Flattening of jointed shield-driven tunnel induced by longitudinal differential settlements

Longitudinal differential settlement of shield-driven tunnel may cause longitudinal bending of the structure, which may further introduce flattening of the tunnel. The flattening of a tunnel as well as the additional internal forces and deformations it causes is seldom studied in the literature. In this paper, analytical solutions are first developed for estimating the additional forces and deformations caused by flattening of a jointed shield-driven tunnel. The effects of tunnel joints, tunnel diameter, lining thickness, ground resistance and original tunnel curvature on the flattening effect of the tunnel are discussed. An application example of a tunnel in Shanghai is adopted to illustrate the flattening of the tunnel. It is found that the flattening effect is significant if the bending radius of tunnel is smaller than 5000 m, and that the deformations and internal forces induced by the flattening effect is comparable with those induce by the earth pressure. Thus, if the flattening effect is ignored, the design may be unsafe. It is important to consider the additional internal forces due to tunnel flattening in tunnel design when differential settlements are expected.

► The flattening effect on tunnel lining is proposed for the first time.
► Flattening effect is significant when bending radius is smaller than 5000 m.
► Ground resistance reduces the moment and shear force due to flattening effect.
► Moment and shear force due to flattening effect are comparable to earth pressure.