Deformation and failure of the Tymfristos Tunnel, Greece

The Tymfristos tunnel, a 1,365 m long road tunnel with an 11 m section diameter, was excavated through the highly tectonized, low strength Flysch formation of central Greece. About 1 year after the completion of the excavation in 1995, convergence of up to 20% followed by failure of the support system was recorded. A 501 m long segment, in which the failure occurred, was reexcavated based on a more detailed study of the ground conditions; however, this second phase was also followed by an up to 6% convergence. The detailed geodetic study of the three-dimensional kinematics of the support shell in relation time, ring closure delays, and support measures reveals an unusual time-history of convergence occasionally continuing even after the ring closure and a nonuniform distortion of the lining. Furthermore, uncontrolled deformation of certain "weak" sections propagated bidirectionally to adjacent, previously stabilized sections producing a "chain effect," responsible for destabilization and resumed convergence of long (similar to90 m) segments.