Stipendiatstilling 3 NGI (2021-2023)

Understanding how soil behaves is vital for building safe and reliable infrastructure, as well as predicting and reducing the consequences of geohazards such as landslides and earthquakes. This is not a trivial task. As the cartoonist and engineer Randall Munroe humorously noted when asked why some sands are softer than others: “We don’t know. No one understands how sand works… Understanding the flow of granular materials like sand is a major unsolved problem in physics.” While this may be a bit exaggerated, it highlights the fact that scientists have yet to develop a general model that reliably predicts the behaviour of sand. Sand grains exhibit a wide range of properties, including size, shape, and roughness, and they can organize themselves in various ways, all of which affect their behaviour. For instance, a cup of sand behaves as a solid when stationary, flows like a liquid when the cup is tilted, and the grains fall through the air like a gas when poured out. Despite the many research gaps that need to be addressed to develop a general theory for sand, this study is particularly motivated by the growing demand for renewable energy from offshore wind farms. The ground conditions for these developments pose significant uncertainties, requiring thorough site investigations and geotechnical laboratory testing of soils to determine parameters sizing and installation of foundations. Offshore structures are also subjected to complex cyclic loading conditions from wind and waves throughout their lifespan, requiring engineering models to predict their behaviour under these conditions. This project aims to understand how sand behaves at the microscale—how the grains move and interact—when subjected to similar loading conditions as offshore structures. This has become possible through experimental methods, such as performing tests inside a CT scanner, and numerical methods, using particle simulators that realistically represent grain shapes. In this project, we conducted loading tests on small sand samples measuring 10 by 20 mm, typically containing ten to twenty thousand sand grains. By taking CT images at different loading stages, we can track grain movements down to a resolution of a few thousandths of a millimetre. We also measure stresses and strains in the material, which allows us to correlate microscopic and macroscopic behaviour. Additionally, the CT images are used to create a “digital twin,” a particle simulation model of the experiment that accurately represents the shape and position of each grain in the sample. This technology provides a unique opportunity to investigate and model sand behaviour. The study shows that the simulation model closely replicates the behaviour observed in physical experiments, which opens the possibility of using simulations to generate data that currently requires significant resources. This is particularly relevant for the development of offshore wind farms, where simulations could supplement or replace some of the laboratory testing needed today. Another exciting application is in space research, where ground conditions for construction on the Moon or Mars need to be investigated, and physical testing is nearly impossible. The study also reveals that sand has a "cyclic memory" at the microscale, especially in the contacts between grains. This means that the structure of the sand is influenced by its load history, which can explain changes in the material's strength and stiffness. The findings suggest that parameters describing the contact network of the sand are essential for developing a general model for sand in the future. (1) New York Times (2020) What Makes Sand Soft?, article by Randall Munroe, Nov. 9, 2020, https://www.nytimes.com/2020/11/09/science/what-makes-sand-soft.html

- Validering og muliggjøring av teknologi for simulering av mekanisk oppførsel av sand / granulære materialer. Eksempler på mulig bruk er hvor kartlegging av grunnforhold er ressurskrevende, for eksempel innen havvind eller på andre planeter (utbygging i verdensrommet) - Økt kompetanse på mikromekanikk som fagmiljø på NGI - Økt grunnleggende forståelse av mekaniske oppførsel av sand under syklisk belastning - Økt internasjonalt forskningssamarbeid mellom NGI, Universitetet i Grenoble (Frankrike), Caltech (USA), EPFL (Sveits) og NTNU