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

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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.