In this study,the results of 1-g shaking table tests performed on small-scale flexible cantilever wall models retaining composite backfill made of a deformable geofoam inclusion and granular cohesionless material were presented.Two different polystyrene materials were utilized as deformable inclusions.Lateral dynamic earth pressures and wall displacements at different elevations of the retaining wall model were monitored during the tests.The earth pressures and displacements of the retaining walls with deformable inclusions were compared with those of the models without geofoam inclusions.Comparisons indicated that geofoam panels of low stiffness installed against the retaining wall model affect displacement and dynamic lateral pressure profile along the wall height.Depending on the inclusion characteristics and the wall flexibility,up to 50% reduction in dynamic earth pressures was observed.The efficiency of load and displacement reduction decreased as the flexibility ratio of the wall model increased.On the other hand,dynamic load reduction efficiency of the deformable inclusion increased as the amplitude and frequency ratio of the seismic excitation increased.Relative flexibility of the deformable layer(the thickness and the elastic stiffness of the polystyrene material) played an important role in the amount of load reduction.Dynamic earth pressure coefficients were compared with those calculated with an analytical approach.Pressure coefficients calculated with this method were found to be in good agreement with the results of the tests performed on the wall model having low flexibility ratio.It was observed that deformable inclusions reduce residual wall stresses observed at the end of seismic excitation thus contributing to the post-earthquake stability of the retaining wall.The graphs presented within this paper regarding the dynamic earth pressure coefficients versus the wall flexibility and inclusion characteristics may serve for the seismic design of full-scale retaining walls with deformable polystyrene inclusions. In this study, the results of 1-g shaking table tests performed on small-scale flexible cantilever wall models retaining composite backfill made of a deformable geofoam inclusion and granular cohesionless material were presented. Two different polystyrene materials were utilized as deformable inclusions. Lateral dynamic earth pressures and wall displacements at different elevations of the retaining the wall accounted for monitored the tests.The earth pressures and displacements of the retaining walls with deformable inclusions were compared with those of the models without geofoam inclusions. Comparisons indicated that geofoam panels of low stiffness installed against the retaining wall model affect displacement and dynamic lateral pressure profile along the wall height. Depending on the inclusion characteristics and the wall flexibility, up to 50% reduction in dynamic earth pressures was observed. efficiency. load and displacement reduction decreased as the flexibility ratio of the wal l model increased. On the other hand, dynamic load reduction efficiency of the deformable inclusion increased as the amplitude and frequency ratio of the seismic excitation increased. Relative freedom of the deformable layer (the thickness and the elastic stiffness of the polystyrene material) played an important role in the amount of load reduction. Dynamic earth pressure coefficients were compared with those calculated with an analytical approach. Pressure coefficients calculated with this method were found to be in good agreement with the results of the tests performed on the wall model having low flexibility ratio.It was observed that the deformable inclusions reduce residual wall consequences observed at the end of seismic excitation thus contributing to the post-earthquake stability of the retaining wall.The graph presented within this paper regarding the dynamic earth pressure coefficients versus the wall flexibility and inclusion characteristics may serve for the seismic design of full-scale retaining walls with deformable polystyrene inclusions.