Facts about Quicksand

This is from the document
'/news-archives/sci/geo/geology/general'.
contributed by: ert@gly.bris.ac.uk (Reggie)

Quicksand is a a condition which occurs when an otherwise static body of sand is "aggitated" in such a way that the grains lift away from one another. As friction is transferred through the sand body via grain-to-grain contacts the separation of grainscauses a dramatic reduction in the shear strength of the material and it starts to behave in a liquid-like manner, i.e. the sediment liquifies.There are a number of "aggitation" processes:

• The upward passage of pore fluid exerts a drag force on the individual grains. If the drag force is great enough it will balance the weight of the grains and they will no longer rest on their neighbours. This processes is often called "fluidisation".

• Vibrations cause grains to move laterally, colliding with their neighbours as they do so. If the vibration energy is great enough the collision rate and grain velocities increase such that the majority of grains are moving and colliding rather than resting statically on their neighbours. The sediment then becomes liquified.

• During earthquakes and other large vibration activity (e.g. storm waves, impact of dense flows, etc.) vibration can causes an initial movement of grains,compaction and pore waters are expelled. The action of the expelled water is combined with the vibration behaviour to produce a complicated liquification process which is often called "liquefaction".

• Shear across a granular body of sediment can cause a similar collision process to that occurring during vibration. Therefore unidirectional shear across a body of sand can also cause liquification. "Granular temperature" models have been used to investigate this process in granular flows.

Silts are generally more easy to liquify than sand and many "quick sands" would be more accurately described as "quick silts".

Some clays rapidly loose there shear strength during vibration. Their shear strength returns once the vibration ceases. This is thixotropic behaviour and is yet another liquification process.

Non-thixotropic clays usually inhibit liquification. However, as clays will deform plastically in their natural state, deformation of clay can often produce structure which are very similar to the structures produced during liquification e.g. load structures.