Experimental and numerical investigation of the behavior of sandy soils with plastic fines under monotonic and cyclic loading

• Applicant: Professor Dr.-Ing. Torsten Wichtmann
• Subject Area: Geotechnics, Hydraulic Engineering
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 550355568

Soil liquefaction is a phenomenon in which water-saturated soil experiences a major loss of effective stress and thus strength and stiffness due to the application of rapid loading, such as earthquake shaking. This often leads to catastrophic consequences, including the loss of lives and civil infrastructure. Traditionally, liquefaction has been primarily associated with saturated loosely packed clean sands. However, a literature review reveals that the majority of historical cases involving catastrophic events triggered by liquefaction have occurred in deposits composed of sands with fines, whether natural or human-made. While the liquefaction of silty sands, i.e. sands containing non-plastic fines, has been extensively investigated during the last decades, less attention has been paid to sands possessing plastic fines. A practical example for the relevance of the liquefaction resistance of sands with plastic fines is related to the recultivation of abandoned opencast mines as lakes, where such mixed soils resulting from the dumping process form the slopes and may liquefy during seismic loading. Another example are the foundations of offshore wind turbines in earthquake-prone regions, considering that marine sands commonly contain a certain amount of fines of either non-plastic or plastic nature. Realistic and reliable predictions of the consequences of seismic loading for geotechnical structures in clayey sands demand an in-depth understanding of the behaviour of such soils under undrained cyclic loading, including the pore water pressure accumulation rates, effective stress paths, stress-strain response and evolution of stiffness and damping. For more complicated boundary value problems like those mentioned above the application of numerical methods is indispensable, necessitating sophisticated constitutive models for the soil. The proposed project intents to thoroughly investigate the behaviour of clayey sand with various fines content under different kinds of loading, to develop a constitutive model adequately describing the experimental observations and to validate this model by the back-analysis of element and model tests. The laboratory program includes index tests, oedometric compression tests, drained and undrained monotonic triaxial tests, undrained cyclic triaxial tests and resonant column tests. A hypoplastic model for sand recently developed in the framework of a collaboration of the groups in Prague and Bochum will be used as the basis for the constitutive modeling, extending it by the equivalent void ratio concept and clay-specific features. Centrifuge model tests on water-saturated slopes under earthquake loading performed in the geotechnical centrifuge in Nantes will be used for validation by means of finite element simulations.

• DFG Programme: Research Grants
• International Connection: Czech Republic
• Cooperation Partner: Professor David Masin, Ph.D.