Experimental investigation of capillary collapse of partially saturated granular soils during saturation

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The capillary collapse is an irreversible volume reduction of partially saturated soils caused by provision of water at essentially unchanging total vertical stress. The cause of this phenomenon is probably, among other factors, the increasing dissolution of the capillary bridges, which is accompanied by a reduction in the stabilizing capillary cohesion and ultimately leads to rearrangements of the grain structure. Loosely bedded, fine-grained soils with a metastable microstructure are particularly at risk in this context. So far, existing studies on capillary collapse focus mainly on the macroscopic investigation, which is why the processes occurring on the micro-scale are still poorly understood. Within the framework of this research project, a better understanding of the physical processes occurring in granular media during capillary collapse on the particle scale is therefore to be gained. For this purpose, three different model soils, which show an increasing degree of complexity due to deviations from the simplest, idealized particle mixture (spherical particles with homogeneous density and uniform size), will be investigated. The starting point of the investigations is a poly disperse packing of industrially manufactured glass beads. In the next step, a model soil of natural, randomly shaped sand particles is investigated. The starting point of the investigations is a poly disperse packing of industrially manufactured glass beads. In the next step, a model soil of natural, randomly shaped sand particles is investigated. Since dump soils from opencast lignite mining are among the most endangered soils in Germany, another model soil will be investigated that contains lignite particles naturally occurring in opencast mine dump soils in addition to the sand grains as an example of a special particle structure (micro-porosity). In situ CT experiments with the help of the three-dimensional image data recorded over time allow insights into the processes taking place at the micro level as well as an analysis of the particle movements, whereby the knowledge gained can contribute significantly to the development of a deeper understanding of the collapse processes during saturation. Microscopic changes, e.g. in the phase distribution and capillary menisci, can be recorded and the relation to the occurring macroscopic collapse deformations can be analyzed. In the planned macroscopic and microscopic experiments, the influence of various boundary conditions such as the applied vertical stress, the lignite content, the irrigation rate and irrigation direction or the matric suction present in the soil will be quantitatively investigated.