The laboratory measurement of soil stiffness in the range of small strains is a very difficult task, due to the insufficient resolution and unsatisfactory precision of laboratory equipment for measurements of loads and displacements. There are opportunities for regular analysis of small-strain soil stiffness in the triaxial apparatus with local transducers. Unfortunately, this methods is typically used only in the research projects because of the high costs. However, using some additional equipment for triaxial apparatus: bender elements facilitated tests with propagation of the shear wave, and consequently calculation of the initial shear modulus. This technique became a simple and a cost-effective one, commonly performed in the geotechnical laboratories around the world. In the requirements of Eurocode 7 there is no comprehensive, detailed description of this research method, but some specialized studies of soil stiffness have been indicated here. For example, techniques which use the seismic wave velocity measurement, particularly for small strains, i.e. less than 0.1%. Therefore, the clear explanation of bender element tests ideas. was considered to be appropriate. This article presents a summary of research methodology in the triaxial apparatus equipped with bender elements, using for the measurements of the shear wave velocity. Examples of experimental results carried out on selected cohesive soils derived from Warsaw area are showed here as well. Moreover, equations of the shear wave velocity and the initial shear modulus as a function of the mean effective stress are proposed.

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