Two sets of dynamic centrifuge model experiments were performed to evaluate the magnitude and distribution of seismically induced lateral earth pressures on retaining structures that are representative of designs currently under consideration by the Bay Area Rapid Transit (BART) and the Valley Transportation Authority (VTA). Two U-shaped cantilever retaining structures, one flexible and one stiff, were used to model the prototype structures, and dry medium-dense sand at 61% and 72% relative density was used as backfill.
The results of the centrifuge experiments show that the maximum dynamic earth pressure increases with depth and can be reasonably approximated by a triangular distribution analogous to that used to represent static earth pressure. In general, the magnitude of the seismic earth pressure depends on the magnitude and intensity of shaking, the density of the backfill soil, and the flexibility of the retaining walls. The resulting relationship between the seismic earth pressure coefficient increment (!KAE) and PGA suggests that seismic earth pressures can be neglected at accelerations below 0.3 g. This is consistent with the observations and analyses performed by Clough and Fragaszy (1977) and Fragaszy and Clough (1980), who concluded that conventionally designed cantilever walls with granular backfill could reasonably be expected to resist seismic loads at accelerations up to 0.5 g.
Conventional seismic design procedures based on the Mononobe and Okabe work that are currently in use were found to provide conservative estimates of the seismic earth pressures and the resulting dynamic moments. Specifically, the BART design criterion for rigid walls appears amply conservative, especially if the normal factors of safety are taken into account. The BART design criterion for flexible walls appears to be somewhat unconservative for loose backfill. However, considering the various factors of safety present in the conventional design it may in fact contain an appropriate level of conservatism.
An important contribution to the overall moment acting on the wall is the mass of the wall itself. The data from the centrifuge experiments suggest that this contribution may be as much as 25%. Given that the conventional analyses methods tend to give adequately conservative results without the separate consideration of the wall inertial effects, the contribution of seismic earth pressures to the overall moment acting on the retaining structures is apparently routinely overestimated. Further analyses are needed to fully evaluate the impact of this observation on the overall design.
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