Person Responsible:

Nicole Hüsener, M. Sc.,                                                                                   Institute of Geotechnical Engineering and Construction Management (B-5)

Supervisors:

Prof. Dr.-Ing. Jürgen Grabe, Dr.-Ing. Maksym Dosta, Prof. Dr.-Ing. habil. Dr. h. c. Stefan Heinrich

Project Topic and Research Questions:

In research project A1 of the thematic focus A "Dry and unsaturated PFS", capillary effects in soils as natural PFS are investigated. The focus lies on the so-called capillary collapse, an irreversible volume reduction of partially saturated soils caused by the provision of water at essentially unchanging total vertical stress. Despite extensive research, however, the processes involved are still not fully understood and will therefore be investigated in more detail as part of the project. In addition, the influence of selected boundary conditions is investigated qualitatively and quantitatively.

Materials and Methods:

To investigate the capillary collapse, multiple experimental methods are applied at different
length scales. On the macro scale, mainly soil mechanical laboratory tests such as single or double oedometer tests are used. In addition, an own customized macroscopic set-up for incremental irrigation was developed, which allows more realistic investigations compared to the common saturation in only one step.

Miniaturized in situ CT experiments are performed to investigate the triggering microscopic processes. Using the obtained high-resolution image data, morphological changes within the sample (such as location and size of water/air clusters, local degree of saturation, etc.) can be evaluated by image analysis. In addition, discrete image correlation allows spatial tracking
of individual particles.

Three different model soils with approx. the same grain size distribution and increasing degree of complexity are examined: a polydisperse glass bead packing, a model soil of natural, randomly shaped sand and a more complex model soil derived from an opencast mine dump soil containing additional porous lignite particles as an example of a special particle structure.

Grain size distribution of investigated model soils

Selected Results:

By means of the oedometer tests, it was found that even a low lignite content (0.3 wt%) seems to have a significant influence on the hydraulicmechanical behavior of a soil. For example, the settlements under load become larger, while the collapse settlements are reduced. In addition, the incremental irrigation tests at negligible loading condition could show that the lignite particles seem to lead to a more uniform, nearly linear collapse behavior over the entire degree of saturation range, while in the case of the pure sandy soil, the settlements were initially lower at low water content, but increased at high degrees of saturation.

In general, it was observed (in both the laboratory and CT tests) that controlling the water volumes injected (uniform small steps) resulted in nearly linear vertical changes in the specimen height, in contrast to suction-controlled wetting tests. Therefore, with sufficiently small irrigation steps, only relatively small settlements can be expected, allowing grain movement and changes in microscopic phase distribution to be tracked using CT scans.

Originally, a numerical simulation of capillary collapse was also planned. Here, the CT image data obtained should ideally serve as the basis for generating the numerical model and, at the same time, be used for later validation of the results. However, when considering the various numerical methods, it was found that modeling is very difficult due to the complexity of the phenomenon and has so far only been possible to a very limited extent. Due to this, numerical investigations were not carried out. Instead, the experimental investigations are to be used to create a data basis that can be used in the future as a basis for the development and validation of a numerical model for the simulation.

Project-related Publications:

• Hüsener, Nicole; Grabe, Jürgen. (2023). Capillary collapse of unsaturated granular soils: experimental investigation and microscale insights. E3S Web of Conferences. 382. 02004. DOI: 10.1051/e3sconf/202338202004. ​​​​​​​
• Hüsener, N. and Grabe, J. (2022): Experimental investigation of capillary collapse of partially saturated granular media. In: Tagungsband zum Workshop des GRK 2462 "Processes in natural and technical Particle-Fluid-Systems" (PintPFS), Veröffentlichungen des Instituts für Geotechnik und Baubetrieb der Technischen Universität Hamburg, Heft 54. Hrsg. von J. Grabe, DOI: 10.15480/882.4640, S. 109-121
• Hüsener, N., Helfen, L., Milatz, M. and Tengattini, A. (2021): Combined X-ray- and neutron-tomography imaging of capillary collapse in unsaturated granular soils. Institut Laue-Langevin (ILL). DOI: 10.5291/ILL-DATA.1-05-53