Expanded polystyrene (EPS) geofoam is a rigid cellular plastic foamed polymeric geosyn-thetic material that has been used in a wide range of geotechnical engineering applications. Due to very low density compared to other conventional fill materials, it can be used as a lightweight fill like higher and steeper embankments, highways, airfields, railroads and so forth. Meanwhile, the behavior of EPS geofoam is so complex and unavailability of all its physical and mechanical properties from lower to higher densities is also another problem. Thereby, the investigation of the shear strength behaviors of EPS geofoam are useful parameters for researchers who are involved in the design and application of EPS geofoam products. In this study, the shear strength behaviors of EPS geofoam using modified direct shear tests at dry and wet conditions were examined thoroughly. The effect of normal stress, density and water submergence on the EPS geofoam shear strength behavior were focused. The tests have been performed on EPS geofoam specimens of three different densities, 12, 15 and 20 kg/m3. The stress-strain response of EPS geofoam under uniaxial unconfined compression loading is found to be nonlinear and a function of specimen size, strain rate, and density. The direct shear test results indicated that the cohesion and angle of internal friction of EPS geofoam increase with an increase in density. However, the cohesion is found to be the major parameter which contributes the shear strength of EPS geofoam. The shear stress and normal stress relationship in direct shear failure envelopes found to be linear. The submergence deceased the internal friction angle of EPS geofoam compared to the dry condition for the same density and under same applied normal stress levels, whereas, the cohesion was slightly increased.
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