Ring-shaped structure of tunnel surrounding rock: Characteristics and load-bearing mechanisms

A comprehensive investigation into the evolution of the load-bearing structure is fundamental for quantitative tunnel design. In this study, we proposed an elastoplastic mechanical model with multiple constraints, derived from the deformation and propagation mechanisms of the surrounding rock under construction disturbances. The model incorporates plastic hardening and softening behavior and provides stress and deformation solutions at different stages. The radial stress is decreased, and the circumferential stress is increased to its peak due to tunnel excavation. Failure is initiated when the peak stress exceeds rock mass strength, and the circumferential stress peak migrates into deeper zones until equilibrium with the load-bearing capacity is achieved. The ring-shaped structure (RSS) of the surrounding rock is formed once a state of stress equilibrium is established. Multiple ring-shaped structures and their associated loading zone exist within the surrounding rock, with each structure bearing only the load from its respective rock stratum. The innermost stable ring-shaped structure serves as the primary load-bearing component. Progressive deformation and instability of this ring-shaped structure are induced by inadequate support stiffness, triggering further failure development until the adjacent ring-shaped structure assumes the rock load. Based on these observations, the "ring-shaped structure" theory is developed to describe the load-bearing structural system of the surrounding rock. This theory not only quantifies the bearing behavior but also links the evolution of ring-shaped structures with stress transfer and progressive failure.