Keywords: waste landfills, seismic stability, base sliding, liners, permanent deformations.

Waste landfills are geostructures characterised by special features, which introduce numerous uncertainties in the design procedure. These uncertainties are related with the stiffness of the waste material (which depends on numerous factors such as the waste compaction, composition and age), with the geometry of the landfill and the characteristics of the base (and side, depending on the geometry) liner system. The base liners are placed at landfills' foundation prior to waste disposal in order to successfully isolate waste material and leachate from the environment. Thus, a low shear strength interface is introduced in the system, providing the potential of development of relative displacements during dynamic loading due to ground shaking.

In this study, the complicated mechanism of interaction between base sliding and the response of a typical above-ground landfill, is investigated by taking also into account the effect of the two-dimensional geometry. For this purpose, a series of finite element analyses have been performed and the effects of parameters such as the waste material properties, the characteristics of the excitation and the shear strength of the interface have been examined and the relevant effect of each of these parameters on system's behaviour is described. The examined cases were also analysed using the model proposed by Westermo and Udwadia [1], aiming to obtain a comparison between the simplified single-degree-of-freedom (SDOF) and the two-dimensional finite element modelling. Finally, the capability of the simplified approach to accurately estimate the dynamic interaction of slip displacement development and dynamic response of the structure is investigated.

An important conclusion that can be drawn is that the response of the examined sliding geostructure is affected, not only by the characteristics of the structure and of the excitation, but of the developed displacements as well. Accordingly, as these phenomena appear to be interrelated, the level of the accumulated displacements is also associated with the response characteristics of the embankment. Furthermore, the potential of large shear strains development within the soil or waste mass (due to high levels of acceleration) cannot be easily avoided. Moreover, the contributing role of the "tuning ratio" (ratio of excitation frequency to eigenfrequency of the structure), not only to the frequency content of the resulting response but to the seismic permanent displacements as well is highlighted. In conclusion, the simple sliding SDOF models seem to be incapable of capturing either the accumulated slip displacements, or the resulting response of geostructure, since they cannot simulate non-symmetric sliding behaviour.

1

B. Westermo, F. Udwadia, "Periodic response of a sliding oscillator system to harmonic excitation", Earthquake Engineering and Structural Dynamics, 11, 135-146, 1983. doi:10.1002/eqe.4290110111

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