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  4. GRK 2462: Experimental and numerical investigation of capillary collapse in unsaturated soils
 
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Projekt Titel
GRK 2462: Experimental and numerical investigation of capillary collapse in unsaturated soils
Förderkennzeichen
GRK 2462/1
Startdatum
May 16, 2019
Enddatum
May 15, 2022
Award URL
https://www.tuhh.de/gbt/forschung/forschungsprojekte.html#c111314
Gepris ID
390794421
Übergeordnetes Projekt
Graduiertenkolleg 2462: Prozesse in natürlichen und technischen Partikel-Fluid-Systemen  
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Funder
Deutsche Forschungsgemeinschaft (DFG)  
Funding Program
Graduiertenkolleg
Institut
Geotechnik und Baubetrieb B-5  
Projektleitung
Grabe, Jürgen  
Mitarbeitende
Hüsener, Nicole
Under natural conditions, soil consists of three phases: soil particles (e.g. sand grains), water and air. Above the groundwater level, the soils are usually in an unsaturated state, in which the individual soil particles are connected to one another via capillary bridges. In unsaturated soils, the cohesion of the grain structure is significantly influenced by what is known as "capillary cohesion". The surface forces of the pore water create suction stresses which additionally press the individual soil grains against one another and in this way lead, among other things, to an increased shear strength of the soil. The size of the suction depends on the degree of saturation of the soil, which is why the capillary pressure-saturation relationship (Water Retention Curve, WRC) is an important key function for characterizing the hydraulic and mechanical behaviour of unsaturated soils.

The capillary cohesion disappears both in the dry and in the saturated state and is therefore also referred to as "apparent cohesion". As a result of natural fluctuations in the water content or the degree of saturation of the soil, for example due to precipitation or a rising groundwater level, there is a risk of saturation due to the increasing dissolution of the capillary bridges and the cohesion of the grain structure. The resulting sudden rearrangement of the grain structure is also referred to as capillary collapse and, under certain conditions, can lead to sometimes very large subsidence of a soil. Loosely deposited, fine-grained soils with a metastable structure are particularly at risk in this context.

When extracting lignite, large amounts of overburden are produced in opencast mines, which are then stored on dumps. Depending on the extraction and transport technology used for the debris, different types of dump are created. However, what all tilted floors have in common is that, in contrast to naturally grown floors, they are often characterized by a particularly loose storage and represent a special case due to the associated high deformation potential. During the open mining operation, the groundwater must also be pumped to great depths under the opencast mine, which is why, after closure, there are particularly large increases in the groundwater, which are very problematic in terms of capillary collapse. Due to the high risk of capillary collapse, which is difficult to calculate, these areas have so far remained largely unused. As part of the research project, a tilted soil from the German open-cast lignite mine is examined, which consists mainly of medium and fine sand and also contains organic components in the form of coal of different sizes.
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