## 2 Project Background and Geological and Hydrogeological Conditions
### Geological and Hydrogeological Conditions
#### 2.2.2 Hydrogeological Condition  
According to the previous survey data, the Shiling-Baihe fault is a concealed fault and runs through the engineering area. The fault stretches from Shiling, Yiyang County, through Cijian, Xin'an County, to Baihe, Mengjin County, with a total length of 55 km. It strikes the northeast and inclines southeast. In order to understand the occurrence of the fracture in the engineering area and the location of the intersection with the axis, the fracture was detected. This geophysical exploration work adopts the magnetotelluric method, and a total of two survey lines are arranged, as shown in Figure 4. The survey line a is arranged on the east side of the tunnel line, 700-1000 m away from the tunnel line, and the survey line is NW-SE, which is basically perpendicular to the Shiling-Baihe fault described in the previous data. It is mainly used to understand the true occurrence of the fault. The survey  
Figure 3: Geology condition of the study section: (**a**) borehole profile of ZK-1 and (**b**) clay rock core 73.5 m below the surface. line is far away from the high-voltage line, which avoids the influence of the high-voltage line on the detection results. The detection line b is arranged at the bottom of the ditch on the west side of the tunnel line, about 110-170 m away from the tunnel line, and the direction is close to N-S, which is roughly parallel to the tunnel line. The length of survey line a is 680 m. From the resistivity profile, it can be seen that there is a top-down low-resistance anomaly at 120-280 m of the survey line a. It is speculated that there may be a high-dip fault fracture zone structure here, and the fault zone and its influence width are about 50 m. The length of survey line 2 is 840 m, which is located at the bottom of Liushugou, and the terrain is not undulating. It can be seen from the resistivity  
Figure 4: Magnetotelluric sounding results: (**a**) magnetotelluric sounding results of line a and (**b**) magnetotelluric sounding results of line b. Chinese in the figure means apparent resistivity. profile that there is a top-down low-resistance anomaly at 480-600 m of the survey line b, and it is speculated that there may be a fracture zone here. According to the geophysical prospecting results, the tunnel is obliquely intersected with the Shiling-Baihe fault at pile number \(8+518\)-\(8+574\), with an angle of 63\({}^{\circ}\) to the tunnel axis, and the width of the fault zone is about 56.0 m. The Shiling-Baihe fault is a normal fault with a strike of NE 60\({}^{\circ}\), a tendency of SE, and a dip angle of 80\({}^{\circ}\). The Middle Pleistocene strata overlying the Shiling-Baihe fault are not cut off, and the fault structure is in a deep concealed state, which is basically not exposed on the surface. The strata on both sides of the fault are sandy clay rock, clay sandstone, sandstone and conglomerate of Tertiary (N) and Neogene (E). The surface of the area to the south of the Shiling-Baihe fault is covered by the Quaternary soil layer, and the Tertiary bedrock is exposed locally. The upper layer of the conglomerate is alternated with a layer of sandstone, and the middle part is a loose sandy pebble layer. The Neogene N sandy clay rock or sandstone is the floor, in which conglomerate and sandstone are good aquifers, with weak cementation, relatively developed pores, and partial argillaceous calcium filling, which creates favorable conditions for groundwater recharge and storage. The survey results showed that the water inflow of the Neogene aquifer in this area was less than 100 m\({}^{3}\)/d when the drawdown was 5 m, and the water-rich area was a water-poor area. The upper part of the north of Shiling-Baihe fault is the Neogene stratum, and the lower part is the Paleogene glutenite aquifer group. The thickness of the stratum is large. In general, the roof of the rock group is about 50-100 m higher than the tunnel floor.