The paper presents the work carried out for the analysis of the hydro-mechanical behaviour of a volcanic ash in unsaturated conditions for the establishment of a landslide eary warning system (EWS) in a Costa Rican quarry. Rapid mass movements involving ash material occurred in the greater quarry area during heavy rainfall events. Laboratory testing and numerical analyses were combined to provide a better understanding of the triggering mechanism and to provide guidelines for the EWS. Along with conventional laboratory analyses, the experimental program involved tests carried out under suction control to assess the volumetric response in unsaturated conditions of the material and to highlight potential collapse-upon wetting phenomena. The behaviour of the material was analysed in a modern framework for soils in unsaturated conditions developed at the EPFL. Numerical simulations were performed with a coupled hydro-mechanical finite elements code in which the mentioned constitutive model is implemented. The analyses allowed a better understanding of the landslide triggering mechanism and provided pore water pressure values which are associated to the failure mechanism and can be used as threshold values for the EWS.