It is almost impossible to do calculations by the effective stresses for unsaturated soils as excess pore pressure remains unknown. Purpose of this research is to verify experimentally the basic statement of the N.N. Maslov’s theory of density-humidity and Fredlund’s theory of absorption for determining strength properties of binder soils and taking into account calculation of slope stability. Tests of soils taken from the collapsed slope of the Hua-Na Hydropower Station (Vietnam) were conducted in stabilometer in consolidated-undrained state. The changes in strength parameters of the unsaturated soils depending on the value of absorption and therefore volume humidity are examined. The stress paths were plotted for these samples; it can be used to determine the stress strain behavior of the soil, found knowing the initial absorption and stress.