Mechanisms and Diffusion Characteristics of Prestress Fields in Deformation-Prone Tunnels under Active Support

Active support systems are increasingly used to control large deformations in soft rock tunnels. Prestressed anchors play a crucial role by creating a wider, stronger prestress area in the surrounding rock. However, limited research has explored their stress distribution in horseshoe cross sections under different rock conditions. This paper first examines the large deformations observed in the Muzhailing Highway Tunnel under strong passive support and analyzes the feasibility and mechanism of active support for surrounding rock control. It then employs the self-developed soft rock similar material in pull-out testing combined with numerical models to analyze the prestress field distribution. Results show that as the surrounding rock deteriorates, both the magnitude and spread of the prestress field decrease. Under the group-anchor effect, stress concentration at the hole wall is alleviated, while the effective prestress between anchors is enhanced, though this effect diminishes with increasing rock grade. To further explore the diffusion effect of prestressed anchor groups, finite element simulations for horseshoe-shaped tunnels were conducted. Findings indicate that increasing the tunnel’s curvature radius leads to a more continuous, uniform annular prestress arch. Additionally, reducing bolt length and spacing helps form a continuous effective compressive stress zone within the reinforced rock mass. These results provide a theoretical foundation for mitigating large deformations and optimizing active support designs in soft rock tunnels.