  Observing and understanding the movement of an intruder through opaque dense
suspensions such as quicksand remains a practical and conceptual challenge.
Here we use an ultrasonic probe to investigate the dynamics of a steel ball
sinking in a 3D dense glass bead packing saturated by water. We show that the
frictional model developed for dry granular media can be used to describe the
ball motion induced by horizontal vibration. From this rheology we infer the
static friction coefficient and effective viscosity that decrease when
increasing the vibration intensity. Our main finding is that the
vibration-induced reduction of the yield stress and increase of the sinking
depth are presumably due to induced slipping at the grain contacts but without
visible plastic rearrangements of grains, in contrast to dry granular packings.
To explain these results, we propose a mechanism of acoustic lubrication that
reduces the inter-particle friction and leads to a decrease of the yield
stress. This scenario is different from the mechanism of liquefaction usually
invoked in loosely packed quicksands where the vibration-induced compaction
increases the pore pressure and decreases the confining pressure on the solid
skeleton, thus reducing the granular resistance to external load.
