In the case of large unstable mountain slopes, the spatial repartition of the saturated zones influence the pressure repartitions in the massif and the slope stability. This contribution proposes an evaluation of water origin and pressure transit time of groundwater flows from hydrochemical and isotopic measurements. The objective is to propose a conceptual model of the groundwater flows and a location of the saturated zones in the slope. The interpretation of time measurements of water pressure in the collapsed zone highlights (i) an evolution of flows with deformation that induces a damaging of the rock properties and (ii) the necessity to connect hydrogeological conditions and geological background in a dynamical system to analyse the pore pressure in the fractured collapsed zone