Effects of vertical acceleration on seismic design of geosynthetic-reinforced soil structures

The stability and permanent displacement of geosynthetic-reinforced soil structures, under the influence of combined horizontal and vertical seismic accelerations, are investigated. The required length and tensile reinforcement of the geosynthetic are determined by considering different possible modes of failure, including tie-back/compound failure, direct sliding and pul-tout. The analysis is conducted by extending the methodology proposed earlier by the authors. While vertical acceleration in the downward direction results in an increased tensile reinforcement by the geosynthetic, acceleration in an upward direction requires a large geosynthetic length to resist direct sliding and tie-back failure. The vertical acceleration has a greater effect on the required tensile reinforcement of the geosynthetic in stabilizing a steep slope when compared to a flatter slope. When the horizontal seismic coefficient exceeds 0.2, the effect of vertical acceleration on the required geosynthetic length becomes significant, and is recommended to be included in design. The proposed methodology of design is verified with the performance of a geosynthetic-reinforced soil retaining wall during the Hanshin earthquake. Effects of seismic coefficients on permanent displacement are illustrated through several selected major earthquake records.