Stiffness parameters for a granite residual soil in Salgueiros station for Porto Metro

Granite residual soils are abundant in the Northern part of Portugal and particularly in the city of Porto. Recently, the construction of the metro for the city allowed extensive geotechnical studies of this type of soil. For one of the stations, Salgueiros, constructed in an abandoned football field, several field tests (CH; SPT; SBP; PMT) and laboratory tests (direct shear, triaxial tests) were performed. Such a panoply of tests allowed obtaining different shear modulus from the various tests and, in some cases, its degradation during strain evolution. The first part of the paper presents the identification of the stiffness parameters derived from the various tests. In the second part, relationships between all the tests are examined, comparing the obtained values with the local design experience. Though the strong heterogeneity of the material, the study evidences a clear evolution with depth of the stiffness parameters, even for material with similar weathering degrees, and indicates, based on the local design behaviour, that shear modulus derived unload/reload loops of the SBP or from the similar loops in triaxial tests could be a good first design value.