## 2 Engineering Geological Conditions
### Engineering Geology  
The bedrock of the study area is bare, and there are loose deposits only in the foothills. The bedrock is a rock combination of the Permian to Carboniferous system. There are significant lithology changes in the study area, and the rock composition has a layered distribution in terms of mechanical strength. Therefore, according to the characteristics of the rock combination in the study area, the rock mass can be divided into five engineering geological types (ET), namely, sandstone, mudstone, limestone, sandstone-mudstone inter-bedded type, and caving ETs (deposited rock mass in goat).  
Based on the principle of engineering geology classification mentioned above and the results of the engineering geological surveying as well as the mapping of the tunnel mountain, the rock types within the depth range of the seam no. 15 from the top of the tunnel to the underground were classified based on their ETs (Figure 2), and a total of 19 ETs were identified from bottom to top.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**System** & **Formation** & **Symbol** & **Thickness (m)** & **Lithological Description** \\ \hline Neogene & & Q & & Loess, laterite, and loose alluvial materials and deposits. \\ \hline Shiqianfeng & P2 sh & 65.0 & & Bright red or mauve sandstone, sandy mudstone, and mudstone. The bottom is K13, generally coarse sandstone. \\ \hline Upper & P2s & 335.0 & & Reddish-brown sandy mudstone and grayish-white sandstone, yellow, or yellow-green mudstone and sandstone. \\ \hline Permian & Lower & P1x & 117.0 & Grayish-white or yellow-green coarse sandstone and yellow mudstone. The top is K10 peach-blossom color bauvite rock. \\ \hline  & & & Gray-black terrigenous elastic rock and coal, containing a total of 6 layers of coal which are, respectively, numbered seam no. 1, 2,... 6 from top to bottom. Among them, seam 3 is one of the coal seams that can be mined in the whole region, and the bottom K7 sandstone marker bed is generally composed of grayish white coarse sandstone of \(\approx\)6–10 m thick. \\ \hline  & & & Grayish-black or gray sandstone, sandy mudstone, limestone, and coal, containing a total of six layers of coal which are, respectively, numbered seam no. 8 to 15 from top to bottom. Among them, seams 8, 12, 13, and 15 can be mined, and the bottom K1 sandstone is generally composed of grayish white feldspar quartz sandstone of \(\approx\)5–15 m thick. \\ \hline  & & & Black mudstone, gray bauvite (mineral) rock, dark gray mudstone limestone, and thin coal seams. The bottom is “Shanxi-type iron ore” or ”Yangquan-type pyrite”. \\ \hline Ordovician & Majiagou & O2m & 600–800 & Limestone \\ \hline \hline \end{tabular}
\end{table}
Table 1: Regional stratigraphy in Yangquan city.  
In the engineering geological mapping of the mountain, the development of mountain cracks was specially investigated and measured, which was mainly carried out on the top of the mountain, and the distribution and strike rose diagram of cracks were plotted (Figure 3).  
Figure 3: Distribution and strike rose diagram of cracks in the mountain of the Tianzimiao tunnel.  
Figure 2: Engineering geological types of the Tianzimiao tunnel.  
The main cause of the mountain cracks is underground coal mining. The strike of cracks is consistent with the direction of the coal mining panel, on the one hand, and on the other hand, the orientation of slope and the early developed joints have an obvious inducing effect on the formation of cracks.