Abstract：Considering the interaction and coordinate deformations between flexible tunnel lining and the adjacent soft ground，the concepts of stiffness resisting differential pressure on tunnel and relative stiffness ratio of tunnel and ground are presented. The corresponding theoretical method to calculate this ratio is also set up considering the variation of joint rotating stiffness with the magnitude of stress. As an application of this theory，based on the constructed Shanghai Metro Line No.2，flexibility index of tunnel lining is quantitatively discussed. The result shows that the lining with thickness of 200 mm should be treated as flexible tunnel and the flexibility index is assumed to be 100%. This conclusion has a good agreement with the qualitative result from the valuating criteria of relative flexibility founded by Peck et al in 1972. For the existing Shanghai Metro tunnel lining with thickness of 350 mm，the calculated flexibility index is 69%，which theoretically demonstrates such an argument that the present Shanghai Metro lining is，to some extent，flexible with rigidity.
