Seismic Earth Pressures on Retaining Structures and Basement Walls in Cohesionless Soils

Observations of the performance of basement walls and retaining structures in recent earthquakes show that failures of basement or deep-excavation walls in earthquakes are rare even if the structures were not designed for the actual magnitude of the earthquake loading. For instance, no significant damage or failures of retaining structures occurred in the recent Wenchuan earthquake in China (2008) or in the subduction earthquakes in Chile (2010) and Japan (2011). To develop a better understanding of the distribution and magnitude of the seismic earth pressures on cantilever retaining structures, a series of centrifuge experiments were performed on model retaining and basement structures with medium dense cohesionless backfill. This paper provides a general overview of the research program and its results. Two sets of centrifuge-scale experiments were carried out on the centrifuge at the Center for Geotechnical Modeling at UC Davis. Three different types of prototype retaining structure were modeled in this research effort as follows: (1) a nondisplacing cross-braced (basement) structure with a stem stiffness of $5.92\times {10}^{10}\mathrm{lb}\text{-}{\mathrm{in}\mathrm{.}}^2$ per ft width ($5.57\times {10}^{05}\text{\hspace{0.17em}kN-}{\mathrm{m}}^2$ per m width) and $1.04\times {10}^{10}\mathrm{lb}\text{-}{\mathrm{in}\mathrm{.}}^2$ per ft width ($9.79\times {10}^{04}\text{\hspace{0.17em}kN-}{\mathrm{m}}^2$ per m width); (2) a nondisplacing U-shaped cantilever structure with a stem stiffness of 5.92 and $1.04\times {10}^{10}\mathrm{lb}\text{-}{\mathrm{in}\mathrm{.}}^2$ per ft width ($9.79\times {10}^{04}\text{\hspace{0.17em}kN-}{\mathrm{m}}^2$ per m width); and (3) a free standing, cantilever retaining wall with a stem stiffness of $2.4\times {10}^{10}\mathrm{lb}\text{-}{\mathrm{in}\mathrm{.}}^2$ per ft width ($2.26\times {10}^{05}\text{\hspace{0.17em}kN-}{\mathrm{m}}^2$ per m width). Overall, for the structures examined [i.e., wall heights in the range 6.1–9.15 m (20–30 ft)], the centrifuge data consistently show that the maximum dynamic earth pressure increases with depth and can be reasonably approximated by a triangular distribution. This suggests that the result of the dynamic earth pressure increment acts near 0.33H above the footing as opposed to 0.5–0.6 H recommended by most current design procedures. The current data also suggest that cantilever walls can resist ground accelerations up to 0.4 g if designed with an adequate static factor of safety.