts the disaster site. The accident occurred in the Chaidar coal mine, located in Gangcha County (see Figure 2a). The accident was caused by the mixing of fissure water and coal ash in the upper part of the working face to form coal slime, which poured into the working face. The mine was an open pit, 160 m deep, and had been backfilled to 80 m when this accident occurred. The backfill part is mainly sandy soil, and the geological conditions are complex and extremely unstable. The accident trapped the victims at a location about 1.2 km from the mine's entrance, the vertical distance between the accident site and the ground surface is 90 m, and the mud flow backward is 70 m. At the time of the accident, 21 miners were working in the mine, 1 person was dead, 1 person was injured, and other 19 workers were trapped in the mine. Due to weather and seasonal factors, there was a constant seepage around the pit (see Figure 2b). The accident occurred at a high altitude and in the frozen soil, the coal mine roof is poor, and the coal seam is powdery. The rescue was extremely difficult. During the progress of drainage and desilting (see Figure 2c), the internal pressure of fluid changes, the unstable balance of coal slime in the upper part of the working face is broken, and coal slime continues to flow into the working face, resulting in secondary disasters (see Figure 2d) of coal slime gushing twice in two hours during the rescue and desilting the rescue team. By the early morning of September 13, the accident rescue work had essentially ended; all 19 trapped people had been found, no vital signs. According to the notice issued by the emergency management department of Qinghai Province ([[http://yjt.qinghai.gov.cn/](http://yjt.qinghai.gov.cn/)]([http://yjt.qinghai.gov.cn/](http://yjt.qinghai.gov.cn/))  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Date** & **Site** & **Casualties** & **References** \\ \hline
4 January 2017 & Defeng County of Henan province & 12 deaths & [12] \\
17 January 2017 & Shuozhou City of Shanxi province & 10 deaths & [13] \\
11 November 2017 & Shenyang City of Liaoning province & 10 deaths and 1 injury & [14] \\
12 January 2019 & Shenmu County of Shaanxi province & 21 deaths & [15] \\
19 June 2019 & Ningwu County of Shanxi province & 6 deaths & [16] \\
28 October 2019 & Nandan County of Guangxi province & 2 deaths and 11 missing & [17] \\
29 February 2020 & Qujing City of Yunnan province & 5 deaths & [18] \\
26 May 2021 & Weishan County of Shandong province & 3 deaths and 3 injuries & [19] \\ \hline \hline \end{tabular}
\end{table}
Table 1: Casualties of coal mine roof fall accidents in various regions of China in recent years.  
Mobile/Article.html?lmid=1&sjid=6712&type=0, accessed on 16 January 2022), all mining enterprises have learned the lessons from the roof fall accident of the Chaidar coal mine, and comprehensively carried out the general survey and treatment of potential disaster-causing factors in coal mines. The legal representative of coal mining enterprises is the first person in charge. All coal mining enterprises are required to strengthen risk analysis, to find potential danger, and to propose treatment measures. Experts from inside and outside the province are called together to investigate the reason behind the collapse and to propose countermeasures to maintain coal mine safety. The final report should be presented to the provincial emergency management department before 31 May 2022.  
Figure 1: Location of the collapsed site: (**a**) Qinghai Province in China; (**b**) disaster site of Haibei Prefecture in Qinghai Province.  
Figure 2: Roof fall collapse of coal mine: (**a**) full view of Chaidar coal mine (recreated based on [40]); (**b**) a continuous seepage (recreated based on [41]); (**c**) borehole desilting; (**d**) secondary disasters (recreated based on [42]).