# Study on the Distribution Law of Crustal Stress in Fault Fracture Area
## 4 Engineering Verification  
This article validates the stress distribution conclusions presented by selecting a certain tunnel as the engineering support. The tunnel's entrance and exit mileage are DK1217 + 793 and DK1255 + 758, respectively, with a total length of 37,965 m. The tunnel is buried at a maximum depth of 1687.85 m and traverses thick layers of Quaternary Holocene and Upper Pleistocene alluvial-proluvial cohesive soil and gravel layers at the entrance, exit, and valley. The tunnel stratum consists mainly of Himalayan granite, diorite, and Caledonian granite, with the exit section featuring the gneiss stratum of the Zhenba Formation of the Nianqing Tanggula Group of the Middle Neoproterozoic. The tectonic mainly consists of fault breccia and crushed rock. The tunnel body passes through approximately 33,895 m of granite and diorite, 3300 m of gneiss, and 770 m of the Quaternary stratum. The cross-section is presented in Figure 10.  
The tunnel body intersects with four regional faults, namely F4-3, F4-5, F61, and F4-6. Among them, the F4-3 fault strikes nearly northwest and intersects the tunnel at a large angle at DK1230 + 100\(-\)DK1230 + 350 section. The width of the fault fracture zone is about 250 m. The hanging wall and footwall strata are Himalayan granite, and the fault zone material is mainly structural cataclasite; The F4-5 fault strikes nearly south-north, passing through the tunnel at a small angle at DK1244 + 750. The hanging wall and footwall strata are Himalayan granite, and the fault zone material is structural cataclasite; The strike of the F6-1 fault is nearly south-north, passing through the tunnel at a small angle at DK1246 + 275\(-\)DK1246 + 575. The hanging wall is Himalayan Diorite, and the footwall is Himalayan granite. The fault zone material is also tectonic cataclasite; The strike of the F4-6 fault is nearly northwest. The hanging wall and footwall strata are Caledonian granite, and the fault zone material is structural cataclasite. Table 3 summarizes the distinctive structural characteristics of these faults based on geological exploration. Furthermore, to determine the initial crustal stress distribution in the tunnel area, hydraulic fracturing was employed to measure the maximum horizontal principal stress value and direction of deep boreholes, as presented in Table 4.  
Figure 10: Vertical section of Sejila Mountain tunnel site. Borehole A5 KSZ-21 is located within fault F4-3, borehole SJLSZ-5 is situated in the strata adjacent to fault F61, and borehole SPSZ-2 is located inside fault F4-6, as shown in Table 4. Due to the presence of crushed rock in the fault fracture zone, the measured stress direction exhibits some variation. A plan view in Figure 11 illustrates the location of each borehole and its corresponding measured maximum horizontal principal stress direction. Based on Table 4 and Figure 11, it is evident that the tunnel site area is subject to tectonic stress, with the main direction of maximum horizontal principal stress being N 50\({}^{\circ}\) E. Upon encountering fault F4-3, where the angle between the \(\sigma_{1}\) direction and the vertical fault strike direction is \(\alpha<45^{\circ}\), the principal stress inside the fault deviates towards the direction parallel to the fault, which is consistent with the principal stress direction N 67\({}^{\circ}\)-83\({}^{\circ}\) E measured at point A5 KSZ-21. When encountering faults F61 and F4-5, as the fault trend is north-south and the angle between the \(\sigma_{1}\) direction and the vertical fault direction is \(\alpha<45^{\circ}\), the main stress of the strata near the fault will slightly deflect towards the direction perpendicular to the fault direction. This finding is consistent with the main stress direction N 64\({}^{\circ}\)-82\({}^{\circ}\) E measured at SJLSZ-5 point.