Abstract: The existence of cavities behind the segments is one of the common disease phenomena in shield tunnels. The cavities not only affect the static mechanics behavior of soil and segment, but also directly affect the dynamic response of the tunnel, which could aggravate the tunnel’s seismic damage. The existing related researches all use the deterministic analysis methods and lack the quantitative evaluation method in terms of probability perspective. Taking a shield tunnel in a rail transit section as an example, considering factors such as the location and size of the cavity, site conditions, and the incident direction of seismic wave, a large number of nonlinear dynamic time-history analyses are conducted based on the soil-tunnel-void interaction using an increment dynamic analysis method. Combined with the theory of tunnel seismic vulnerability, the seismic vulnerability is studied for the shield tunnel with the cavity disease. It is found that: the size of the cavity, the site conditions and the incidence direction of seismic wave have an important influence on the tunnel seismic vulnerability. As the cavity size increases, the plastic deformation of the soil adjacent to the cavity increases substantially, the eccentricity extent of the segment section increases, and the load-bearing performance decreases, and the affected area is about 3?5 times the cavity size. The cavity behind the segment increases the vulnerability of the tunnel structure. The increase of vulnerability is more obvious with the void size increasing. The impact of the cavity on the structure vulnerability varies with different locations of the cavity. When the cavity size is relatively large, the impact extent shows the following trend with a descending order: the side wall, shoulder, and crown cavity. As site condition worsens, the tunnel vulnerability increases and the influence of the void on tunnel vulnerability also enhance. Under the transverse and vertical incident directions of seismic ground motions, the tunnel vulnerability increases nonlinearly with the increasing of the cavity size. Although the damage probability under vertical ground motion is less than that of transverse ground motion, the increasing extent is significantly larger and more sensitive to the cavity size. It is worth to noting that for the three cavity locations and the two incident directions of ground motion, the larger cavity size may significantly affect the seismic performance of the tunnel structure.

Key words: shield tunnel, cavity, site conditions, incident direction of seismic wave, seismic damage, vulnerability curve