Face stability of conventional shallow tunnels in heterogeneous soil: the case-history of Tunnel Duphot

Maintaining the stability of the tunnel face is an essence to ensure safety during underground construction at any depth and through any ground condition. Specifically in the case of urban environments, where tunnels are typically shallow and often driven in increasingly difficult ground conditions. The objective of this dissertation is to assess face stability of conventional shallow tunnels in heterogeneous soil. The structure under examination is Tunnel Duphot, an urban tunnel commissioned to fulfil the role of connecting historical buildings in the heart of Paris. The research started from the theoretical study of the physical properties of the Parisian subsoil to understand the cause of face stability observed in situ that was apparently in contrast with the results provided by the analytical methods. The Parisian Basin is a geological basin of sedimentary rocks in which sediments were deposited during various geological eras. In particular, the Alluvions de la Seine are a quaternary overload of the third geological period of the Cenozoic era presenting a very heterogeneous character with sandy-gravelly passages and finer or even silty passages. In condition of partial saturation, one of the main phenomena to be identified in heterogeneous soil as this one, is the matric suction which represents the negative pressure that can result into a short-term resistance under conditions of unconfined compression. In order to study this phenomenon, several geotechnical investigations have been carried out and a three-dimensional finite element analysis has been implemented. It was concluded that the suction combined with the extreme heterogeneity of the soil, contributed to the overall short-term stability of the excavation face due to the support function manifested in the increase in the effective tensional state performed by some layers towards others.