Seismic Responses of Reinforced Soil Retaining Walls and the Strain Softening of Backfill Soils

Considering that most geosynthetic-reinforced soil structures (GRS) are backfilled with well-compacted granular soils exhibiting evident strain-softening, it is expected that the mobilized soil strength beyond the peak would affect the response of GRS structures if the earthquake-induced deformation is large. This issue is particularly relevant if GRS structures are to be designed against seismic loading based on permanent displacement. In this study, a calibrated finite-element procedure was used to investigate the influences of strain softening of backfill soils on the deformation and reinforcement load of wrapped-face GRS walls. Dense Toyoura sand, a medium dense Japanese silty sand, and loose Fuji River sand were used as backfills of model GRS walls, which were subjected to sinusoidal excitation ranging from 0.1 to $0.6g$. From the study, it is found that the permanent deformation of GRS walls was attributed to the compaction of backfill, smeared shear deformation of soil, shear deformation along slip surfaces, and free-field displacement in the retained earth if it was adequately deep. The slip surfaces were related only to the peak soil strength. The maximum reinforcement load was directly related to the strain softening of backfill soil. Soils with larger peak strength but smaller residual strength could result in larger reinforcement load in strong earthquakes. The results imply that using residual strength along the slip surface determined from peak strength may be more appropriate if GRS structures are to be designed against rare-event strong earthquake loading.