Mechanical behaviour of an unsaturated dynamically compacted silty sand

This paper deals with the main results of a research project carried out at the University of Naples “Federico II” [RAMPINO, 1997], aimed at analysing the influence of soil suction on the behaviour of compacted granular soils with a non-negligible fine component. The study has been carried out by two experimental apparatuses for unsaturated soils (a stress path triaxial cell and an oedometer) on a dynamically compacted silty sand, used in the core of the Metramo river dam in Italy. The experimental program includes tests run at constant and variable suction (in the range 0÷400 kPa), following various loading paths. Suction has significant influence on soil response and in general improves the soil’s mechanical properties. By increasing suction, compressibility is reduced and stiffness and shear strength increase, following an exponential law with decreasing gradient. However, the hypothesis of saturated material is not at all for safety’s sake, since the soil can experience collapse upon wetting, an occurrence absolutely unpredictable on the basis of Terzaghi’s effective stress principle. The observed behaviour is framed in the most widespread elasto-plastic models for unsaturated fine-grained soils, giving some interesting starting points for their extension to granular compacted materials.