Figure 2.
The diagrams present tunnel cross section sketches before and after failure with associated depth measurements. Part a shows the condition before failure with the tunnel centre at a burial depth of 33.5 metres, the upper segment average depth at 36.4 metres, and the lower segment average depth at 38.1 metres. The tunnel lining is shown as a circular outline with surrounding segments maintaining a relatively uniform profile. Part b shows the condition after failure where the upper segment average depth is 33.5 metres and the lower segment average depth is 35.2 metres. The tunnel lining is distorted with visible displacement of segments and a thin space preceding the upper segment, indicating deformation and structural change after failure.Failure of two parallel running tunnels, Shanghai Metro Line 4 (source: Zheng et al. (2017))