Back analysis of time-dependent behaviour of a test gallery in claystone

This paper presents a back analysis of the time-dependent behaviour of a monitored tunnel in Switzerland. It concerns a specific section of the Mont Terri reconnaissance gallery which was excavated in creeping rock. From a theoretical point of view, Lemaitre's viscoplastic model was implemented in the FLAC 2D finite-difference numerical code. Particular attention was paid to the stability of the numerical solution for such an explicit mode of resolution. Lemaitre model parameters were successfully fitted on creep tests and then on in situ convergence measurements after a complete numerical simulation. Based on the comparison of strain and stress measured in the lining with the calculated values, time-dependent behaviour can finally be extrapolated over a period of approximately 10 years. Scale effects between results of in situ and laboratory fittings are also discussed. The aim of these investigations is to justify the interest of Lemaitre's model for the analysis of time-dependent behaviour of underground structures over a long period of time.