Vito Foresta

Suction Controlled Laboratory Tests on Undisturbed Pyroclastic Soil: Stiffnesses and Volumetric Deformations

This paper summarizes the preliminary results of a study on the effects of suction on stiffness and compressibility of ashy layers which cover the Pizzo d’Alvano relief (Sarno-Southern Italy). These soils, similarly to other pyroclastic soils, show some peculiar characteristics, as a remarkable secondary settlement, a metastable structure and a limited range of suction variation. At first, the influence of secondary settlement on the long-term deformability is presented. The influence of suction on the apparent preconsolidation stress and, in general, on the stress-strain behaviour of such soils is also shown by means of suction controlled tests. Finally, the metastable structure of these soil is investigated by means of a suction controlled oedometer apparatus. Results show that a reduced time of load permanence, in oedometer tests, is suitable to determine both the entity of primary settlements and the law of variation of secondary settlements of solid phase. More time is needed for the complete measure of the water content variation. The effect of suction is evident on the apparent preconsolidation stress value and on both the primary compressive index (Ct) and the creep coefficient (Cα). The influence of suction on the deformability parameters of the solid skeleton seems to vanish at high stress levels. Isotropic triaxial tests show a general anisotropic behaviour of the soil within the apparent preconsolidation range although a not so clear influence of suction on this behaviour is observed. Comparing the collapse tests on undisturbed and remoulded specimens, it is possible to identify a metastable structure in the first ones and, on the contrary, a lacking of collapse in the others.

Penetration tests in shallow layered unsaturated pyroclastic soil deposits of Southern Italy

The paper deals with a recent extensive in-situ and laboratory investigation carried out in a site characterized by thin layered unsaturated air-fall volcanic (pyroclastic) soil deposits, which are very steep and prone to slope failures turning into catastrophic flows (Cervinara, Southern Italy). The field campaign was based on iron-rod drillings, penetration tests and hand-excavated shafts, beside to the collection of undisturbed soil specimens used for laboratory tests. The real sequence of the soil layers was identified in more than two hundreds verticals. In addition, while a series of available literature formulas are detected as unreliable for these soils, the paper propose new reliable empirical correlations between the soil friction angle (of three lithotypes) and both the number of blows of the penetration test and the vertical stress.

Application of a generalized plasticity constitutive model to a saturated pyroclastic soil of Southern Italy

The paper explores the capability of the advanced Pastor-Zienkiewicz constitutive model to accurate simulate the deformability, the shear strength and the static liquefaction of an air-fall volcanic (pyroclastic) soil, which was involved in catastrophic landslides of the flow type. The calibration and validation of two distinct versions of the PZ constitutive model is done for both undisturbed (loose) and remoulded (dense) saturated specimens. Either drained or undrained triaxial tests are properly reproduced through the model. The potentialities deriving from the use of an advanced constitutive model are pointed out.