# Stability Control of Multilayer Roof Strata in the Large Mining Height Gob-Side Entry: A Case Study
## 3 Roof Separation Analysis of Bolted Joint Rock Mass
### Numerical Analysis of Roof Separation in the Gob-Side Entry
#### 3.2.2 Result Analysis  
The area above the backfill in the large mining height gob-side entry consists of a compound roof with relatively low strength, which is generally fractured with low stiffness and strength. The vertical stress distribution in the backfill area is shown in Figure 8. As the distance away from the backfill increases, the vertical stress in the immediate roof above the backfill area increases first and then decreases. The location of peak stress remains relatively constant, positioned near the backfill side adjacent to the gob. Additionally, the vertical stress in the immediate roof changes more rapidly on the gob-side, while it changes more slowly on the head entry-side.  
Before the backfill structure begins to bear the overlying loading, the compound roof above the gob-side backfill area is subjected to several forces, e.g., the advanced abutment pressure from the working face, unloading pressure due to mining, support from the mining shield and concentrated loads from individual hydraulic props. After the backfill structure starts bearing the overlying loading, the roof is subjected to dynamic pressure caused by initial roofving, rotation and subsidence of key roof blocks. To study the failure characteristics of the roof in the backfill area of the large mining height gob-side entry, the distribution of plastic zone in the 22301 working face in Tunlan Mine is shown  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Rock Strata**} & **Bulk Modulus** & **Shear Modulus** & **Cohesion** & **Friction Angle** & **Tensile Strength** \\  & **(GPa)** & **(GPa)** & **(MPa)** & **(\({}^{\circ}\))** & **(MPa)** \\ \hline Overlying strata & 8.9 & 6.1 & 5.0 & 42.0 & 2.8 \\ Basic roof & 11.9 & 8.2 & 4.5 & 38.0 & 4.6 \\ Immediate roof & 3.2 & 2.0 & 3.8 & 29.0 & 2.6 \\ Coal layer & 3.7 & 1.5 & 2.4 & 25.0 & 0.2 \\ Immediate floor & 2.5 & 1.2 & 1.4 & 20.0 & 1.0 \\ Basic floor & 11.9 & 8.2 & 34.0 & 51.0 & 2.0 \\ Underburden strata & 8.9 & 6.1 & 5.5 & 34.0 & 2.8 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Rock parameters in the simulation.  
Figure 7: Numerical simulation model.  
in Figure 9. After the backfill structure is constructed, the roof in the backfill area begins to undergo compressive deformation as the main roof rotates and subsides. Before the backfill starts bearing the overlying loading, the roof in the backfill area experiences tensile failure due to the collapse of the overlying strata. Once the backfill begins to bear the overlying loading, the roof returns to a two- or three-directional stress state. As the main roof continues to rotate and subside, the roof in the backfill area primarily undergoes shear failure.  
Figure 8: Vertical stress distribution in the backfill area.  
Figure 9: The distribution of plastic zone in the 22301 large mining height gob-side entry.  
Based on the above analysis, if the roof in the large mining height gob-side entry undergoes a severe crash and collapse after experiencing the two dynamic pressure stages, the support resistance may not be effectively transmitted from the support structures to key roof blocks. As a result, the key roof blocks cannot be cut along the outer side edge of backfill, thus leading to increased rotational subsidence of the roof. Consequently, severe damage will occur in the roof and coal pillar sides, making it difficult to maintain the intact structure of reserved gob-side entry. Therefore, maintaining the stability and integrity of compound roof above the backfill structure is critical for the success of the large mining height gob-side entry.