The use of geosynthetics is one of the most important advances in recent decades. Among the different families of geosynthetics, the high-density expanded polystyrene foams (EPS) are gaining diffusion in the field of geotechnics in several applications like: reducing the weight of embankments, reducing stresse on retaining walls, insulation of structures on expansive soils, as dampers for seismic loads, etc. taking advantage of its low unit weight and high compressibility. This paper presents the results of an experimental program based on several tests performed on high density EPS samples in a novel cubical flexible boundary true triaxial device. The results show that this type of material has a complex mechanical behavior on deformation and shear strength parameters, which is important to be considered when the design of geotechnical works.