Viscous ISA Modell for clays under cyclic and monotonic loading

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The stress-strain behavior of soft clayey soils is governed by their viscosity. Viscosity is the main cause for the time-dependend behavior of soft clays, which is of great importance for the evaluation of long-term performance of foundation structures or soil-structure interactions in geotechnical engineering problems. Phenomena like creep, relaxation and rate dependence are the main causes for e.g. subsequent tilt or settlement of structures built on soft clayey soils or earth slides. Considering the important consequences of time-dependent phenomena, some viscous models have been developed in the past. The visco-hypoplastic model assumes the equivalence between the viscous strain rate and plastic strain rate so that only one is considered in the model. Hence it leads to an over-estimation of the peak of the shear stress. The most important drawback of these models is that they lack on simulating the accumulation of the plastic strain and/or the pore water pressure build up, which has fatal consequences for the soil behavior under cyclic loading.The Modified Cam Clay model has been as well adopted to account for viscous phenomena named as UH model. This model is not able to describe relaxation or creep after unloading, because the constitutive model shows elastic behavior after unloading.Therefore, the primary task of this project will be to develope a model within the ISA platform suitable for clays, which should be extended to account for time-dependent phenomena without loosing the possibility to describe time-independent behavior as a feature of the same model. Moreover the proposed model should describe well the cyclic behavior of viscous clayey soils for different stress/strain amplitudes. For this purpose, a new state variable accounting for the number of cycles the soil has been experienced will be introduced. Thus, the historical elements of the soil will be included in the constitutive model. Finally, the extended model will be calibrated for different loading speeds in laboratory tests conducted on Kaolin and Lower Rhine Clay, which will provide an information about the time-dependent behavior of clays in general as well as about their behavior under cyclic loading.