Evaluation of the Hydraulic Hysteresis of Unsaturated Pyroclastic Soils by in Situ Measurements

The triggering mechanism of debris flows and mudflows involving unsaturated soils has been extensively covered in the literature. It is associated with heavy rainfalls occurring during wet periods, when the pore water pressure regime is critical for landsliding. Having said this, full understanding of slope failure conditions needs a fair analysis of groundwater flow and, therefore, a proper hydraulic soil characterization as fundamental basis of the analysis. At this regard, laboratory characterization could be not sufficient: the soil water retention curve (SWRC) is hysteretic and the paths followed in situ, in terms of water content and matric suction, depend on the preceding hydraulic history. This aspect has to be accurately taken into account for analysis and prediction. In order to explore the hydraulic hysteretic behaviour of pyroclastic soils, in this paper pairs of water content and matric suction measurements registered at the same depth in an instrumented unsaturated pyroclastic slope are compared to a number of retention curves obtained in the laboratory.