Geomechanical characterization of a highly heterogeneous flysch rock mass by means of the GSI method.

Over the last years, several researchers have investigated heterogeneous rock mass behaviour using the Geological Strength Index (GSI) classification system in conjunction with the Hoek–Brown criterion. Because of the variable composition of such rock masses, the availability of both petrographic and mechanical features of the intact rock is essential in order to carry out geomechanical studies. Starting from the detailed evaluation of the strength and deformability of the intact rock, the GSI method was applied to the Sanremo flysch formation, which is widely outcropping in Western Italy and involved in many engineering projects. It is characterized by both a remarkable lithological heterogeneity and a structural complexity. Petrographic analysis and laboratory tests allowed the definition of the main constituent lithotypes and the identification of their mechanical properties, whereas geomechanical surveys were performed in order to estimate the GSI values and the strong/weak ratio. Based on the frequency of strong and weak layers, a ‘‘weighted average’’ of intact rock values was used in estimating the rock mass parameters. Three main rock mass classes were identified, and a basic engineering geological map of the study area was prepared.