In-situ Stress Analysis and Rockburst Risk Assessment of Liupanshan Tunnel of Bailong River Water Diversion Project

[Objective] Taking the deeply buried Liupanshan tunnel of the Bailong River Diversion Project as the research background, this study aims to systematically investigate the distribution characteristics of the in-situ stress field along the tunnel alignment and to assess the rockburst risk of hard-rock sections under high-stress conditions, thereby providing a scientific basis for early-stage project planning, construction safety control, and support design. [Methods] A comprehensive approach combining field testing, numerical simulation, and theoretical analysis was employed. (1) Geological survey and deep-borehole hydraulic fracturing tests were conducted to obtain measured in-situ stress data from representative boreholes along the tunnel alignment. (2) Based on the measured data, a three-dimensional geomechanical model was established, and a multivariate regression inversion method was used to invert the initial in-situ stress field of the entire study area. (3) By integrating the Russenes criterion, the Turchaninov criterion, and the criterion of the ratio of chamber damage depth, a multi-criterion comprehensive evaluation of rockburst risk was conducted for the hard-rock sections. [Results] (1) In-situ stress characteristics: Along the tunnel alignment, horizontal stress was dominant, and the maximum horizontal principal stress was mainly oriented in the NEE-EW direction, forming a small angle with the tunnel axis, which was favorable for surrounding rock stability. The stress magnitude increased with depth and was significantly influenced by faults and valley topography, resulting in localized stress concentration and stress differentiation. (2) Inversion verification: A comparison between the measured data and inversion results from the deepest borehole ZK2301 showed good agreement in the deep zone, verifying the reliability of the inversion model. (3) Rockburst risk: The multi-criterion assessment indicated that rockburst risk increased with burial depth along the tunnel alignment: no rockburst at depths ≤197 m; weak rockburst at depths of 197-344 m; moderate rockburst at depths of 344-629 m; and strong rockburst at depths >629 m. Sections with strong rockburst risk accounted for a relatively large proportion of the entire tunnel, mainly concentrated in hard-rock zones with high burial depths. [Conclusion] The in-situ stress along the Liupanshan tunnel is generally at a moderate-to-high level, providing conditions conducive to rockburst occurrence. The selected tunnel axis orientation is reasonable and favorable for surrounding rock stability. However, strong rockburst risk exists in hard-rock sections with large burial depths. Therefore, it is recommended that appropriate rockburst monitoring and mitigation measures should be implemented for high-risk tunnel sections during construction. This study provides critical guidance for the safe construction of the project and offers valuable reference for rockburst prediction and prevention in similar deeply buried hard-rock tunnels.