# Stability Control of Multilayer Roof Strata in the Large Mining Height Gob-Side Entry: A Case Study
## 2 Engineering Background  
### Geological Layout and Rock Characteristics  
The 22301 working face in Tunlan Mine is located at a depth ranging from 357 to 458 m, with an average coal seam thickness of 4.75 m and an average dip angle of approximately 4\({}^{\circ}\). The shortest distance to the goat of the previous 22118 working face is 36 m, leading to significant mining pressure on the current drift layout. Both the southern and northern sides of the working face consist of solid coal. The layout of the mining and excavation operations is shown in Figure 1.  
The average strike and dip lengths of the 22301 working face are about 2652 m and 205 m, respectively. Mining activities are conducted along the No. 2 coal seam. Above the coal seam is a 0.2 m-thick layer of black carbonaceous mudstone, characterized by low strength, easy fragmentation and susceptibility to collapse, which can be regarded as a fake roof since it has no bearing capacity. Above the fake roof, the rock strata is the immediate roof. In general, this layer is firstly exposed and has bearing capacity during mining operations, and it plays a crucial role in the overall stability of the mine roof structure [27]. The immediate roof consists of two rock layers: a 0.6-m-thick, muddy siltstone, with well-developed bedding, and a localized 1.2-m-thick layer of grayish-white fine sandstone. The main roof is 2.9 m thick siltstone with hard cemented structure. The immediate floor of the No. 2 coal seam is composed of 0.3 m thick sandy mudstone, while the old floor is 4.68 m thick siltstone. The overall strength of the surrounding rock in the roadway is low, making it prone to fragmentation and collapse, which are typical characteristics of a compound roof. The conditions of the coal seam roof and floor are summarized in Table 1. \begin{table}
\begin{tabular}{c c c c} \hline
**Rood and Floor** & **Rock Strata** & **Thickness** & **Characteristics** \\ \hline Main roof & siltstone & 2.9 & The light gray fine-grained feldspar quartz sandstone, with features gentle wavy bedding. \\ \hline \multirow{4}{*}{Immediate roof} & grayish-white fine sandstone & \multirow{2}{*}{1.2} & \multirow{2}{*}{Localized siltstone layer is highly developed.} \\ \cline{2-2}  & dark grey muddy siltstone & & & \\ \cline{1-1} \cline{2-2}  & carbonaceous muddy & & & \\ \cline{1-1} \cline{2-2}  & carbonaceous mudstone & & & \\ \hline \multirow{2}{*}{Immediate floor} & grayish-white fine sandstone & \multirow{2}{*}{1.2} & \multirow{2}{*}{Localized siltstone layer is highly developed.} \\ \cline{2-2}  & dark gray muddy siltstone & & & \\ \cline{1-1} \cline{2-2}  & carbonaceous mudstone & & & \\ \hline \multirow{2}{*}{Immediate roof} & grayish-white fine sandstone & \multirow{2}{*}{1.2} & \multirow{2}{*}{Localized siltstone layer is highly developed.} \\ \cline{2-2}  & fine sandstone & & & \\ \cline{1-1} \cline{2-2}  & carbonaceous mudstone & & & \\ \hline \multirow{2}{*}{Immediate floor} & grayish-white fine sandstone & \multirow{2}{*}{0.6} & \multirow{2}{*}{Fossils of roots and leaves can be found, and in the middle section, there are thin layers of fine sandstone and coal seams, as well as sandy mudstone bands.} \\ \cline{1-1} \cline{2-2}  & carbonaceous mudstone & & & \\ \hline \multirow{2}{*}{Fake roof} & black carbonaceous mudstone & \multirow{2}{*}{0.2} & \multirow{2}{*}{This layer has low strength, is fragmented and prone to collapse.} \\ \cline{1-1} \cline{2-2}  & & & & \\ \hline \multirow{2}{*}{Immediate floor} & gray mudstone & \multirow{2}{*}{0.3} & \multirow{2}{*}{The dark gray sandy mudstone contains plant root fossils, with pyrite flakes at the base, interspersed with thin layers of fine sandstone.} \\ \cline{1-1} \cline{2-2}  & & & & \\ \hline \multirow{2}{*}{Main floor} & siltstone & \multirow{2}{*}{4.7} & \multirow{2}{*}{The black-gray siltstone is uneven in texture and contains fragmented plant root fossils.} \\ \cline{1-1} \cline{2-2}  & & & & \\ \end{tabular}
\end{table} 
Table 1: Roof and floor conditions of coal seam.