Catching up with creep in geomaterials

Creep is a time dependent process during which materials accumulate strains under the influence of permanent stresses, as commonly observed in slopes where gravity generates a long-time downhill movement of the soil. Large infrastructure can also introduce load in the soil, which become subject to creep and can potentially affect the stability of the structure itself.

Actually, creep remains the most overlooked mechanism in granular materials, and is often ignored and poorly understood. In fact, we still don’t fully understand the underlying mechanisms occurring at the grain scale. In coarse grained geomaterials (such as dry sands) the process is erroneously considered negligible, however, at larger stress magnitudes the effects are dramatic. Fine-grained geomaterials such as clays have very large creep rates causing huge additional costs for infrastructure, tunnels and other foundations, which often are unforeseen.

The rate-dependent behaviour, such as creep and relaxation, of natural occurring granular materials is strongly linked to their micro-structure, crushing and grain surface properties. The general hypothesis in the discipline is that the inter-particle force distribution continues to change under constant external load, probably because of small variations of grain geometrical properties. Also, continuum observations (traditional displacement measurements at the sample boundary) indicate that the process is grain size dependent, i.e., assemblies of smaller grains lead to significantly larger on-going displacements in the assembly. The physicochemical inter-particle behaviour and structure evolution of natural saturated clays is markedly different from the frictional contacts in the coarse grained geomaterials. Also the micro-structural origin of creep in this materials still are largely unknown.

The objective of this work is to put in place a methodology for the micro-structural study of creep using synchrotron radiation. We propose to open the rich new field of 3D+time high-speed micro-mechanics with two simple experiments to define two extremes of the field: 1. large grains and subtle creep effects and 2. fine grained matter, such as clay, where creep is significant. The aim is to close the gap between the temporal scale of behaviour in our materials and the temporal scale in observations thanks to the high-speed high-resolutions facilities available at synchrotrons.

The originality of the project is the application of new imaging techniques available only in synchrotrons to study important and difficult-to-observe processes in geomaterials.

The expected breakthrough therefore is to extend the current knowledge of rate-dependent behaviour in geomaterials by following their microstructural evolution using high-speed tomography in two textbook examples of geomaterials: sand and clay.