In situ X-ray CT imaging of transient water retention experiments with cyclic drainage and imbibition

Abstract

The water retention curve (WRC) represents a key function in unsaturated soil mechanics as it can be applied for the modeling of the hydro-mechanical behaviour of unsaturated soils. The macroscopic WRC is characterised by different phenomena, such as hysteresis upon cyclic drainage and imbibition as well as a multitude of hydraulic scanning paths upon a reversal of water flow direction. With the help of modern X-ray computed tomography and hydraulic experiments that can be performed in a CT scanning environment, so-called in situ CT experiments, we image cyclic drainage and imbibition in a sand on the pore scale in order to quantitatively measure and study the change of microstructure and capillary state variables, characterising capillary effects in unsaturated granular soils. The measured pore scale data can then be related to the macroscopic WRC. To our knowledge, for the first time very different capillary state variables, such as interfacial areas, contact lines and contact angles, could be extensively measured in high detail for various hydraulic cycles in our experiment. Besides the experimental procedure, the wealth of measured data will be comprehensively presented and discussed and finally shared with the research community for further investigations as it is believed to yield deep insights into the microstructural origins of water retention behaviour with its hydraulic hysteresis and clues for effective stress in unsaturated granular soils.