Parametric Study and Optimization of Longitudinal Bolting in Tunnel Face Reinforcement

Longitudinal bolting has emerged as an effective pre-confinement technique to reinforce tunnel faces, offering advantages over alternative methods by directly enhancing stability in the excavation direction and improving load transfer to the surrounding ground.Despite its practical use, the influence of key design parameters remains insufficiently quantified for optimization.This study addresses this gap through a three-dimensional numerical model that incorporates soil-bolt interaction, with the soil represented by the Mohr-Coulomb failure criterion and the bolts modeled as linear elastic elements.A systematic parametric study was performed to evaluate the effects of bolt density, embedded length, axial stiffness, and soil strength on tunnel face stability.The results demonstrate that increasing bolt density significantly reduces face extrusion and axial displacement, although the rate of improvement diminishes beyond 0.25 bolts/m .Force distribution analysis revealed three distinct zones: confinement (0-0.5R),anchorage (0.5R-2.5R), and inert (>2.5R).The factor of safety was shown to reach its optimum at a bolting density of 0.25 bolts/m , providing a balance between reinforcement efficiency and material economy.These findings not only clarify the mechanisms governing longitudinal bolting efficiency but also deliver practical guidelines for the design and optimization of tunnel face reinforcement systems.