A comparative study of GSI chart versions in a heterogeneous rock mass media (Marão tunnel, north Portugal): a reliable index in geotechnical surveys and rock engineering design

The rock mass quality in underground excavations and or tunnelling is a key issue in the design of the most suitable support to ground stability. This work aims to discuss the applicability of the latest version of the geological strength index (GSI|₂₀₁₃) in an anisotropic media during ongoing deep underground excavation by drilling and blasting. The study encompasses a comprehensive geological, geotechnical and geomechanical characterisation and evaluation of the excavation faces in the Marão tunnel (north Portugal). In addition, 305 m (two underground sections) of tunnel were mapped in 74 consecutive face underground excavation advances. Geological, geotechnical and geomechanical data relating to the site investigation were collected and computed in each new face advance. The scanline sampling technique was applied on exposed rock surfaces, a geotechnical analysis of the discontinuities was performed, and the block size of the rock mass was determined. Geotechnical and geomechanical zoning of the studied sections were continuously carried out considering the knowledge of the site rock mass characteristics and behaviour. In addition, a comparison was performed between the version of GSI|₂₀₁₃ and its earlier version (namely, GSI|₉₈) and correlated with rock mass rating. The study will contribute to a better understanding of GSI|₂₀₁₃ and its applicability in geoengineering projects in design or excavation work stages.