Development of a high-cycle accumulation model for granular soils with crushable particles

• Applicants: Dr.-Ing. Meisam Goudarzy; Dr.-Ing. Merita Tafili; Professor Dr.-Ing. Torsten Wichtmann
• Subject Area: Geotechnics, Hydraulic Engineering
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 526158236

A high-cyclic loading, characterized by a large number of cycles and small strain amplitudes, can lead to an accumulation of permanent deformations in the soil that may endanger the serviceability of foundations. A highly topical example are the foundations of offshore wind turbines (OWT), where the high-cyclic loading results from wind and waves. The OWT tilting must be kept within small tolerances to ensure the operation requirements. For a prediction of the permanent deformations in the design phase, high-cycle accumulation (HCA) models are most promising because they are based on extensive laboratory investigations and enable studies of the whole soil-structure interaction for any type of foundation. In many regions of the world, OWTs have to be founded in calcareous marine sands, that means granular materials with potentially crushable particles. Particle breakage leads to an irreversible change of the particle size distribution and thus affects the mechanical behaviour of the granular material. Grain crushing occurs during both the installation of the OWT foundation and the subsequent high-cyclic loading in the operation phase, and can significantly affect the OWT long-term response. However, due to a lack of respective investigations the influence of particle breakage on different aspects of the behaviour of crushable granular materials under high-cyclic loading is unclear so far. The objective of the project is the development, validation and exemplary application of a HCA model for granular soils with crushable particles. A detailed experimental investigation with high-cyclic loading on triaxial samples of a calcareous sand will be performed, varying several parameters as initial relative density, average stress, stress amplitude, loading frequency and drainage conditions (drained / undrained). Tests with a multidimensional cyclic loading and with a variation of amplitude, average stress and polarization between multiple packages of cycles are also included in the experimental program. The amount of particle breakage will be quantified by sieve analysis before and after a test and by grain shape analysis. Based on the experimental results, a HCA model for granular materials with crushable particles will be developed, considering a damage variable quantifying the amount of particle breakage. For the implicit phases of an FE calculation with the HCA model, two low-cycle constitutive models considering particle breakage will be further developed competitively, based on additional oedometer, triaxial and RC tests on the calcareous sand. The calibration and validation of the developed constitutive models based on the element tests is followed by an exemplary application in a FE parametric study considering a typical OWT monopile foundation under lateral cyclic loading. In a second phase pile installation effects will be investigated and the validation of the constitutive models will be extended to the back analysis of model tests.

• DFG Programme: Research Grants