Abstract: Both high technology and high risk are involved in construction of the subsea tunnel, especially in poor ground condition sections. Xiamen Xiang’an tunnel is the first subsea tunnel in China mainland district. The tunnel is designed as a three-tube tunnel with 6-lane shuttle and the deepest point about 70m under sea level, built by means of boring and blasting method. The stratum of site is mainly composed of granite with some poor ground conditions including completely or strongly weathered granite in land and shallow section near to both portals as well as several significant faults in seabed. It is very important to achieve risk management in poor ground condition sections during tunneling in this case. Both the principle of risk occurrence and the method of risk evaluation for the construction of subsea tunnels are discussed based on the basic theory of risk analysis and evaluation. According to the geological prospecting, structure design and planned construction method in detail design phase of Xiamen Xiang’an tunnel, both typical geological risks and representative construction risks during tunnelling in poor ground condition sections were identified and evaluated by means of the method proposed in this paper. Countermeasures for reducing the risk level were proposed in order to implement a risk management to ensure the safety and quality of tunnelling.

Key words: subsea tunnel, poor ground condition section, geological risk, construction risk, risk evaluation, Countermeasures