Mobility in immersed granular materials upon cyclic loading

We study the mobility of objects embedded in an immersed granular packing and subjected to cyclic loadings. With this aim, we conducted uplift experiments whereby a horizontal plate is embedded in the packing and subjected to a vertical cyclic force oscillating between zero and a maximum amplitude. Tests performed at different cyclic force frequencies and amplitudes evidence the development of three mobility regimes whereby the plate stays virtually immobile, moves up steadily, or slowly creeps upwards. Results show that steady plate uplift can occur at lower force magnitudes when the frequency is increased. We propose an interpretation of this frequency-weakening behavior based on force relaxation experiments and on the analysis of the mobility response of theoretical viscoelastoplastic mechanical analog. These results and analysis point out inherent differences in mobility response between steady and cyclic loadings in immersed granular materials.