Study on the Influence Law of Tunnel Primary-Support Arch Frame on Rock-Bolt Support Function

Abstract

This study reviews the development of research on systematic-rock-bolt effectiveness in soil tunnels and clarifies the mechanism of rock-bolt inefficiency. Field statistics show that rock-bolt axial forces in soil tunnels are generally low or even compressive, especially at the arch crown, indicating limited contribution to overall stability. Comparative analysis further suggests a strong correlation between arch-frame support and rock-bolt inefficiency. Numerical simulations and physical model tests were conducted to investigate the influence law and mechanism of arch frames on rock-bolt mechanical behavior. Results show that, compared with the bolt-only scheme, the combined bolt-and-arch-frame scheme reduces bolt axial force by about 90%, and some crown bolts become compressive under shallow-burial conditions. The main mechanism is that the high stiffness of the arch frame restrains surrounding-rock deformation, thereby reducing the relative displacement required to mobilize bolt axial force; under high lateral pressure, the arch-shaped structure also transfers load toward the crown and alters the local stress state. Within the investigated parameter range, the inhibitory effect becomes stronger as the surrounding-rock grade decreases. Although arch frames effectively control surrounding-rock stability in soil tunnels, they weaken bolt performance, with the most pronounced inhibition at the spandrel. Differently from previous studies mainly focused on field observations or single-condition analysis, this paper establishes a multilevel evidence chain for systematic-rock-bolt inefficiency in soil tunnels by combining field statistics, numerical simulation, and physical model tests, thereby providing a new analytical perspective for optimizing combined support systems.