Flexural behaviour of clays: Experimental investigations, parametrisation and numerical simulation

• Applicant: Professor Dr.-Ing. Christos Vrettos
• Subject Area: Geotechnics, Hydraulic Engineering
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 437211075

Clay soils have the ability to absorb a certain amount of tensile stress. Typical applications are clay mineral barriers in landfills. The topic is being given a new relevance in connection with the novel and sustainable concept of the “landfill-on-landfill”, which can cover the need for landfill space in a land preserving way.The prediction of the stability of clay mineral barriers against crack development and their functionality with respect to sealing requires knowledge i) of the loading due to the imposed deformations as well as ii) of the plastification limits and the strength. Although insight gained from previous investigations allows a rough estimate of the resistance of clays to imposed bending actions, a widely accepted deformation verification method to assess the ductility reserves on the basis of standardized testing methods is missing.The aim of the research is to fill this gap. Experimental investigations, constitutive modelling and numerical methods are combined for this purpose. Soil mechanical tests on typical clays, in particular triaxial tests, constitute the basis for the material modelling by means of an own constitutive model for partially-saturated cohesive soils considering also temperature effects. They are supplemented by direct triaxial tensile tests to determine the tensile strength as well as by Resonant Column tests to determine the stiffness at low strain levels. The main part of the investigations constitutes beam bending tests on a modified test stand. Preliminary studies have already identified important aspects that should be considered. Besides established measurements techniques, the deformation behaviour will be captured by novel non-destructive testing and imaging methods, and subsequently analysed and reproduced by means of numerical simulations. Based on the results of a cross-material investigation, an elasto-plastic constitutive model will be enhanced by a multi-modulus material approach with a tension cut-off and nonlinear compressive and tensile deformation moduli will be derived on this basis. The tests will be carried out at temperatures of 20°C and 40°C. The existing laboratory devices are already equipped with this option and tested; for the beam bending tests, the new climate chamber installed at the TU Kaiserslautern will be used.The outcome will comprise a validated method for the experimental analysis, modelling and simulation of the deformation behaviour of clays under bending-tensile loading. Based on this, the influence of the stress state in clay may be assessed and transferred to the deformation verification of deep seated intermediate mineral clay seals. The research project builds on previous own research work on the mechanics of partially saturated cohesive soils and is intended to make a further contribution to this.

• DFG Programme: Research Grants