Collapse and runout of granular columns in pendular state

It is well known that even small amounts of liquid can strongly modify the mechanical behavior of granular packings in static and dynamic conditions. This experimental work, therefore, focuses on the collapse of columns of wet granular materials in the pendular wetting regime. Different from previous studies, where idealized spherical materials (glass beads) are typically used, here experiments on irregular wet calcium carbonate particles (coarse sand) were carried out and compared with glass sphere results. Particles of different sizes (in the range 0.8-5 mm) were mixed with water from 0% to 4% w/w and poured in a rectangular box. Flow was then triggered by removing a lateral wall of the box. The measured runout distances showed marked differences between the two types of materials which could not be explained only in terms of particle shape or capillary forces. Ring shear tests and 3D tomographic reconstructions of the liquid distribution in the materials highlighted the role of additional mechanisms related to liquid spreading at the particle surface, inter-particle friction, and contact lubrication.