Hydro-mechanical coupling in unsaturated granular media studied at the grain scale

Partially saturated soils are three-phase mixtures of soil particles, and water and/or air in the pores.

At the interfaces between water and air, menisci are formed on the solid surface of the grains as commonly observed in a test tube or a straw. The curvature of the air-water interface of the water caught between the particles creates a “suction” (negative water pressure) that hold the particles together thereby increases the strength and stiffness of partially saturated soils over the strength of the same material dry or fully saturated.

This phenomenon of increase in strength is commonly experienced by children who will instinctively use wet sand for building their sand castles. In more serious situations, the degradation of suction forces due to the infiltration of water and/or shearing can lead to a drastic loss in strength, sometimes with tragic consequences – a typical cause of landslides.

This project aims to reveal some of the key microscopic mechanisms involved in the behaviour of partially saturated soils, i.e., microstructural changes during deformation up to failure as well as water distribution in pore space and its relation to suction levels.

The adopted strategy compares experimental and numerical (DEM and FEMxDEM) results in order to improve and refine the local laws of interaction between solid and liquid phases implemented in these codes.

Two kinds of materials are studied and were chosen to facilitate the dialog between experiment and modelling:

• an “ideal” material made of spherical sapphire/ruby beads, which micro structure perfectly fits the modelled material,
• a natural (and local) geomaterial, Hostun sand, with a more complex microstructure but closer to real conditions.

To characterise the hydro-mechanical couplings in these two geometrical configurations, the samples are imaged during either triaxial compression tests or wetting-drying experiments using micro x-ray Computed Tomography and neutron tomography (a technique that gives better results when it comes to water imaging). A trinarisation of the images will be performed to distinguish the three phases (see below), and to characterise the evolution of water distribution, as well as to follow the evolution of the grain network. In the case of the spherical material the trinarisation will be facilitated by a tool that takes into account the geometry of the particles.

Two scales are investigated: the nano scale (an assembly of hundreds of elements) and the micro scale (an assembly of thousands of elements).

These techniques will provide a set of parameters such as the evolution of the local density, grain positions, the local degree of saturation, liquid bridges characteristics (position, volume, connectivity…) that can be connected to the macroscopic loading, to the stress inside the material and to the suction effect.

This work has the potential to become a fundamental physical background for modelling deformation and failure in partially saturated soils through giving a better understanding of the contribution of suction to the stiffness and strength of geomaterials.