Particular features of the physical and mechanical characteristics of certain Phlegraean pyroclastic soils

This paper deals with the mechanical properties of a number of pyroclastic soils derived from volcanic eruptions (pozzolan and pumice) in the Campanian region of central Italy. Although the analyzed samples come from the Phlegraean and Vesuvian areas, the conclusions and observations on their mechanical properties can be considered similar for most volcanic soils. The behavior of this kind of material is somewhat peculiar and indicates that pozzolan can be considered as an intermediate soil with technical characteristics between granular and cohesive soils. Pumice, one component of pozzolan, undergoes the process of grain crushing even at low stress levels, which influences the shear strength of this material. The failure envelope determined for dry and saturated specimens is non linear, such as the strength parameter (φ′) that is a function of the confining pressure or consolidation pressure. In addition, some surprising findings have emerged from the experimental results, such as the tendency of the pozzolan to swell in the absence of the pumice, high compressibility and high shear strength, high porosity beside low hydraulic conductivity, high values of the angle of friction of pumice, at constant volume, for both the “mixed grain size” and for each granulometric class fraction.Finally it is worth stressing that the study discussed in this article aims to provide a further contribution to the identification of those factors related to the alteration of the physical and mechanical characteristics of these types of soils which could trigger instability phenomena, especially during rainfall, affecting areas of the world where there is a presence of volcanoes.

► Tendency of the pozzolan to swell in the absence of the pumice ► High compressibility and high shear strength ► High porosity beside low hydraulic conductivity ► High values of the angle of friction of pumice for both “mixed grain size” and each single granulometric class fraction