Recent changes in codes and improved understanding of strong ground motions have led to increased demands in the seismic design of retaining structures. Correspondingly, the dynamic loads computed using currently available design procedures significantly exceed loads used in the design of most of the
existing retaining structures, suggesting that they may have been significantly underdesigned. Field evidence from recent major earthquakes fails to show any significant problems with the performance of retaining structures designed for static earth pressures only. In view of this, an experimental and analytical study of seismic earth pressures on cantilever retaining structures was performed to address the apparent discrepancy between theory and actual performance.
Two sets of dynamic centrifuge model experiments were performed to evaluate the magnitude and distribution of seismically induced lateral earth pressures on retaining structures and to study the seismic response of retaining wall–backfill systems. Two U-shaped cantilever retaining structures, one flexible and one stiff, were used in each experiment to model prototype structures representative of retaining-wall designs currently under consideration by the Bay Area Rapid Transit (BART) and the Valley Transportation Authority (VTA). Dry medium-dense sand at 61% and 72% relative density was used as backfill. The results obtained from the centrifuge experiments were used to develop and calibrate a two-dimensional (2-D) nonlinear finite element (FE) model built on the OpenSees platform. The finite element model was used to further study the seismic response of retaining wall–backfill systems and to evaluate the ability of numerical modeling in capturing the essential features of the seismic response observed in the centrifuge experiments.
In general, the magnitude of the observe d seismic earth pressures depends on the magnitude and intensity of shaking, the density of the backfill soil, and the flexibility of the retaining walls. Specifically, the results of the centrifuge experiments show that the maximum dynamic earth pressures increase with depth and can be reasonably approximated by a triangular distribution analogous to that used to represent static earth pressures. Hence the current practice and assumption that the result ant of the dynamic earth pressu res acts at 0.6–0.7H is not consistent with the experimental results. A similar conclusion was reached by Nakamura (2006) based on centrifuge experiments on gravity walls. An important contribution to the overall dynamic moment acting on the wall is the mass of the wall itself. The data from the centrifuge experiments suggest that this contribution can be substantial. Moreover, the dynamic earth pressures and inertial forces do not act simultaneously. The experimental results show that when the inertial force is at its local maximum, the overall dynamic moment acting on the wall reaches its local maximum as well, while the dynamic earth pressure increment is at its local minimum or is around zero. This observation contradicts the Mononobe-Okabe hypothetical assumptions and suggests that designing retaining walls for maximum dynamic earth pressure increment and maximum wall inertia is overly conservative. The relationship between the seismic earth pressure increment coefficient ( ΔKAE) at the time of maximum overall wall moment and peak ground acceleration obtained from the centrifuge experiments suggests that seismic earth pressures can be neglected at accelerations below 0.4 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 be expected to resist seismic loads at ac celerations up to 0.5 g. The finite element model results using nonlinear soil model parameters are in good agreement with centrifuge results and are consistent with the observed trends. The results of finite element modeling with denser soil parameters showed that the seismic earth pressures decreased on the or der of 23–30%, suggesting that seismic earth pressures may not be a significant issue in good soil conditions. This aspect of the problem requires further experimental and analytical evaluation.
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