Performance-based three-level fortification goal and its application in anti-dislocation countermeasures: A case study of Shantou Submarine tunnel

The dislocation momentum is the design basis for anti-dislocation to tunnel when a tunnel crosses an active fault. The influence of different dislocation levels on tunnel performances is not clear. Thus, based on seismic activity parameters at the site of interest and probability of fault dislocation, probability fault displacement hazard analysis (PFDHA) methodology was introduced in this paper to ascertain the fault dislocation level under different exceeding probabilities (63%, 10%, and 2%–3%). Then, based on the definition of different ground motion strength and fortification goals of the tunnel, a three-level fortification goal with different performance requirements of the tunnel was proposed. The first attempt to use the proposed indexes including the maximum dislocation of the tunnel and maximum relative deformation of the tunnel was tried to evaluate deformation and failure states with an experimental approach. Subsequently, the feasibility of the three-level fortification goal was further investigated according to the self-defined qualitative description and quantitative indexes in the segmental design and sectional expansion tunnels comprehensively. The results show that the fault dislocations relying on PFDHA at the site of the Shantou Submarine Tunnel are firstly ascertained as 0.04, 1.8, and 2.4 m respectively. Taking the fault dislocation as model input values into account, the dislocation mechanism of the tunnel under the three levels was revealed. More importantly, judging from the dislocation performance requirements of the three-level fortification goal, the tunnel deformation and failure states are mitigated by adopting the countermeasures. The sectional expansion design can well meet the requirements without the restriction of a strong earthquake, while the effectiveness of the segmental tunnel can be proved under frequently occurred and fortification earthquake. The final research results are expected to provide a new fortification goal for anti-dislocation hazard evaluation on expansion design in high-intensity seismic regions and segmental design in slight and moderate-intensity seismic regions.