Mobility in granular materials upon dynamic loading

This Thesis explores the mechanisms controlling the mobility of large objects embedded in granular materials. The goal is to establish which loading conditions lead to their motion and to identify the underlying physical mechanisms. To this aim, laboratory experiments were conducted using a canonical mobility test, which involves uplifting a plate embedded in a dry granular packing vertically. The experimental conditions were varied in order to evidence the effect of the presence of water in the granular packing, and the effect of dynamic loadings whereby the plate is moved at different velocities or subjected to a cyclic force. Three main discoveries emerged from these experiments. The first discovery is the development of a drag force instability when the plate is driven at a constant and slow velocity, which vanishes at higher velocities. The second discovery is a visco-elastic dynamics developing in immersed packing, which is evidenced by drag force relaxation dynamic and a velocity-driven increase in drag force. The third discovery is a drop in packing resistance when the plate is subjected to a low magnitude cyclic force. The physical origin of these mobility responses is consistently analysed and rationalised, considering elementary mechanical processes. The conclusions of this Thesis form a fundamental basis to advance a variety of engineering discipline pertaining to foundation design, excavation techniques and bulk material handling