An engineering geological assessment with respect to the TBM excavation, within a highly heterogeneous system of formations known as the "Athens Schist" is presented. The method considers mainly the rock mass competence on the basis of criteria related to lithology, tectonic deformation (gracturation-foling-shearing), weathering and rock mass quality as well as geometrical-structural and groundwater criteria. Hence it calibrates the rock mass interaction during excavation using the specific TBM of the Athens Metro project, with an emphasis to potential face failure and overbreaks. It is based on a statistical-minded extrapolation of factors that govern the behaviour of the ground, when subjected to mechanised boring. The final objective is to identify which tunnel sections exhibit "friendly", "moderate"' or "adverse" tunnelling conditions with respect to overbreak development and propagation and to provide a basis for the selection of appropriate treatment and for alternative excavation methods.