Research purposes： The Xiamen Xiang'an Subsea Tunnel is the first subsea tunnel built in the China mainland. As the geological conditions were complex, the accidents of water and mud bursting and the collapse easily happened when the service tunnel of the Xiamen Xiang'an Subsea Tunnel crossed monzonite veins. It was one of the difficult points in construction and it also never been met in the previous tunnel construction. Therefore，there was no construction experience to draw. During the construction process, the feasible excavation methods and supporting program had to be used along with the advanced geological prediction, monitoring and measurement in order to ensure the safe and fast construction.
Research conclusions ； (1) By making the feasible geological prediction，the accurate geological situation was obtained. By making the FLAC3D numerical simulation and comparison of the excavation schemes with bench method, the 6m step height was easily chosen. Based on the controls of the stratum deformation and tunnel stability, the reasonable 2 m excavation footage was adjusted with the help of the FLAC3D numerical simulation, and the reasonable step length was 10m. (2) In order to meet the requirements of the strength, stability and durability, the composite lining was adopted to the service tunnel of the Xiamen Xiang an Tunnel. (3) For the monozonite veins section, by monitoring and measuring the subsidence of the crown and the convergence of the cave，the rationality of the scheme and the effectiveness of specific measures were verified．(4)The key technology system of the excavation，support，geological prediction and monitoring and measuring for the Xiamen Subsea Tunnel can provide the referenee to construction of submarine tunnel across monozonite veins section or weathered rock section．
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