Dynamic characterization of soils and soft rocks of the Central Archeological Area of Rome

Abstract

The paper presents the results of in-situ and laboratory tests aimed at defining the cyclic properties of soils and soft rocks of the Central Archeological Area of Rome in the framework of the seismic microzonation study of the area. The small-strain shear modulus \(\text{ G }_{0}\) (or analogously shear wave velocity \(\text{ V }_\mathrm{S}\)) and the curves expressing shear modulus G and damping ratio D variation with shear strain amplitude were investigated. A large amount of in-hole tests integrated with active surface wave tests were utilized to characterize the small-strain stiffness of the lithotypes identified in the area. Small-strain stiffness values determined by geophysical tests were further compared with those measured in the laboratory showing different behaviors of soils and soft rocks. The effects of sample disturbance and degree of jointing, for soils and soft rocks, respectively, was invoked to explain the observed differences. The shear modulus and damping ratio versus shear strain amplitude curves were determined by means of resonant column and cyclic shear tests, both simple and torsional. The experimental results were compared with literature data on similar soils highlighting some peculiar behaviors. In particular the role of fine matrix in sandy soils and organic matter content of clays on the cyclic properties was stressed. The results showed that an increase in fine matrix and organic content results in a stronger linearity and lower damping ratio.