Reformulation and Extension of an Elastoplastic Constitutive Model for Sandy Soils accounting for Cycling Loading and Large Deformations

当代岩土工程问题的精确和有效的数值模拟需要精细的本构模型，其中包括有限应变理论以及复杂的循环加载机制的土壤。为此，过去已经开发了许多模型。特别是对于砂，现有的模型都不能被认为是一个详尽的解决方案。其中最具潜力的模型之一是CSSA模型（临界状态沙模型）。本研究项目解决了其不足之处。本研究项目旨在将关于任意曲线坐标系的有限应变理论纳入模型。此外，该模型的加载机制的平均有效应力的变化（帽机制）是增强占循环加载。进一步发展的结果是ECSSA模型（扩展CSSA模型）。在已经完成的项目第一阶段，ECSSA模型已经在数学和机械上制定。此外，其实施的背景下，大变形问题的数值方法已经制定。此外，还为验证和测试ECSSA模型做好了准备。在本申请所考虑的项目第二阶段，对再生砂样品进行了实验室测试。通过一系列的排水循环三轴压缩试验，研究了砂土在等应力比循环压缩下的特性。这对于完成ECSSA模型上限机制的增强是必要的。此外，进行了单调和循环三轴试验，以确定ECSSA模型的模型常数。由于ECSSA模型是从有限变形理论发展而来的，因此，实验室试验的进行和评估与该理论一致。在实验工作的基础上，ECSSA模型循环帽机制的开发完成。确定了试验砂的模型常数，并利用室内试验结果对模型进行了验证。在研究项目的最后一步，ECSSA模型进一步验证和测试，将其应用于边界值问题，功能循环加载和大的土壤挠度。对于这里要研究的问题，存在来自模型测试或其他模拟方法的参考解决方案。

Precise and efficient numerical simulation of contemporary geotechnical problems requires elaborate constitutive models for the soil which incorporate the finite strain theory as well as sophisticated mechanisms for cyclic loading. To this end, a lot of models have been developed in the past. Particularly for sands none of the existing models can be considered an exhaustive solution. One of the models which offer the greatest potential is the CSSA-Model (Critical State Sand Model). Its shortcomings are resolved by this research project. The research project aims at incorporating the finite strain theory with regard to arbitrary curvilinear coordinate systems into the Model. Furthermore, the model's loading mechanism for changes of the mean effective stress (cap mechanism) is enhanced to account for cyclic loading. The outcome of the further development is the ECSSA-Model (Extended CSSA-Model). During the already finished first phase of the project the ECSSA-Models has been formulated mathematically and mechanically. Moreover, its implementation in the context of numerical methods for large deformation problems has been worked out. In addition, preparations for the validation and testing of the ECSSA-Model have been done.During the second phase of the project, which is to be considered in the present application, laboratory tests on reconstituted sand samples are conducted. By a comprehensive series of drained cyclic triaxial compression tests the behavior of sand under cyclic compression at constant stress ratio is investigated. This is necessary to finish the enhancement of the cap mechanism of the ECSSA-Model. Furthermore, monotonic and cyclic triaxial tests are conducted to determine the model constants of the test sand for the ECSSA-Model. Since the ECSSA-Model evolves out of the finite deformation theory, the laboratory tests are carried out and evaluated in a manner consistent with that theory.On the basis of the experimental work the development of the ECSSA-Model's cyclic cap mechanism is finalized. Subsequently the model constants of the test sand are determined and the model is validated by means of laboratory test results. In the last step of the research project, the ECSSA-Model is further validated and tested by applying it to boundary value problems that feature cyclic loading and large soil deflections. For the problems that are to be investigated here reference solutions from model tests or other simulation approaches exist.