Effects of undrained strength anisotropy on surface subsidences induced by the construction of shallow tunnels

The implementation of an anisotropic soil model that allows consideration of the variation of undrained strength due to anisotropy is described. This analysis is then used to identify the significance of strength anisotropy on the prediction of deformation behaviour of a shallow tunnel. It is found that, for the case of an unlined tunnel, attention should be given to the effect of strength anisotropy particularly for a soil possessing type "K" anisotropy (i.e., where the smallest undrained strength occurs at an angle θ other than the vertical or the horizontal, and the smallest value usually occurs at θ = 45°). On the other hand, for a lined tunnel, the effect of strength anisotropy upon the surface settlement profile will depend upon the size of a so-called "gap" parameter. The gap represents the net effect of loss of ground and some "workmanship" factors in a plane strain finite element analysis. Increasing the gap eases the restrictions imposed by the tunnel lining upon possible soil deformations, thereby increasing the effect of strength anisotropy. However, it is shown that for a lined tunnel with a moderate value of gap, detailed consideration of strength anisotropy may not be necessary. Key words: strength anisotropy, tunnelling, predictions, surface subsidences, finite element method, soil model, soft clays, vane strength.