Full 3D investigation and characterisation of capillary collapse of a loose unsaturated sand using X-ray CT

The aim of this paper is to characterise in 3D the capillary collapse phenomenon using X-ray Computed Tomography (X-ray CT) during water infiltration into a partially saturated soil. To understand the mechanisms leading to capillary collapse, we progressivelly saturated a specimen of sand by controlling the water pressure using the negative water column technique. During this imbibition process, we followed the granular structure using X-ray CT. The microstructure was analysed to assess the volume of water filling the pores and deformation of the granular skeleton using Volumetric Digital Image Correlation tools. Matheron's granulometry was used in parallel to characterize the initial microstructure and its evolution during the imbibition. We show that the collapse phenomenon can occur in a clean sand and can be controlled continuously with the negative water column technique. The volume change of the specimen at local scale started at a particular water content which coincided with the coalescence of capillary bridges between grain clusters. Gravity effects leading to a non-negligible gradient of the hydrostatic pressure along the specimen's height were observed and induced a vertical gradient of strain. Localisation of the vertical strain on conical surfaces and of the volumetric strain and water content at the bottom corner of the specimen appeared during the imbibition process. These localisations are thought to be due to an inhomogeneity of the initial density or/and an effect of cell walls facilitating the sliding of grains and the provision of water along preferential paths. However, in spite of those localisations, macroscopic measurements at the scale of the sample were representative of the local behaviour of the unsaturated sand.