Stiffness Degradation and Yielding of EPS Geofoam under Cyclic Loading

This paper summarizes major findings from a laboratory study based on stress-controlled cyclic uniaxial compression tests with initial deviator stress conducted on expanded polystyrene (EPS) geofoam. The experiments involved nonelasticized EPS cylinders of various densities subjected to various static deviator stresses and cyclic loading conditions to provide a detailed description of the geofoam cyclic stress-strain response in the viscoelastic and visco-elasto-plastic domains. Experimental results characterizing the viscoelastic behavior were compiled into a normalized Young’s modulus and damping ratio versus cyclic axial strain amplitude relationships. A unified formulation that can be used to determine the normalized modulus degradation curve for a given static deviator stress level and geofoam density (within $15–25\mathrm{kg}/{\mathrm{m}}^3$ ) was developed. Examination of the yielding behavior of EPS geofoam under cyclic loading revealed a nonlinear increase in the accumulated plastic axial strain with the increasing number of applied loading cycles. Additionally, larger cyclic deviator stress amplitudes and lower loading frequencies resulted in larger plastic axial strains accumulated under the same number of cycles.