Mechanism of the inclined and advanced rockbolt support system and dynamic evaluation of its reinforcement range in mechanized tunneling for high-speed railways

Abstract

To tackle the challenges of tunnel over-excavation, the Chongqing-Kunming High-speed Railway has optimized rockbolt arrangements by replacing traditional radial rockbolts near the tunnel face with pre-stressed inclined rockbolts that are positioned more forward and set at angles of 45° to 60°. This adjustment thereby aims to enhance the stability of the tunnel face, delay steel arch installation, and expand operational space for drilling equipment. Focusing on the case of the Jinyunshan Tunnel, this study employs finite difference modeling (FDM) to analyze the distribution of support stress fields under varying conditions, thereby enabling a quantitative assessment of the rockbolt-reinforced zones. This quantitative evaluation allows for an effective assessment of the feasibility and safety of implementing delayed support sections. Furthermore, triaxial compression simulations that incorporate prestressed rockbolts reveal the impact of confining pressures on the properties of anchored rock masses. Additionally, field and laboratory tests were conducted to further evaluate the effectiveness of tunnel deformation control, the enhancement of surrounding rock stress, and the practical support capabilities of inclined rockbolts. The research results indicate that: (1) The support system establishes an elevated minimum principal stress zone near the tunnel face, enhancing physico-mechanical parameters of the anchored rock as the minimum principal stress increases. (2) With diminishing tunnel face spatial effects, prestress diffusion extends from the vault to the surrounding rock. (3) In the early stages of excavation (0 ∼ 4 m from the tunnel face), inclined rockbolts outperform radial rockbolts by providing more timely support to improve the stress state of the delayed support zone. (4) Critical factors such as burial depth, lateral pressure coefficients, prestress, and rockbolt angle significantly influence the stress field. When the stress in the rockbolts does not exceed their yield strength, optimal support efficiency and cost-effectiveness can be achieved by using 5-meter-long rockbolts installed at a 60° angle.