Engineering Geological Considerations in Tunnelling through Major Tectonic Thrust Zones - Cases along the Egnatia Motorway, Northern Greece

Abstract.

Tunnelling through tectonic shear zones is always a challenge due to the geometrical complexity and the presence of poor quality rock masses with high variability in their occurrence. Based on the experience from the Egnatia motorway, in Northern Greece, the paper aims to give some general concepts, on the main failure mechanisms of the deformed materials and discuss guidelines for the design. Two case studies are presented, the first when the rock mass is brecciated derived from brittle rocks and the second when the rock mass is sheared from the compression of weak rocks with plastic characters. In the first case the rock mass is composed of interlocking angular pieces of strong rock and can exhibit satisfactory stability under confined conditions. Unless immediate support is provided, the cohesionless rock mass does ravel in tunnel excavation. In the second case heavily sheared clayey flysch exhibits squeezing behaviour even under tens of meters of overburden. In this case there is a variability of the material and it is not suggested to provide a number of support categories for each of the erratically alternating various geotechnical conditions that may be present but it is preferable to design an excavation and support system and sequence which are capable to deal with the worst anticipated conditions.