Expanded polystyrene (EPS) is widely used in works where there are soils with low support capacity, bridges and/or viaducts and road widening. This is due to its properties such as low specific mass, high strength and low compressibility to high EPS specific masses. Therefore, it is widely used in conjunction with soils. By analyzing the possibilities of rupture, it is observed that the greater probability of failure occurs at the interface between the materials. In this way, it is essential to analyse the direct shear parameters, especially the interface friction, in order to verify external and internal stabilities by the action of horizontal loads. Thus, this work evaluated the shear strength parameters of EPS in different specific masses (18, 28 and 33.5 kg/m³) with sandy and clayey soils interfaces measured at the optimum water content obtained by the compaction test (Standard Proctor). In the laboratory, specific mass tests for characterization and direct shear tests at EPS/Soil interfaces according to ASTM D3080 (shear box of 100×100 mm) were performed. Five normal stresses (11, 22, 33, 44 and 55 kPa) and speed of 0.2 mm/min were applied. Peak shear stresses were evaluated by two ways: peak stress as the changing stress from elastic to plastic (inflection of stress versus displacement curve) and peak as the maximum stress obtained in the test. The main results show an increasing trend of interface friction with the increase of the specific mass of the EPS.
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