The contact mechanics of sand and rock grains

The contact mechanics of sand and rock grains can be critical in civil or petroleum engineering problems that require a discrete not continuum approach e.g. debris flow, oil well behaviour, but the state-of-the-art in particulate soil mechanics lies in experimental (x-ray CT) and numerical (discrete element-DEM) studies of grains where simple, smooth shapes and simplistic contact laws are often assumed. It is probable that in the very long term, as computer power increases, all geotechnical modelling will be discrete. The supervisors' current grant on ballast aims at recreating realistic particles for DEM, but developing constitutive laws for the inter-particle contact would require another, purely fundamental, project. Grains of geological origin have surface asperities (0.1-10 m) and complex shapes. The only contact models available were derived for engineered surfaces, for normal loading and many in the 1970s. In available models for rough surfaces, it is either assumed that asperities deform elastically, or plastically, or both, without consensus, but neither works well for soil/rock grains.

One needs to take a step back to be able to generate the appropriate constitutive laws for contact between soil/rock grains. This requires designing particle-to-particle tests to investigate specific aspects of contact behaviour, characterising the contact surfaces and developing constitutive laws from elastic and plastic theory but informed by these results. Baudet has developed a novel method to quantify grain surface roughness and its evolution with loading based on advanced fractal theory. She intends to involve Dr Murthy, tribologist/soil mechanics from the prestigious Indian Institute Science with which collaboration is encouraged by UCL Global Engagement office. She and Murthy have established strong links through Royal Society and DUO-India Fellowship schemes. UCL soil laboratory possesses a unique apparatus for inter-particle testing, built by Coop (fig.1), able to control and resolve loads (1-1,000N with 0.01N resolution) and displacements (0-5mm with 0.02m resolution) for normal and tangential loading, and an advanced optical profilometer for roughness measurements.