This paper presents experimental analysis and numerical modelling aimed at improved understanding and prediction of the hydro-mechanical behaviour of volcanic ash at various states of saturation. Results from a comprehensive experimental program are presented in order to characterize the response of the material in terms of matric suction and confining stress changes. The evolution of the yield stress at different suction levels has been quantified. The volumetric response with suction variations allowed the analysis of the collapse upon wetting behaviour. Water retention and permeability are also addressed. Tests results are used to calibrate a constitutive model based on the effective stress concept extended to partially saturated conditions. The ability of the model to reproduce and anticipate the ash behaviour is tested with an articulated stress path.