Quicksand
Quicksand (also known as floating sand ) is a suspension of sand in water . Although it appears to be somewhat solid when undisturbed, it behaves like a special type of liquid under the influence of pressure : a non-Newtonian fluid . The fine-grained sand of some sandbanks on the coast , in which run-up ships sink and get stuck, is also referred to as grinding sand in the seaman's language .
functionality
In the event of vibrations or superimposed pressure, the grains of sand lose contact with one another and move in their watery environment without major resistance. Dry sand would compact and solidify under these circumstances.
The very small pores between the grains of sand prevent the liquid from draining off quickly ( gravel is much more permeable and consequently does not show this phenomenon). The entire suspension - as can also happen with Quickerde and Quickton - briefly changes to a “liquefied”, thixotropic state that lasts until the grains of sand come into contact again after the pore water has been squeezed out.
The effect can only occur if:
- the grains of sand were freshly poured up or relocated, so the grains are still loosely related to one another without cement and the pore spaces easily fill with water;
- enough water is available to completely fill the pore spaces: on rivers in sedimentation zones with sandy sediment (i.e. more in the lower reaches of the river), at springs with constant water supply and in the coastal areas of lakes and seas.
A water-retaining subsoil can almost entirely prevent the rapid drainage of the squeezed out water and the reconsolidation of the sand body if the terrain is appropriate.
Occur
Since quicksand requires a lot of water, it can almost only be found in bodies of water. The drifting sand, which is often portrayed in films and books, is almost impossible, especially since it is often shown in dry, sandy deserts such as the Sahara, which are naturally very dry.
The quicksand is easy to recognize in sand pictures : Even larger amounts of water are trapped in the freshly trickling sand, the sand compacts immediately when you knock on it. The sand grains rub against each other, the suspension is thickened by the shear and the internal resistance - the viscosity - of the suspension increases. Tilting movements, on the other hand, revive the liquid state.
hazards
Quicksand has a reputation for being life-threatening as soon as one gets into it, especially through exaggerated depictions in adventure literature and films. That is only partially true. The high overall density of the water-sand dispersion makes it practically impossible to sink deeper than about the stomach, since the density of the human body is close to that of water; Quicksand, on the other hand, has at least twice the density. Complete drowning is therefore impossible, but there is a risk of hypothermia at low temperatures . In mud flats too , sinking into quicksand when the tide comes up can be a death trap.
The fine-grain quicksand can be dangerous for ships if they run into a sandbank. The low load-bearing capacity of the sand is exacerbated by the movements caused by waves and tidal currents , creating swirling “maelstrom” that can be buried further by the run-up ships. Examples of accidents in which the ships could not be freed are the Fides and the Ondo in the Großer Vogelsand in the Elbe estuary.
Freeing yourself from quicksand without help can be difficult or almost impossible, depending on the nature of the subsoil and the depth of the sinking. If the legs are already sunk too deeply, it is recommended to move as little as possible, as the sand comes to rest after a while and the body is easily distended. In the affected areas, the rescue workers are equipped with pumps that liquefy the mixture by means of water injection so that a rescue is possible.
Quicksand effects ( soil liquefaction ) can cause secondary damage in earthquakes if larger areas of thixotropic sand are also affected - for example in the earthquake of Kobe in 1995 (Japan) or the sinking of Port Royal (Jamaica) in 1692.
"Dry" quicksand
In the scientific journal Nature published Detlef Lohse ( University of Twente in Enschede ) and co-authors in 2004 an experiment that could explain the "dry" quicksand.
By means of an air stream that was blown through a perforated container with the finest sand (silt), a very loose packing of the sand layer could be achieved. Then a falling, metal-filled ball sank into the material and sometimes created a real splashing fountain. In light of their results, the researchers speculated that air eddies in the desert could deposit sand in a similarly loose pack that even people, camels or vehicles could sink into it - as is often shown in adventure films. However, caution is advised when simply transferring the laboratory experiments to the desert scale: In the sand layer in the laboratory, which is just a few centimeters thick, the weight of the sand that presses on the structure of the sand grains is minimal. It is different when it comes to layers of sand 3, 4 or more meters thick, which are a prerequisite for the complete sinking of a person or a camel: One cubic meter of dry sand already weighs around 1.5 tons. With each new layer of sand that is deposited on an existing layer, the pressure on the grains of sand increases and the sand begins to settle ( compaction ). An extremely loose sand structure, as produced in a laboratory on a small scale, changes its physical properties very quickly when it is loaded with tons.
Web links
Individual evidence
- ↑ Mahlsand , Duden, accessed on August 6, 2018.
- ↑ Carl Grevener: Material: Danger quicksand . May 15, 2018 ( planet-wissen.de [accessed on August 10, 2018]).
- ↑ Carl Grevener: Quicksand - dangers in the sand pit. 2013.
- ↑ The drama on the "Großer Vogelsand" ( memento from September 26, 2012 in the Internet Archive ) , NDR.de, December 5, 2011.
- ↑ Elbe mouth: Big bird . DER SPIEGEL 49/1962, December 5, 1962.
- ↑ Hans Georg Prager: Savior without fame: the adventure of sea emergency aid. Sutton Verlag, 2012. p. 28.
- ↑ Detlef Lohse, Remco Rauhé, Raymond Bergmann, Devaraj van der Meer: Granular physics: Creating a dry variety of quicksand. In: Nature. Vol. 432 = No. 7018, 2004, pp. 689-690, doi : 10.1038 / 432689a .