Quick sand and Quick clay

Asmae Kaldoun - April 2nd 2007

Abstract

Quick sand is the generic name for unstable soils reputed to trap anyone
who treads on it. Popular wisdom has it that one should not move when
trapped in quicksand, as motion makes one sink in even deeper and that
once trapped, it is difficult to escape. Here we provide an explanation
for these observations by studying the most commonly encountered form of
natural quicksand. We show that a spectacular liquefaction of the
material occurs when a stress is applied to the material: the
liquefaction is the reason why one sinks away, and it is more pronounced
for larger stresses. By constructing "laboratory quicksand", we
demonstrate that the liquefaction is due to the structure: quicksand is
a loose granular packing of sand particles stabilized by a clay matrix
that forms a particulate gel. The stress liquefies the clay matrix, and
the granular assembly collapses, ex pulsing water. This results in a
densely packed system that practically impossible to dilate: it is for
this reason that once trapped it is difficult to get out of quicksand.
A sinking test demonstrates that, due to buoyancy, it impossible to
drown in the quicksand.

Quick clay is a soil that changes from solid soil to a liquid mass when
it is solicited. And it has been the main cause of many deadly land
slides in Canada, Russia, Alaska, Norway and Sweden. We were able to
reproduce the land slide phenomenon in the laboratory by using natural
quick clay from Norway. We conclude that the flow behavior of the quick
clay presents, beside the three usual regimes observed with yield stress
materials, a fourth regime, we called land slide regime, because in this
state the quick clay can travel very quickly over large distances. The
rheological response of the quick clay shows that the samples present a
critical concentration. After which spectacular liquefaction happens by
adding only 1% of water to the sample. The oscillatory measurements show
that the elasticity of the sample increases and reaches a maximum by
adding salt.