Numerical analysis of tunnel response during excavation in squeezing rock by using two constitutive models

Two constitutive models (SHELVIP and 3SC) have been developed in order to describe the squeezing behaviour during tunnel excavation. SHELVIP couples the general theory of elasto-plasticity with a time-dependent component. In the stress space an external non-hardening plastic surface defines the locus of plastic strains development, while an internal stress-hardening viscoplastic surface establishes the onset of viscoplastic strains. The 3SC model consists of an elastic element for the time independent reversible response, a viscoelastic Kelvin element for primary creep (time-dependent reversible strains) and a viscoplastic element for secondary creep (time-dependent non reversible strains). Following a short presentation of the two constitutive models, the case study of the Saint Martin La Porte access adit, along the Lyon-Turin Base Tunnel, is taken as representative with the main purpose to analyse the response of a novel yielding support system which has been devised to cope with the very severe squeezing conditions encountered.