Theoretical modelling of water retention curves based on the X-ray micro-CT observations

A theoretical water retention curve (WRC) model was developed based on X-ray micro-computed tomography (CT) image analysis, capturing microscopic water retention states. By subtracting CT images, drainage and imbibition clusters were identified, and their shapes and distances from soil particle surfaces were quantitatively analysed. The analysis showed that during drying, water drains from the pore centre in a simple shape, whereas during wetting, water imbibes near soil particle surfaces in complex shapes before filling the pore centre. Based on these findings, a theoretical WRC model with scanning curves was constructed using a unit cell assembly approach, considering density and particle size distribution. It was assumed that suction changes induce drainage and imbibition in each unit cell, with the overall saturation determined by summing water content across all unit cells. Unlike previous models that applied the same approach for both behaviours, this model uses different models: drainage follows a hemisphere model, while imbibition follows an inscribed sphere model. This difference enables the model to represent scanning curves, thereby improving the qualitative agreement with observed water retention behaviours.