A physical and numerical investigation on the stability of shallow tunnels in strain softening media

The behaviour up to failure of shallow underground openings is discussed on the basis of some laboratory, small-scale model tests and of their finite element simulation. The experimental results are first illustrated. They were obtained from two-dimensional (plane strain) and three-dimensional tunnel models tested under standard gravity conditions. Then, the phenomenon of strain localisation that characterizes the medium surrounding the model tunnels is discussed, recalling two alternative approaches for its numerical interpretation. On this basis, a finite element procedure for strain softening analyses is outlined and applied to the simulation of the tests in both two- and three-dimensional regimes. The comparison between experimental and numerical results leads to some conclusions on the influence of strain localisation on the overall behaviour of shallow tunnels and on the stability of their headings.