A study on the subsoil of the Tower of Pisa based on results from standard and high-quality samples

In this paper, an updated geotechnical characterization of the soils underlying the leaning Tower of Pisa is presented, based on results from a recent experimental investigation as well as on results from previous investigations. Careful analysis of the cone penetration tests carried out in the past have permitted a detailed distinction between the cohesive and the cohesionless portions of the upper deposits. The mechanical behaviour of the soils has been investigated through a large number of laboratory tests performed on samples retrieved with standard tube samplers and with a large diameter thin-walled sampler. The mechanical behaviour of the soft upper clayey deposit is shown to be rather sensitive to sampling disturbance; the large diameter sampler allowed for the observation of a high compressibility that had not been evidenced in the previous investigations. A comprehensive set of soil parameters is derived for the clay strata. Normalized triaxial stress paths describe a state boundary surface similar to that predicted by the simple Modified Cam Clay model fitted on the experimental data. Also, the undrained shear strength predicted by the model is shown to be in good agreement with the experimental results obtained from standard unconsolidated undrained triaxial compression tests. Values of the small-strain shear modulus determined experimentally compare satisfactorily with those given by an empirical relationship. A profile for the small-strain shear modulus is proposed accordingly.