Sand collapsing

[ILikeChemistry]

I was at the beach one afternoon in Australia Adelaide, I was young so I didin't think much of it but when I was running towards the shore (About average belly button deep) the sand under my feet was collapsing, I don't really remember it all that well but i'm pretty sure it lasted around 3-5 seconds, I was really scared so I attempted to sprint underwater. The only thing I could really think about (All I know about) is that it could have possibly been Angel Shark(s) but like I said it was in the afternoon so I doubt it. Thanks in advanced!

[Ophiolite]

Beaches are irregular. Shallow drainage channels are cut in the generally flat surface at low tide and, in the absence of significant wave action, will persist at high tide. If you walked on to the edge of one of these channels it would likely have collapsed under your weight.

 

Based on the very limited information you have provided that is the best explanation I can come up with.

[ILikeChemistry]

Beaches are irregular. Shallow drainage channels are cut in the generally flat surface at low tide and, in the absence of significant wave action, will persist at high tide. If you walked on to the edge of one of these channels it would likely have collapsed under your weight.

 

Based on the very limited information you have provided that is the best explanation I can come up with.

Thank you! It was a really weird experience, definitely a first for me as well.

[J.C.MacSwell]

It could have been a quicksand effect, from you disturbing the sand, a slight upward fow of water through the sand, or combination there of:

 

https://en.wikipedia.org/wiki/Quicksand

[Ophiolite]

It could have been a quicksand effect, from you disturbing the sand, a slight upward fow of water through the sand, or combination there of:

 

https://en.wikipedia.org/wiki/Quicksand

I don't know much about quicksand, but surely this effect would not be present when the sand is under a couple of feet of water already, as is the case here. Your thoughts?

[studiot]

Hello, ILikeChemistry and welcome to Science Forums.

I see you are in Oz and now offline. Sorry I missed you.

 

The phenomenon you describe is called 'liquefication' and is a purely physical action. No ghoulies or ghousties are involved.

I'm sure it was pretty frightening and I understand (perhaps the biologists here will give more information) that some marine organisms use this effect.

But I do not think that an animal source would be more than quite local.

 

Liquefaction occurs through both saturation of the sand and vibration.

I'm sorry I can't find a year 8 explanation here is a much more advanced one.

 

http://web.hku.hk/~junyang/pdf/Yang_Savidis_JGGE.pdf

 

Essentially sand can support loading pressure when dry but when wet or vibrated the sand grains flow away from the source of loading like a liquid.

 

Sand is a non cohesive soil.

 

This means that there is no chemical interaction between the grains so any transmission of force is like a dry stone wall.

(Dry stone walls are made without mortar)

Such a material cannot support tension.

It can only support compression.

So when the material is loaded by your weight, the bursting force under your feet (which is tensile) pushes the grains aside.

 

Hope this helps.

 

[J.C.MacSwell]

I don't know much about quicksand, but surely this effect would not be present when the sand is under a couple of feet of water already, as is the case here. Your thoughts?

I think you would still get it, the disturbance/vibration, certainly saturation if already underwater, and possibly an artesian effect for some uplift. It does not have to be much to lose much of the little shear or static friction forces the sand particles might otherwise maintain. Any water displaced by the feet adds to the effect.

[studiot]

A slight digression but you might like to try to get this book from your local library.

 

The Self-Made Tapestry

 

by Philip Ball

 

Now issues in three volumes there's lots of accessible stuff in there for you including discussion of sand flow,

up to and including why you should not overload a cement mixer.

 

https://www.amazon.co.uk/s/ref=nb_sb_noss?url=search-alias%3Dstripbooks&field-keywords=philip+ball+tapestry+of+nature