## 2 Project Overview and Geological Conditions of the Area  
The Ridge and Valley Province of Chuandong is located in the east of the Sichuan Basin. Affected by the distribution, geometric characteristics and fault boundary conditions of the rigid basement and brittle layer detachment layer in the Sichuan Basin, Ridge and Valley Province of Chuandong, has become the most orderly mountain range combination area in China and also the most Jura-type fold mountain in the world [18; 19; 20; 21]. In this area, there are more than 30 northeast-southwest aligned parallel mountains. Figure 1a shows the location of the project, which illustrates that the project will be constructed under significantly difficult geological conditions between Huaying Mountain and Tongluo Mountain. Meanwhile, the stratum here is the red beds of the eastern Sichuan Basin, which are continental sedimentary rocks consisting of alternating red mudstone and celadon sandstone or sils [22; 23].  
Figure 1: (**a**) Guobo underground station located in the complex geological environment of the Ridge and Valley Province of Chuandong; (**b**) longitudinal section and (**c**) typical X–sections and corresponding stratum of Guobo Station.  
The Jinshan tunnel area has a low hill landform, with a ground elevation of 225\(\sim\)380 m and a relative elevation difference of 40\(\sim\)120 m. Meanwhile, the natural slope angle is between 10-30\({}^{\circ}\), with some steep gullies. The Guobo underground station will be built along the Jinshan tunnel by employing the drilling and blasting method, which is a double-layer underground railway station with a total length of 284 m. Figure 1b shows the longitudinal section, typical X-sections and corresponding stratum of Guobo Station. The initial geological survey report indicates that the surrounding rock of the station's project area belongs to grades IV\(\sim\)V, and the strata are mainly mudstone in the main body section and mudstone intercalated with sandstone in the portal section. From the inlet to the outlet, the dip of the rock stratum gradually turns to the northwest, then to the southwest, and finally to the northwest, and the dip angle of the rock stratum roughly slows down. Meanwhile, two sets of rock mass joints exist in the project area; their occurrences are N85\({}^{\circ}\)W/68\({}^{\circ}\)NE and N80\({}^{\circ}\)W/90\({}^{\circ}\), respectively. From the X-section, it can be seen that the Guobo station belongs to a super large section with an excavation width of 23.993 m and an excavation height of 22.278 m. In the preliminary planning of the station, it is proposed to adopt the initial support mode of a combination of shotcrete and erecting steel arches. In addition, advanced support is also considered in some areas to improve the mechanical properties of the rock mass.  
As shown in Figure 2, the boreholes in the project area of the station are divided into the body section and the portal section. The borehole range of the body section is from HMDK15 + 388 to HMDK15 + 673, including an intensive boring area from HMDK15 + 493 to HMDK15 + 553. Three rows of boreholes are arranged longitudinally; one row is arranged in the centerline of the tunnel, two rows are arranged at 13 m on each side of the centerline of the tunnel, and 22 boreholes are bored in each row. A total of 66 boreholes are drilled in less than 300 m, and the borehole density of the body section is extremely high. Meanwhile, there are two boreholes, 4-4 and 4-6, at the tunnel entrance section. The boreholes reveal that the lithology of the rock cores taken from borehole 4-4 is sandstone and that the lithology of the rock cores taken from the other boreholes is mudstone.  
Figure 2: _Cont_.