## 2 Analysis of First Weighting of Main Roof  
### Working Face Overview and Strata Behavior Monitoring  
To analyze the roof structural characteristics of thin bedrock under thick loose layer during shallow coal seam mining, we took the no. 12401 panel of \(5^{-2}\) coal seam in Shendong mining area as the engineering background. The length of working face was 260 m with a dip angle of 1-3\({}^{\circ}\), and the average burial depth was 120 m. The thickness of the bedrock was 70 m on average. The thickness of the loose layer was 50 m on average. The thickness of the bedrock fluctuated in a small range. The roof was mostly mudstone, fine-grained sandstone, and siltstone, with an average compressive strength of 23.15 MPa. The floor was mainly siltstone, and the rock mass was complete with an average  
Figure 1: The main roof is cut off and the immediate roof is crushed [40]. compressive strength of 26.35 MPa. The thickness of the coal seam was 7.1-17.35 m, with an average of 7.2 m, and the designed mining height was 7.0 m. The comprehensive histogram is shown in Table 1. The working face adopted 150 sets of ZY18000/32/70D electro-hydraulic control shield-type hydraulic supports. The measured first weighting interval of the main roof was about 54 m, and the average of periodic weighting interval was 15.8 m.  
Figure 2 shows the monitoring curve of support resistance during the first weighting of working face, and Figure 3 shows the variation curve of working resistance of supports during the first weighting. The working resistance of the support was small and the change range was not large, most of which was 25.2-30.0 MPa, found by analyzing the curves in Figures 2 and 3. The support resistance of no. 55-120 in the middle of the working face increased rapidly when the working face advanced about 53 m, and the maximum value reached 47.3 MPa (red area in Figure 4). There was frequent opening of the safety valve, rapid roof sinking, and great danger of support crushing. The rib fall of the coal wall was seriously cracked, and the average rib fall depth was 200-400 mm. After that, the weighting entered a continuous stage. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Geotechnical Name** & **Columnar** & **Thickness/m** & **Burying Depth/m** \\ \hline Sand layer & & 40 & 40 \\ Mudstone & & 7.7 & 47.7 \\ Silstone & & 8.1 & 55.8 \\ Medium grain sandstone & & 7.11 & 62.91 \\ Fine sandstone & & 9.1 & 72.01 \\ Silstone & & 12.1 & 78.11 \\ Fine sandstone & & 6.46 & 82.57 \\ Silstone & & 4.37 & 94.94 \\ Fine sandstone & & 4.69 & 96.63 \\ Mudstone & & 3.68 & 103.31 \\ Fine sandstone & & 4.72 & 108.03 \\ Mudstone & & 5.86 & 113.9 \\
5–2 coal seam & & 6.2 & 120 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Lithology of the no. 12401 working face. When the working face advanced about 60 m from the open-off cuts, there were obvious vertical and horizontal movement cracks on the surface, as shown in Figure 4. The subsidence was between 210 mm and 235 mm, and the average was 224 mm. The sinking amount was between 210 mm and 235 mm, with an average of 224 mm. The monitoring results showed that the roof damage caused by the first weighting directly affected the ground surface, and there were obvious step subsidence and dynamic load phenomenon in the process of weighting. The measured first weighting interval of the main roof was about 54 m, which was larger than that of the general mining height. The support resistance increased sharply during first weighting, and the weighting in the middle of the working face was strong. The dynamic loading phenomenon was obvious, with the average dynamic load coefficient being 1.81. The roof moved violently, causing the safety valve of the support to open frequently, and there was great danger of support crushing. Therefore, it was necessary to analyze the applicability of the hydraulic support with a rated working resistance of 18,000 kN under the conditions of large mining height. Figure 4: Vertical and horizontal movement of ground cracks.