On the behaviour of flexible retaining walls under seismic actions

Abstract

This paper describes an experimental investigation of the behaviour of embedded retaining walls under seismic actions. Nine centrifuge tests were carried out on reduced-scale models of pairs of retaining walls in dry sand, either cantilevered or with one level of props near the top. The experimental data indicate that, for maximum accelerations that are smaller than the critical limit equilibrium value, the retaining walls experience significant permanent displacements under increasing structural loads, whereas for larger accelerations the walls rotate under constant internal forces. The critical acceleration at which the walls start to rotate increases with increasing maximum acceleration. No significant displacements are measured if the current earthquake is less severe than earthquakes previously experienced by the wall. The increase of critical acceleration is explained in terms of redistribution of earth pressures and progressive mobilisation of the passive strength in front of the wall. The experimental data for cantilevered retaining walls indicate that the permanent displacements of the wall can be reasonably predicted adopting a Newmark-type calculation with a critical acceleration that is a fraction of the limit equilibrium value.

Some remarks on the seismic behaviour of embedded cantilevered retaining walls

“…As expected, the displacements of the walls increase as the intensity of the applied earthquake increases. However, as shown by Conti et al (2012) on the basis of centrifuge tests, the walls can accumulate significant displacements (u/H . 1-2%), even for accelerations lower than the critical one, and hence before the available soil passive resistance is fully mobilised in front of the wall.…”

“…These displacements cannot be computed by a conventional Newmark (1965) analysis, that is, assuming a yield acceleration equal to the critical value provided by the limit equilibrium analysis. Following the procedure adopted by Conti et al (2012) for the interpretation of centrifuge dynamic tests, a Newmark calculation was carried out for each analysis, in which the yield acceleration, a c N , was found by trial and error to match computed and numerical displacements at the end of each earthquake. As an example, Fig.…”

“…The yield acceleration, a cN , required to match the computed final displacement is only a fraction of the critical acceleration a c that corresponds to the complete mobilisation of the soil passive strength; in other words, had the Newmark analysis been carried out using a c , the displacements of the wall would have been zero. Figure 11 shows the computed values of a cN as a function of the maximum acceleration a max , both normalised by the limit equilibrium value of the critical acceleration, a c : Data from centrifuge tests (Conti et al, 2012) have been also included in the same figure for comparison. Both numerical and experimental data indicate that a c N /a c increases with the maximum acceleration applied, up to about a max /a c ¼ 1 .…”

“…Results from both numerical (Callisto & Soccodato, 2007 and experimental Zeng & Steedman, 1993;Conti et al, 2012) work on the dynamic behaviour of embedded retaining walls, however, have shown that the Newmark approach does not describe the observed behaviour satisfactorily. In fact, if the critical acceleration of the system is computed with standard limit equilibrium methods, such as those adopted in European countries and the USA (Blum, 1931;Padfield & Mair, 1984;King, 1995;Powrie, 1996;Day, 1999;Osman & Bolton, 2004), seeking the pseudo-static coefficient corresponding to which limit conditions are attained in the system, two findings are of major concern.…”

“…On the basis of centrifuge dynamic tests carried out on pairs of embedded propped and cantilevered walls in dry sand, Conti et al (2012) have shown that a Newmark analysis carried out using the limit equilibrium value of the critical acceleration would yield displacements that are much smaller than observed, as the analysis would overlook the displacements experienced by the wall before the acceleration reaches the limit equilibrium critical value. According to the authors, the observed behaviour may be justified by a stress redistribution and a progressive mobilisation of the soil strength on the passive side of the wall produced by the earthquake.…”