Abstract: In order to investigate the loading state of the combined structure of inner tension ring-segment-surrounding rock in tunnel boring machine (TBM) pressurized water conveyance tunnel, a three-dimensional refined finite element model of the composite structure comprising the inner tension ring-segment-surrounding rock in a TBM pressurized water conveyance tunnel has been established for the Class V TBM pressurized water conveyance tunnel of the Rongjiang-Guanbu water diversion project. The study has yielded important findings. The reinforcement of the tunnel structure with internal tensile steel ring can effectively control the deformation of the tunnel, reduce the tensile stress of the segment and the range of tension zone, and improve the bearing capacity of the composite structure. Under the action of internal water pressure, various parameters such as compression stress, vertical deformation, joint opening degree, stress of connecting bolts, stress of the inner tension ring, and stress of anchor rods all decrease compared to conditions without internal water pressure. However, the tensile stress of the segment and joint misalignment increase by 19.68% and 39.25%, respectively. The main reason is that the overall expansion of the tunnel structure is caused by internal water pressure. This underscores the need for strengthened safety monitoring during water filling operations. Under the combined action of external water and soil pressure and internal water pressure, the change of surrounding rock type lead to increased stress of connecting bolt, the misalignment of segment joints and the stress of anchor bolt by 37.11%, 15.29% and 14.75%, respectively. This highlights the importance of monitoring in the transition area of surrounding rock types where the load-bearing capacity is poorer. In addition, under the combined action of external water and soil pressure and internal water pressure, the load sharing rate of surrounding rock, segment, inner tension steel ring and anchor bolt is 21.38%, 43.08%, 24.01% and 11.53%, respectively. The load sharing rate of inner tension steel ring is 34.06% higher than that without internal water pressure. The effect of internal water pressure improves the load sharing effect of the inner tension steel ring. The load sharing ratio of surrounding rock in composite structure decreases with the increase of surrounding rock types (Class Ⅲ, Class Ⅳ, Class Ⅴ), and the load sharing ratio of class Ⅲ surrounding rock is 16.96% higher than that of Class Ⅴ under the same load. These research findings provide valuable theoretical reference for lining design and late reinforcement measures of similar tunnel projects.

Key words: water conveyance tunnel, composite structure, segment lining, inner tensile steel ring, mechanical properties, sharing rate