This project is focused on large deformation behavior of soil – structure interfaces under different boundary conditions. The interface behavior is a basic field of soil mechanics that is crucial for the appropriate description of the phenomena in the contact between granular bodies and structure. The interface tests are usually carried out under boundary conditions related to constant normal stiffness (CNS). In this project, the novel boundary condition, called constant dilatancy (CDIL), is introduced. Application of constant dilatancy condition produces a significant difference in vertical strain - shear strain behavior in comparison to the standard CNS tests. Constant dilatancy condition can allow for more detailed insight into the macroscopic interface behavior. Currently, to the best of PI knowledge, no research regarding constant dilatancy boundary conditions was made. From soil mechanics point of view, the newly presented boundary condition allows to go beyond currently using practices of interface testing and theoretical description. For instance, it is possible to obtain more crucial stress paths (higher increase of shear and normal stress during shearing) than in CV test in drained conditions. The stress-displacement behavior in constant dilatancy criterion will be under investigation. The importance of the project is also related to the influence of stress history (OCR) on the interface behavior. The effect of grain crushing within the interface, especially for highly overconsolidated sand samples, will be also investigated. Finally, constant dilatancy condition and testing procedure will allow for more detailed insight into the soil-steel behavior. Presented project extends current knowledge of soil-structure interface behavior and facilitates understanding of basic interface behavior. Findings and outcomes of this project facilitates land and off-shore pile design, which is important aspects for Northern Poland and energy industry.