Microstructural factors affecting the static and the cyclic resistance of a pyroclastic silty sand

In this note the undrained cyclic behaviour of a pyroclastic silty sand typical of the Neapolitan area, in southern Italy, is analysed through an extensive laboratory investigation. Monotonic and cyclic triaxial tests were carried out on undisturbed samples and on reconstituted soil mixtures. The latter were prepared keeping the grain size distribution of the natural soil as reference and replacing either non-plastic ash with a plastic fine (PCR) or the pyroclastic sandy fraction with a siliceous material (SAR). All the tests were carried out at the same consolidation relative density and confining stress with the purpose of studying the influence of peculiar constitutive factors, such as fabric, particle breakage and non-plastic fines content on the undrained cyclic behaviour. The results highlight that the structure significantly enhances the liquefaction resistance of the natural material with respect to that of reconstituted samples. Furthermore, the non-plastic fine ash was observed to increase the cyclic resistance of the pumice sand with respect to that of the same coarse matrix with added plastic fines. Finally, no appreciable evidence of grain crushing was detected in any of the tested soils containing pyroclastic sand.