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Soil collapse may occur passing from unsaturated to saturated conditions, thus causing major problems, among which one can mention a poor performance of the structures or the occurrence of landslides turning into flows. The mechanisms of the soil collapse have been studied at macroscopic scale since many years, while few observations at microscopic level are available. In this work, the mechanisms of capillary collapse were investigated for a volcanic (air-fall) pyroclastic soil of Southern Italy, which is characterized by an open metastable structure and is frequently involved into catastrophic rainfall-induced landslides. The experimental investigation was performed through X-ray Computed Tomography, which allows reconstructing 3D images of the specimen from the spatial distribution of the linear attenuation coefficient. The tests were carried out on coarse sand. During the tests, the specimens were loaded by its self-weight without any external load, and the suction was gradually reduced until the specimen collapse occurs. The aims of the experimental program were: i) follow the transformation of the specimen’s microstructure; ii) evaluate the variation in terms of water content, porosity and grains spatial distribution; iii) analyse the effect of grain size distribution on the development of capillary forces and mass forces. The experimental evidences outline that, for the coarse pyroclastic sand, the collapse occurs at a very low suction, while it is not mandatory to reach the complete saturation

Grain scale mechanisms for capillary collapse in a loose unsaturated pyroclastic soil