Increasingly complex design systems require an individual approach when determining the necessary design parameters. As soils are characterized by strong strain-dependent nonlinearity, test methods used to characterize the subsoil should be carefully selected, in terms of their "sensitivity" as well as suitability for the analyzed type of problem. When direct measurements are not available, while design calculation models require specific parameters, indirect parameter estimation may be used. This approach requires calibration and validation of empirical correlations, based on well documented database of tests and case studies. One of the parameters often used, when analyzing soil-structure interaction problems, is the shear stiffness of the soil and its strain-dependent degradation. The aim of the article is to present the procedure for description and evaluation of soil stiffness based on field tests (CPTU, DMT and SDMT) and a large number of reference curves obtained from laboratory tests (TRX) for selected soil types. On the basis of the given algorithm, it is possible to obtain a stiffness module G0 value at any level of deformation, based on in-situ tests.

The non-linearity of the modulus in the zone of small deformations has become one of the three basic concepts of modern soil mechanics, together with “effective stresses” or “critical state”. It is therefore necessary to obtain suitable parameters to describe these phenomena through the development of modern measuring equipment and newresearch methods. Limitations in the availability of the research area or research equipment indicate the need to create a data set, in the formula of regional assessments. The article presents a compilation of data on the deformation characteristics of soils covering about 75% of the country’s area, which are the most common subsoils for building. Descriptions, images of microstructures, and a record of mechanical parameters are presented for various age-old glacial clays and marginal clays and loesses. Emphasis is placed on parameters obtained from triaxial tests, including the determination of the shear modulus at small deformations obtained from BET measurements. In combination with the patented solution of sample strain measurement, complete deformability curves of the tested samples were obtained, indicating model reference curves developed for the above soil types. The statistically significant amount of data collected allowed the creation of a specific portfolio for selected soils as a starting point for assessing deformability. This corresponds to the current expectations regarding the characteristics of the behaviour of the substrate in the full spectrum of stresses and deformations, obtained from different types of tests, which, as in the case of soil stiffness degradation, together allow the correct determination of the necessary parameters.