Study on Deformation Characteristics and Countermeasures of the Shallow-buried Tunnel Portal Section in Silty Clay

To investigate the deformation characteristics of surrounding rock in the shallow-buried tunnel portal section in silty clay, field monitoring was conducted based on the Yanjiazhai Tunnel project in Yunnan. Variations in crown settlement, tunnel convergence, and surface settlement were analyzed. An orthogonal experimental design based on the numerical model was employed to perform a sensitivity analysis of countermeasures. The results indicate that the surrounding rock deformation of the tunnel portal section in silty clay is characterized by significantly greater crown settlement compared to tunnel convergence, with a maximum cumulative settlement reaching 919 mm. The time-history curve of the tunnel crown can be divided into three stages: rapid growth, continuous growth, and stable development, with an average deformation rate of 0.78 mm/d during the stable stage. The inclination of the upper step for foot-locking anchors exerts the most pronounced influence on crown settlement. When the upper and middle bench inclinations exceed 20° and 30°, respectively, crown settlement decreases significantly with increasing inclination. A synergistic effect exists between bolt length and inclination angle, as longer foot-lock bolts enhance the crown settlement control effect associated with greater inclination.