Viscous ISA Modell for clays under cyclic and monotonic loading

软粘土的应力-应变特性受其黏度的支配。黏性是导致软粘土随时间变化的主要原因，在岩土工程问题中，黏性对评价基础结构的长期性能或土-结构相互作用具有重要意义。蠕变、松弛和速率依赖等现象是导致建在软粘土或滑坡上的结构随后发生倾斜或沉降的主要原因。考虑到时变现象的重要后果，过去已经发展了一些粘性模型。粘-欠塑性模型假定粘应变率和塑性应变率相等，因此模型中只考虑一个应变率。因此，它会导致对剪应力峰值的过高估计。这些模型最大的缺点是缺乏对塑性应变累积和/或孔隙水压力累积的模拟，这对循环荷载作用下土体的行为具有致命的影响。修正的Cam Clay模型也被用来解释称为UH模型的粘性现象。由于本构模型在卸荷后表现出弹性行为，该模型不能描述卸荷后的松弛或蠕变。因此，该项目的主要任务将是在ISA平台内开发一个适合粘土的模型，该模型应该扩展到考虑时间相关现象，同时又不失去将时间无关行为描述为同一模型特征的可能性。此外，所提出的模型应能很好地描述粘性粘性土在不同应力/应变幅值下的循环特性。为此，将引入一个新的状态变量，用于计算土壤所经历的循环次数。因此，土壤的历史因素将包括在本构模型中。最后，扩展模型将在高岭土和下莱茵河粘土的实验室测试中进行不同加载速度的校准，这将提供关于粘土的一般时间依赖行为以及循环加载下的行为的信息。

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.