ABSTRACT
Recent experience of metro construction in Porto has highlighted the existence of particular failure mechanisms in weathered granite linked to the complex and heterogeneous characteristic of the ground due to its spatial variability as well as discontinuities filled with weak materials. This experience shows that failure mechanisms of the ground surrounding a shallow tunnel excavated by a TBM in an urban environment are not limited to the classical, predictable, soft ground modes of wedge failure in the excavation face and arching failure of the tunnel profile (i.e. roof collapse or fall until a stable arch is formed), but are also conditioned by local instability in the excavation face triggered by hydraulic and/or mechanical shocks and progressive failure of initial cavities due to over-excavation or non-complete filling of the tail void by longitudinal grouting, manifesting finally as a sudden brittle collapse. It may be concluded that a correct understanding of these particular failure modes by all parties involved is essential for mitigating the risks of construction in urban environments.
