The paper deals with the mechanical behaviour of a natural volcanic silty soil sampled from steep slopes. This soil is very loose and unsaturated over most of the year due to climate conditions. Thus so-called wetting collapse and static liquefaction may occur during rainfall. Major issues are posed once the slides turn into flows with high destructive potential. However, the modelling of the constitutive behaviour is challenging and not yet available in the literature for this soil. A recent Generalized Plasticity Model was selected as it is capable to adequately take into account the effects of change in soil porosity, bonding related to the matric suction normalized versus soil porosity, and static liquefaction proneness. The model is calibrated for 37 saturated/unsaturated laboratory tests, and the performance of the model is assessed quantitatively. It is newly shown that the model – with one single set of constitutive parameters – is capable to well describe the soil mechanical response, in unsaturated and saturated conditions, experienced by the soil in different laboratory devices and along different stress paths. Those insights provide a theoretical framework for designing further laboratory tests, improving the understanding of this complex natural soil, and implementing better modelling of landslides of the flow-type.
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