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

毛管塌陷是指在总竖向应力基本不变的情况下供水引起的部分饱和土的不可逆体积减小。这种现象的原因可能是，除其他因素外，增加溶解的毛细管桥，这是伴随着减少稳定的毛细管凝聚力，并最终导致重新排列的晶粒结构。在这种情况下，具有亚稳微观结构的松散层状细粒土尤其危险。迄今为止，已有的研究主要集中在宏观研究，这是为什么发生在微观尺度上的过程仍然知之甚少。在这个研究项目的框架内，更好地了解颗粒介质中发生的物理过程，在颗粒尺度上的毛细塌陷。为此，将研究三种不同的模型土壤，由于偏离最简单的理想化颗粒混合物（密度均匀、尺寸均匀的球形颗粒），这些土壤的复杂程度不断增加。调查的起点是工业制造的玻璃珠的多分散填料。在下一步中，研究自然随机形状的砂粒的模型土壤。调查的起点是工业制造的玻璃珠的多分散填料。在下一步中，研究自然随机形状的砂粒的模型土壤。由于露天褐煤开采的排土场土壤是德国最濒危的土壤之一，因此将研究另一种模型土壤，该模型土壤除含有沙粒外，还含有露天矿排土场土壤中天然存在的褐煤颗粒，作为特殊颗粒结构（微孔）的示例。借助随时间推移记录的三维图像数据，原位CT实验可以深入了解微观层面发生的过程以及颗粒运动的分析，从而获得的知识可以大大有助于更深入地了解饱和期间的坍塌过程。可以记录微观变化，例如相分布和毛细弯月面的变化，并可以分析与发生的宏观塌陷变形的关系。在计划的宏观和微观实验中，将定量研究各种边界条件的影响，如施加的垂直应力，褐煤含量，灌溉率和灌溉方向或土壤中存在的基质吸力。

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.