The Unified Theoretical Model for Predicting the Full-Cycle Stability of Hydraulic Tunnels

The stability of hydraulic tunnels is crucial to the safety of water conservancy and hydropower engineering. Previous studies mostly focused on evaluating the stability of hydraulic tunnels within a certain stage. This study presents a full-cycle stability prediction of hydraulic tunnels from the construction to the operation stages. The corresponding unified theoretical model is established, where the installation delay of lining is considered. On the premise that the rock satisfies the Hoek–Brown strength criterion, the mechanical responses of hydraulic tunnels are classified into six cases to discuss, according to the development laws of elastoplastic deformation of surrounding rock during the whole stage. The analytical solutions for the stress and displacement of surrounding rock are provided in each case, and the mathematical expressions are proposed for determining the critical support pressure (no plastic zone generated), minimum support pressure (no loosening zone generated) and critical water pressure (exchange of the directions of the maximum and minimum principal stresses), respectively. Furthermore, the reliability of the theoretical derivation is well verified by implementing it in two engineering projects and comparing it with previous solutions. Finally, a comprehensive parametric investigation is conducted based on the proposed theoretical model, including the geological strength index (GSI), disturbance coefficient ([Formula: see text] and water pressure ([Formula: see text]. It can be found that both the increase in the GSI value and the decrease in the [Formula: see text] value can cause a decrease in the plastic radius of the surrounding rock, thus leading to a decrease in the tunnel displacement. The directions of the principal stresses in the surrounding rock may exchange and the tangential tensile stresses will generate as the water pressure increases. The required critical water pressure becomes higher with a higher GSI value. The increases in the loosening radius of surrounding rock and the critical support pressure can be observed as the GSI value decreases. The results of this study can provide useful guidance for the design of hydraulic tunnels.