P Brabhaharan, Dr G Fairless, reviewed by H E Chapman, Opus International Consultants Ltd (EQC funded project 99/419)
Research was carried out to assess the displacement performance of retaining walls, and the significance of vertical earthquake shaking, which can occur in near fault areas. The research included a literature review and numerical analyses using a numerical model of a Reinforced Earth wall.
A literature review indicated that vertical earthquake shaking has not been seriously considered until recently. Recent papers highlight the importance of vertical shaking to the assessment and design of retaining walls. No model studies have been carried out to verify the effects of vertical shaking or to assess parameters that might be important.
The study showed that a wide range of displacements can occur in different earthquakes, even when they have similar peak ground accelerations. The energy content was found to be a more influential parameter to assess wall displacements in earthquakes.
The vertical shaking had a significant effect of the displacement of the wall, when they energy content of the earthquake was significant, or where the frequency of the earthquake shaking was such that it was similar to the natural response frequency of the wall. This indicates that the frequency content of the earthquake and resonance effects can be important. The magnitude of the displacements depend on both the energy content of horizontal and vertical shaking are not significantly large (say distant from earthquake sources), then the vertical accelerations are shown to have little effect.
The results suggest that there is a possibility that vertical shaking may increase the flexibility of the retaining structure, and modify its natural period. Where this shifts the period of a structure to a frequency with significant energy content in an earthquake, this can lead to resonance effects and hence greater displacements.
Currently design is based on pseudo-static methods using horizontal peak ground accelerations. The study shows the importance of the energy, frequency content and vertical shaking of earthquakes. This is important for design of retaining systems supporting other structures. Further research is recommended to assess the performance of different wall systems under earthquakes with different characteristics, and to develop appropriate design parameters and methods where vertical shaking is likely to be a major component.