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Mainly, two divisions are encountered in the literature regarding constitutive models: elasto-plasticity and hypoplasticity. In addition, barodesy and the framework of generalized plasticity are used. The main disadvantage of nearly all models is the failure to reproduce the mechanical behaviour under cyclic loading without restrictions. A model which deals neither with the true hypoplastic nor the true elasto-plastic approach will be proposed herein. Moreover, it combines and establishes the unification of both frameworks and it can be assigned to both of them. It introduces a so called historiotropic surface, responsible for the memory of the soil, but no consistency condition. It also introduces a hypoplastic strain rate, but incorporating the distance to the new surface for its intensity and the back stress tensor. Hence, both the intergranular strain as well as the fabricdilatancy tensor are omitted, even though the eight-shaped stress hysteresis at cyclic mobility is well reproduced.

In the past decades, the necessity for detailed earthquake microzonation studies was recognized worldwide. Therefore, different approaches were established and applied. Unfortunately, the majority of these approaches are not based on pre-existing field data but require extensive seismic measurements and investigations. Furthermore, these approaches incorporate non-linearity inadequately and cannot take groundwater level changes into account. For this purpose, notably numerical models are most suitable. These models require a good knowledge of the local geological conditions (especially of the uppermost unconsolidated units), information about the geotechnical parameters of these units, and a hydrogeological model of the investigated area. Most of this information can be obtained from geotechnical investigations and surveys that have already been carried out in most densely populated areas. In a case study for Bucharest City, non-linear analyses were performed using software that is based on the visco-hypoplastic constitutive law. The results indicate that groundwater level changes have an important influence on duration and amplitude of ground response and thus should be considered for seismic microzonation studies. This approach can be used to display site effects and to identify different microzones taking different earthquake magnitudes and groundwater levels into account. KEY WORDS: microzonation, site effect, visco-hypoplasticity, non-linearity,

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