It is well known that soils are made by a solid matrix and by voids, usually interconnected, filled by water and/or air. Starting from this intrinsic constitution, springs out a great difficulty in the schematization of the behaviour of these complex materials, particularly when voids are very small and, teherefore, the permeability values are very low. In addition to these problems, well known from the beginning of geothechincs, there are the ones derived from the highlighting of the intrinsic variability of soils. The possibility to have numerous and detailed data allowed to point out these variability in stochastic models based on the spatial knowledge of expressed average tendencies and of the variances. This knowledge allows to apply to the soils concepts of security based on collapse probability or, likewise, evaluating the reliability.