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

Considering the fact that most geosynthetic-reinforced soil structures (GRS) are backfilled with well-compacted granular soils which exhibit evident strain-softening, it is expected that the mobilized soil strength after 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 subject to sinusoidal excitation ranging from 0.1g to 0.6g. From the study, it is found that the permanent displacement of GRS walls was attributed to compaction of backfill and retained earth, smeared shear deformation over the wall, shear deformation along slip surfaces, and free-field displacement in the retained earth if it is 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 lead to larger reinforcement load in strong earthquakes. It is therefore more rational to design GRS walls against strong seismic loading using the residual soil strength if the slip surfaces are based on the peak soil strength and the motion amplification/de-amplification is properly taken into account.