In-situ stress characteristics in the project area of a large hydropower station on the Northern margin of the Eastern Himalayan Syntaxis

To understand the distribution patterns of in-situ stress and fault stability in the engineering area of a large hydropower station on the northern margin of the eastern Himalayan syntaxis, hydraulic fracturing stress measurements and three-dimensional stress field inversion analysis were conducted. This revealed the stress distribution characteristics in key structures such as the underground powerhouse and water diversion tunnels. The results indicate: (1) The principal stress relationship generally follows S_H>S_v>S_h, with horizontal stress dominating—indicative of a strike-slip stress regime. The dominant orientation of the maximum horizontal principal stress is NEE, consistent with the principal compressive stress direction derived from focal mechanism solutions. This suggests that the current strike-slip shear stress environment is primarily controlled by the NE-directed compression of the Indian Plate against the Eurasian Plate. (2) Measured principal stresses increase linearly with depth: SH ranges from 3.0 MPa to 11.0 MPa, and Sh from 2.0 MPa to 6.7 MPa, corresponding to gradients of 1.82 MPa/100 m and 0.72 MPa/100 m, respectively. Compared to the Tibetan Block, mainland China, North China, and the northern segment of the North-South Seismic Belt, the overall stress level in the study area is relatively low. (3) Fault hazard analysis based on the Mohr-Coulomb criterion and Byerlee’s Law shows that the stress magnitude on the Jiali Fault within the project area does not reach the critical threshold for shallow crustal fault slip instability, indicating a relatively stable state. (4) Inversion results reveal that the maximum horizontal principal stress along water diversion tunnels and in the underground powerhouse ranges from 3.9 MPa to 11.0 MPa, gravitational stress from 5.8 MPa to 10.7 MPa, and minimum horizontal principal stress from 4.5 MPa to 7.8 MPa. The azimuth of the maximum horizontal principal stress varies between 48° and 66°. The tunnel axis predominantly intersects the orientation of the maximum horizontal principal stress at large angles, which adversely affects the stability of the surrounding rock mass.