Small strain behaviour of a pyroclastic soil in unsaturated conditions

The paper presents part of the experimental results of a wide research program carried out the University of Naples Federico II (Italy) on a pyroclastic soil retrieved from a shallow deposit located in Cava dei Tirreni, 20 km South-East of Naples (Italy). Resulting from volcanic activity, the soil has a grain size distribution ranging from gravel (pumices) to a non-plastic sandy silt and a vacuolar nature of soil particles. The basic mechanical properties of this pyroclastic soil have been widely studied with conventional laboratory tests and usefully integrated by means of extensive in situ testing. This paper focuses on the small strain behaviour of the mentioned soil in unsaturated conditions. Tests have been carried out by using a suction controlled RCTS cell, investigating the volumetric behaviour through isotropic stress paths while measuring almost continuously mechanical and dissipative properties of soil at small strain. Samples have been reconstituted by dry air pluviation technique and then saturated and freezed at constant volume. The results collected along several isotropic stress-paths including compressions and drying/wetting single stages and cycles are presented and interpreted in terms of evolution of Go and Do with net stress, suction and with the plastic volumetric strains caused by the mean net stress and suction. The observed mechanical response is interpreted highlighting the influence of the peculiar nature of the soil and discussing the extension to the unsaturated state of the predictive criteria typically used to estimate the evolu-tion of Go and Do of saturated plastic soils with mean effective stress.