Rio Tinto Tunnel of Line C of the Metro do Porto, about 1km long and currently at design stage, will pass with shallow overburden through granites and schists weathered to various degrees. The intrusive interface between the two lithological domains was subject to intense shearing and contact metamorphic processes. Site investigation for the tender design phase involved standard techniques including core drilling, standard penetration testing, Lugeon and LeFranc permeability testing and sampling. The application of complementary methods such as refraction seismic survey, detailed outcrop mapping and aerial photo analysis allowed for the development of a project-specific rock mass classification and a detailed geotechnical model. A rock mass classification system was developed with reference to international standards, including the RMR and GSI standards, but with additional emphasis on description of rock mass behavior and anticipated ground related risks. With all descriptive and interpretative elements summarized in individual classification sheets the project-specific rock mass classification represents a conclusive site characterization. The correlation of data from various sources and statistical evaluation of all data allowed for re-producible interpretation and classification processes. Detailed description of rock mass behaviour, failure modes and potential ground relate risks supply the information required for a transparent tender design. Such an approach gains in significance as contractual claims in large civil engineering contracts become increasingly common.