Seismic Earth Pressures on Retaining Structures in Cohesive Soils

Observations of the performance of many different types of retaining structures in recent earthquakes show that failures of retaining structures, including braced excavation supports and basement walls, are rare even if the structures were not designed for the actual intensity of the earthquake loading. Therefore, an experimental and analytical study was undertaken to develop a better understanding of the distribution and magnitude of seismic earth pressures on various types of retaining structures. This report is a second in the series and presents the results of centrifuge model experiments and numerical analyses of seismic response of retaining structures with cohesive backfill. The experimental results show that the static and seismic earth pressures increase linearly with depth and that the resultant acts at 0.35H-0.4H, as opposed to 0.5-0.6H assumed in current engineering practice. In general, the total seismic load can be expressed using Seed and Whitman’s (1970) notation as: Pae= Pa +ΔPae, where Pa is the static load and ΔPae is the dynamic load increment. In level ground, the dynamic load coefficient can be expressed as Kae=½γH2(0.68PGAff/g) for basement walls and ΔKae=½γH2(0.42PGAff/g) for cantilever walls. These results are consistent with similar experiments performed in cohesionless soils by Mikola & Sitar (2013). In sloping ground the seismic coefficient is ΔKae=½γH2(0.7PGAff/g), which is consistent with Okabe’s (1926) Coulomb wedge analysis of the problem. However, the numerical simulations and Okabe’s (1926) limit state theory suggest that the resultant acts between 0.37H-0.40H for typical values of cohesion. Overall, the results also show that typical retaining walls designed with a static factor of safety of 1.5 have enough strength capacity to resist ground accelerations up to 0.4g