Modeling of soil-structure interaction during tunnelling in soft soil

This paper discusses the modelling of loading induced by tunnelling in soft soil. Only two dimensional analyses are considered. Two kinds of modelling are analysed and compared : a simple decrease of the stress vector applied at the periphery of the excavation (described by a scalar parameter lambda named the "deconfinement" rate), which is supposed to take into account in a very simple way the complex interaction between the lining and the surrounding soil, and a more complex modelling taking into account four different phases simulating the different kinds of interaction between the tunnel and the soil (deconfinement, weight change of the excavated zone, pore pressure applied on the lining, grout injection arid consolidation of the grout). This comparison is achieved using two constitutive models : an elastic-perfectly plastic model (the Mohr Coulomb ("MC") model) and a more complex elastic-plastic model considering hardening mechanisms (the "CJS" model). The results obtained show that, if the analysis is achieved only to evaluate the surface settlements, the first simple modelling can be used with a good approximation, but only if the CJS model is considered. If more complete results are needed, in particular the loading applied on the lining and the displacement of the lining, it is necessary to consider the second more complex modelling. The two kinds of modelling are compared on a simple case of shallow tunnel with homogeneous soil conditions, and then, on a real site where numerous data have been obtained during the construction of the metro of Lyon. This site benefitted from coordinated research in which our Laboratory was in charge of the numerical modelling.