Triaxial tests are common laboratory methods to study the mechanical properties of soils. According to international practice, it allows determining the reliable strength and stiffness properties. This research paper describes the results of statistical analysis of the deformation parameters for clay soils obtained from triaxial tests. The research focused on clay deposits of the Quaternary, Jurassic and Carboniferous periods of diverse genesis. The results of 992 consolidated isotropic drained triaxial tests of clay soils in Russia (Moscow) and Belarus (Minsk) were analysed. More than 50% of the tests were carried out under unloading/reloading conditions. As a result, empirical equations enabling evaluation of the effects of physical properties and stress state on stiffness of clay soils with different age and genesis were proposed. Comparison of accomplished tests of Quaternary and Jurassic soils from Thailand, Europe and the USA showed that stiffness for overconsolidated soils is in the same range as soils from Moscow and Minsk sites. The performed studies revealed the values of the Hardening soil model m-parameter depending on soil forming factors and its preconsolidation degree. In overconsolidated soils, values of the m-parameter are on average twice less than in normally consolidated or lightly overconsolidated soils. Proposed equations can be applied for preliminary estimation of the stiffness parameters for finite element method calculation, as well as used in geotechnical models that allow variability, horizontal and vertical distribution of stiffness to be taken into account. In general, geotechnical engineers may utilize the obtained results by applying them to design of complex soil models.