Quantitative Stability Assessment of Tunnels Using the Stability of Opening Index (SOI)

Abstract

Assessing tunnel stability based solely on strength values cannot fully reflect the structure's load transfer efficiency. This research introduces a quantitative assessment using the "Stability of Opening Index" (SOI), a dimensionless index calculated from the ratio of the energy directly causing deformation at the opening to the total energy absorbed by the structure. It utilizes cumulative displacement data from Finite Element Method (FEM) analysis, and its validity is confirmed against the failure behavior observed in laboratory tests. The study found that the SOI value has a significant quantitative correlation with the structure's stability. Therefore, SOI serves as an effective quantitative tool for engineers to tangibly compare tunnel designs, considering both geometric shapes and reinforcement systems.