Rainwater infiltration is the mechanism that leads or predisposes unsaturated pyroclastic slopes to failure by reducing matric suction and hence reducing the shear strength. Therefore, groundwater flow analysis is an important tool to investigate slope stability as well as predict conditions caused by heavy rainfall. Analysis and prediction quality can be greatly improved by in situ monitoring. In this framework, this paper describes the results of an ongoing experimental research project on mudflows, based on monitoring of a test site in a typical geological environment in western Campania. The site was selected to carry out extensive laboratory and in situ experimentation, consisting of field monitoring of climatic conditions, matric suction and water content. The suction data span about 4 years, while water content data cover about 2 years. Considered together, these data allow clear identification of seasonal fluctuations of the hydraulic properties in the subsoil, while giving interesting information on the hysteretic hydraulic behaviour of pyroclastic soils. Suction measurements were elaborated to obtain groundwater heads in the subsoil and then equipotential lines and flow vectors.

In many geotechnical applications, especially in the study of weather-induced landslides, a reliable soil hydraulic characterization in unsaturated conditions is required. Currently, the experimental techniques that neglect the hydraulic hysteresis represent the greatest limitation to landslide forecasting. In this paper, a procedure to obtain an unsaturated soil hydraulic characterization on natural pyroclastic samples is proposed and verified. The approach enables the evaluation of the soil hydraulic properties along the main drying path and wetting/drying cycles to fully quantify the effects of the hydraulic hysteresis. Pyroclastic soil samples collected at a test site at Mount Faito in the Campania region (southern Italy) were tested. The experimental investigation consisted of a sequence of testing phases: a constant-head hydraulic conductivity test, a forced evaporation test followed by several wetting–drying cycles, and a drying test in a pressure plate apparatus. The hysteretic model proposed by Parker and Lenhard (1987) was adopted to fit the data, while inverse modelling of the forced evaporation tests allowed to derive the model parameters. Therefore, the main drying and wetting branches and the soil response to drying and wetting cycles from any reversal point were reproduced with the model, which suitably described the hysteretic behaviour of the pyroclastic soil under all conditions and along all paths.