In many old B grade adventure movies, a substantial number of baddies perished by stumbling into quicksand where they became trapped before quickly submerging below the surface to drown in the sand. Is this really possible?
When we stand on the surface of dry sand or soil, it supports our weight because the grains are tightly packed and the friction between the grain particles creates a force chain. If sand or soil becomes moist, the water creates a capillary attraction between grains that allows the grains to clump more closely together. When we stand on moist sand or soil, our weight pushes out the water between the grains and we sink until we reach the depth where the grains are tightly packed enough to support our weight.
Quicksand is saturated sand or soil and it may occur where there is abundant water such as shorelines, swamps, river banks, beaches and above underground springs. When sufficient water flows upward through the sand or soil, the water is trapped and forces the grains apart, reduces the friction between grains which creates a suspension of grains floating in water. This soupy mixture may appear solid, but it behaves like a liquid and is unstable when disturbed. If a person stands on quicksand its structure collapses, the grains sink to the bottom and the water floats to the top, the person's weight cannot be supported and they sink until they reach a level where their weight equals the weight of the sand and water mix that they have displaced. Fortunately humans are only half as dense as quicksand so are only likely to sink to their waist rather than disappearing into the depths. The vacuum forces of quicksand are very powerful and the best way to escape is to move legs around slowly to allow water to re enter and reduce the viscosity of the sand.
Earthquakes can also liquefy soil. Liquefaction occurs when wet ground is shaken violently causing the soil particles rearrange into more dense and compact clumps; eventually the water pressure becomes greater than the forces holding the particles of soil together. When this happens the soil loses its strength and structure and behaves like a liquid hence the term liquefaction. The result is a layer of thick, silty liquid sludge covering the ground.
Liquefaction causes extensive damage, it may result in landslides and fissures. The loss of soil strength means that buildings and structures lose their support, underground pipes become buoyant and float and a thick layer of silt spreads over the ground surface. Liquefied soil can also act as a quicksand trap, a recently discovered fossil site containing numerous preserved bones of mammoths, mastodons and bison occurred when an earthquake liquefied sediment in a shallow lake, trapping all the unfortunate animals standing in the lake at the time.