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Retaining Wall (2010-2011)

Seismic Isolation of Earth Retaining Structures Using EPS Geofoam – Centrifuge Testing

Principal Investigator: Adda Athanasopoulos-Zekkos (University of Michigan, Ann Arbor)

PROJECT SUMMARY

Earth retaining structures constitute an important component of many civil engineering works. In earthquake prone areas an earth retaining structure must be designed to be able to withstand the seismic earth pressures in addition to the static ones. It is known that in the case of strong ground motion the combined (i.e. static plus dynamic) earth pressures may be more than two times higher, compared to the static pressures. The appropriate design against the increased lateral -static plus dynamic- loading results in a significant increase in the construction cost. For this reason a method for the seismic earth pressure reduction (or isolation) would be particularly welcome by the civil engineering profession and construction industry (for both new and existing structures).

The subject of the proposed study is the extension of previous investigation on the effectiveness of an EPS geofoam compressible inclusion as a seismic isolator against lateral earthquake earth pressures. In this method a layer of EPS geofoam (playing the role of a compressible inclusion) is placed between the back face of the wall and the backfill material. During earthquake loading the backfill seismic pressures are first applied to the EPS layer. This layer acts as a buffer (due to its greater compressibility) absorbing the major part of the thrust and transferring only a portion of it to the retaining structure. The effectiveness of the above method of seismic isolation of retaining structures has been the subject of investigation at the Laboratory of Geotechnical Engineering of the Department of Civil Engineering of the University of Patras, during the last ten years. This research has focused on numerical analyses of the response of retaining structures seismically isolated using EPS geofoam compressible inclusions. The results of numerical analyses indicate that it is possible to achieve a particularly effective seismic isolation of earth retaining structures, provided the density and the thickness of compressible inclusion is appropriately selected.

The objective of the proposed research is to combine results from centrifuge tests with those of numerical analyses to develop a design methodology for seismic isolation of earth retaining structures. To achieve this objective the two proposed research tasks include:

  1. Performing two centrifuge tests on small scale physical models of retaining structures with and without being seismically isolated by EPS compressible inclusions under dynamic loading and comparing the results in terms of the earth pressures measured behind the retaining structures
  2. Performing extensive parametric numerical analyses using the Finite Element and Finite Difference method, will be conducted to study the effect of important parameters concerning
    1. The compressible inclusion (shape, thickness, density)
    2. The retaining structure (type, height, flexibility, permissible displacement, backfill slope)
    3. The base excitation (harmonic loading of variable frequency and intensity, recorded accelerograms)
    4. Type of analysis (equivalent linear, non-linear inelastic). A number of numerical analyses will also be contacted for modeling published shaking table tests, for additional validation of the method of analysis used in the present investigation
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