, soil mechanics
Quicksand is not a special type of sand. It is a hydrogeological
Quicksand occurs when an upward hydraulic gradient
reaches a critical level, at around 1.0 meters/meter. What happens is that gravitational forces in the soil mass are balanced by upward seepage force
. The inter-granular strength of the soil particles drops to zero, and the soil is said to be in the quick
condition. If the critical gradient is exceeded, the surface will appear to be "boiling" as the particles are moved by the upward flow of water. In the natural environment, quicksand is commonest at the bottom of hills. It can also occur where a confined aquifer
has surface contact.
Quicksand is a natural phenomenon, but it can occur in engineered soil works, such as near sheet pilings. In engineered earth structures, appearance of quicksand (e.g. downstream of an earth dam
) can signify a design or construction fault as well as the impending catastrophic failure of the structure. It is a very bad sign.