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Author Topic: What makes sand soft?  (Read 5462 times)

Offline Laura_Kelly

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What makes sand soft?
« on: 05/01/2010 02:39:28 »
What makes sand soft? Is is friction between particles?

Why is sand soft when dry and when very wet, but firmer when only damp?


 

Offline Soul Surfer

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What makes sand soft?
« Reply #1 on: 05/01/2010 23:20:07 »
An interesting question.

When sand is very dry the grains can easily roll and slip over each other.  When sand is very wet the water also allows it to move reasonably freely but not a freely as when it is dry because of the viscosity of water in the gaps. When sand is just damp and contains a mix of air and water the surface tension of the water droplets between the grains comes into play and this effectivley sticks the grains of sand together and makes it much more solid.

The shape of the grains in the sand can also affect its flowing properties if the grains are sharp and angulsr they with flow less easily than if they are weathered and round.  Builders tend to stock two different sorts od sand, sharp and soft dependant on what sort of job the sand is to be used for.
« Last Edit: 06/01/2010 10:43:42 by Soul Surfer »
 

Offline RD

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What makes sand soft?
« Reply #2 on: 05/01/2010 23:27:20 »
What makes sand soft?

The shape of the grains ...

Quote
The study of individual grains can reveal much historical information as to the origin and kind of transport of the grain. Quartz sand that is recently weathered from granite or gneiss quartz crystals will be angular. It is called grus in geology or sharp sand in the building trade where it is preferred for concrete, and in gardening where it is used as a soil amendment to loosen clay soils. Sand that is transported long distances by water or wind will be rounded, with characteristic abrasion patterns on the grain surface. Desert sand is typically rounded.
 People who collect sand as a hobby are known as arenophiles* or psammophiles*.
http://en.wikipedia.org/wiki/Sand#Study

* or "nuts".
« Last Edit: 05/01/2010 23:28:54 by RD »
 

Offline Laura_Kelly

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What makes sand soft?
« Reply #3 on: 06/01/2010 03:46:14 »
Aside from the shape of the grains and the water content, what other factors do you reckon affect how soft sand is?
 

Offline daveshorts

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What makes sand soft?
« Reply #4 on: 06/01/2010 10:01:30 »
Sand flows easily when very dry or very wet, but is quite stiff when only damp, mostly because of surface tension.

if you have a drop of water on the table and dip the end of a thin strip of paper into it the paper will stick, but if the paper is in a bucket of water it won't stick to anything, this is because water molecules are attracted to one another and so they try to minimise the water's surface area. so if you pull the paper away you form more surface and the water pulls back trying to minimise its surface.

The sand particles stick together best when there is a little bit of water joining theparticles, which is surrounded by air, so that when you move the particles apart you increase the surface are of the water, and the water pulls back.

Other things which can affect the hardness of sand include fibres mixed in like tree roots, the particle size, mineralization (water soluble minerals depositing between the grains etc
 

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What makes sand soft?
« Reply #4 on: 06/01/2010 10:01:30 »

 

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