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 (C_t) and the creep coefficient (C_α). 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.