## 3 Results  
### Analyzing Development and Evolution of Caving and Fracture due to Overburden  
#### 3.1.1 Analyzing Development and Evolution of Caving due to Overburden  
In the process of coal seam mining, the overlying strata deform to varying degrees. The field distribution of their failure due to overburdening during the process of advance of the working face is shown in Figure 13.  
The working face advanced along an upward slope of the surface from the open-off cut. As shown in Figure 13, when the working face had advanced 48 m, initial fracture occurred in the two strata of the main roof, exhibiting the general state of the \"Voussoir beam,\" and the block of rock after fracture formed a relatively stable structure of a \"three-hinged arch.\" When the working face had advanced 56 m, the initial collapse of strata S2 and S3 occurred, where the height of the collapse was 11 m and the angle of collapse was 28.2\({}^{\circ}\). The collapsed blocks of rock were stacked at the bottom of the coal seam. At this time, the mining fissures were mainly distributed in the two strata S2 and S3. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Symbol** & **Description** & **Loess Layer (S4)** & **Silstone (S3)** & **Mudstone (S2)** & **Coal (S1)** & **Griststone (S0)** \\ \hline \(\gamma\) (KN/m\({}^{3}\)) & Volume–weight & 18 & 25.3 & 25 & 14 & 26.12 \\ \(R\) (cm) & Minimum radius of particles & 20 & 20 & 20 & 20 & 20 \\ \(R_{\text{max}}\)/Rmin & Particle radius ratio & 1.6 & 1.6 & 1.6 & 1.6 & 1.6 \\ \(E^{*}\) (Gr/Pa) & Effective modulus of flat joint & 0.03 & 0.0693 & 0.224 & 0.05 & 18 \\ \(K^{*}\) & Rigidity ratio of flat joint & 2 & 2 & 2 & 2 & 2 \\ \(\alpha_{c}\) (MPa) & Average tensile strength and & 0.08/ & 0.5/ & 0.8/ & 0.25/ & 2.2/ \\ standard deviation of flat joints & 0.02 & 0.125 & 0.2 & 0.0625 & 0.55 \\ \(C\) (MPa) & Average cohesion and standard & 0.2/ & 2/ & 1/ & 0.5/ & 10/ \\ deviation of flat joints & 0.05 & 0.125 & 0.25 & 0.125 & 2.5 \\ \hline \hline \end{tabular} Note: In other numerical models, E generally represents the elastic modulus, and K generally represents the bulk modulus. In order to distinguish these similar symbols, this paper uses E\({}^{*}\) to represent the effective modulus of flat joint and K\({}^{*}\) to represent the rigidity ratio of flat joint.  
\end{table} 
Table 1: Microscopic mechanical parameters of rock formation. Figure 13: Evolution of collapse of overlying strata during the advance of the working face. When the face had advanced 64 m, the mining fissure began to develop on the surface of the body of the slope, and a longitudinal tension-induced crack appeared above the coal wall, ahead of the direction of advancement of the working face, and above the side of the open-off cut. As the working face kept advancing, the overlying strata of coal seam were destroyed by the Voussoir beam. When the face had advanced 128 m, the collapse zone was close to the surface, and the arched support developed to the surface. Because of the load of the loess layer, the blocks of rock of the overlying strata were continuously crushed, filling the goaf and compacting overlying rocks in the middle of the collapse zone. When the face had advanced 144 m, the first periodic weighting of the main roof occurred. Following this, because the surface load layer continued to thicken, the original Voussoir beam structure could not withstand the pressure of the overlying rock layer, and the two strata S2 and S3 of the main roof gradually lost their supporting capacity and completely collapsed on the coal seam floor. The Voussoir beam structure was destroyed and the main roof formed the \"step beam\" structure.