Due to the complex nature of rock masses, geomechanical classifications are often used to estimate indirectly strength parameters, such as the Geological Strength Index(GSI), developed by Hoek. This work intends to compare the rock mass strength parameters assessed by the GSI index with those obtained in direct shear tests carried out on large blocks. The rock mass in study is characterized by the dominant presence of carbonaceous phyllite, by the intense folding and weathering, by its ductile behavior and by presenting a dominant failure mode controlled by the rock mass properties. The tests were conducted in a servo controlled direct shear equipment on four undisturbed samples of 30 × 30 × 30 cm3. It was found that the rock mass strength parameters assessed by the GSI index were significantly conservative compared to the results of the shear tests. Based on these results, the input parameters of the generalized Hoek & Brown failure criterion were calibrated using the RocLab software.
Due to the complex nature of rock masses, geomechanical classifications are often used to estimate indirectly their strength parameters, such as the Geological Strength Index (GSI).
Nowadays, this tool is globally recognized and it is widely used in the definition of projects which involve the mechanical behavior of rock masses.
However, as it is suggested on these methodologies, itis necessary to check if during the construction, the geotechnical structure behaves according to the initial predictions. Otherwise, modifications on the models must be performed in order to obtain a more realistic representation.
This work intends to verify the rock mass strength parameters assessed using the GSI index by comparing these results with those obtained in direct shear tests carried out on large blocks (large enough to be considered representative of the rock mass in study).
The rock mass in study is mainly composed by carbonaceous phyllite and its particular characteristics, described in the further sections, made possible to obtain reasonable samples, representative of the rock mass.