In-situ stress is a fundamental parameter for seismic research, deep resource development, and underground engineering. However, in soft rock formations with strong rheological behavior, the hydraulic fracturing(HF) method cannot directly determine the maximum horizontal principal stress(σ_H), and the anelastic strain recovery(ASR) method often suffers from large uncertainties due to the difficulty of accurately determining the anelastic strain recovery compliance. To address these limitations, this study proposes a novel approach for in-situ 3D stress in soft rock formations that integrates the hydraulic fracturing method with the anelastic strain recovery method. The minimum horizontal principal stress(σ_h) is obtained through hydraulic fracturing tests, while the anelastic strain recovery compliance is back-calculated from ASR data on recovered core samples, enabling the complete in-situ 3D stress tensor to be reconstructed. This combined method has been successfully applied in the Ningjin salt cavern gas storage project, confirming its reliability and applicability in deep soft rock formations. The proposed approach provides a new technical solution for acquiring in-situ stress in soft rock formations and offers valuable support for salt cavern gas storage construction, unconventional hydrocarbon development, and wellbore stability evaluation.

In order to determine the in situ stress state of the newly built hydropower station project in Jiacha county, and understand the activity of the eastern segment of the Yarlung Zangbo River fault zone, we obtained the in situ stress data in the engineering field by using the self-developed deep-hole stress measuring equipment based on the hydraulic fracturing method. We analyzed the fault activity by employing the Mohr-Coulomb failure criterion and the collected in situ stress data along the eastern segment of the Yarlung Zangbo River fault zone. The results showed that: (1) The magnitudes of the maximum horizontal principal stress S_H ranged from 6.07 MPa to 37.62 MPa, the values of the minimum horizontal principal stress S_h were from 3.13 MPa to 20.33 MPa at the depth from 122.75 m to 418.75 m, and the dominant direction of the measured maximum horizontal principal stress was nearly NNE. (2) The stress regime was mainly characterized by S_H > S_h > S_v (S_v is the vertical principal stress), which was prone to reverse faulting. (3) Most of the measured and collected Mohr stress circles intersected the failure threshold line with friction coefficient equal to 0.6, which indicated that the eastern segment of the Yarlung Zangbo River fault zone was in a high level of fault slip instability. Furthermore, the fault slip risk of the west section from Gongga to Langxian was higher than that of the east section near Linzhi along the eastern segment of the Yarlung Zangbo River fault zone.