Delayed failure at the Messochora tunnel, Greece

Geodetic and extensometer data are used to shed light on the poorly known effect of delayed tunnel deformation. Four weeks after full section excavation and without any evidence of gradual strain accumulation, significant convergence of the tunnel walls and cracking of the lining occurred along a 36 m long, weak rock zone in the Messochora tunnel (Greece). Deformation extended also to nearby, previously stabilized sections. This event is in variance with predictions of an exponential-type pattern of decrease of strain accumulation based on theoretical and field evidence; it can only be explained if we accept that at the weak zone the tunnel was at a critical stability level, and that some small-scale interventions, which would otherwise have no effects, triggered a new phase of deformation transferred to nearby sections. Evidence from this tunnel indicates that, especially in weak rock zones, post-excavation stabilization may only be transient, depending on the balance between stresses and the combined strength of the rocks/lining.