GEODECO has developed a probabilistic procedure for an efficient seismic hazard evaluation at soil sites with uncertain properties. This methodology couples PSHA with multiple non-linear dynamic analyses of the soil deposit subject to real ground-motion records. In order to overcome the limitations of conventional linear-equivalent soil amplification models GEODECO adopts the most advanced non-linear modelling capabilities in order to predict realistically the site response. Our methodology accounts for soil non linearity and the diffusion phenomena that take place in the saturated porous medium. The constitutive relationships used to model the soil behaviour are based on flow plasticity, and their parameters are calibrated by fitting the available experimental data.