Shifting Sands/Soils and Landscape-evolution

[Backseat Driver]

https://phys.org/news/2021-06-shifting-sands-soils-landscape-evolution.html?

Shifting sands, creeping soils, and a new understanding of landscape evolution
by Erica K. Brockmeier, University of Pennsylvania

A new study published in Nature Communications finds that piles of sand grains, even when undisturbed, are in constant motion. Using highly-sensitive optical interference data, researchers from the University of Pennsylvania and Vanderbilt University present results that challenge existing theories in both geology and physics about how soils and other types of disordered materials behave.

Most people only become aware of soil movement on hillsides when soil suddenly loses its rigidity, a phenomenon known as yield. "Say that you have soil on a hillside. Then, if there's an earthquake or it rains, this material that's apparently solid becomes a liquid," says principal investigator Douglas Jerolmack of Penn. "The prevailing framework treats this failure as if it's a crack breaking. The reason that's problematic is because you're modeling the material by a solid mechanical criterion, but you're modeling it at the point at which it becomes a liquid, so there's an inherent contradiction."

Such a model implies that, below yield the soil is a solid and therefore should not flow, but soil slowly and persistently "flows" below its yield point in a process known as creep. The prevailing geological explanation for soil creep is that it is caused by physical or biological disturbances, such as freeze-thaw cycles, fallen trees, or burrowing animals, that act to move soil.

In this study, lead author and Penn Ph.D. candidate Nakul S. Deshpande was interested in observing individual sand particles at rest which, based on existing theories, should be entirely immobile. "Researchers have built models by presuming certain behaviors of the soil grains in creep, but no one had actually just directly observed what the grains do," says Deshpande. (snip)

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[niyad]

1. many years ago, a huge section of the highway to Lake Tahoe from the Carson City side

Gave way and fell down the mountain,. I had driven that section only a short time prior to the collapse, and had noticed a long crack in the road, but did not think much of it. It was decided that the winter storms, the thaw/freeze cycles, had. caused it.

[mopinko]

3. do you live up that way?

my only brother is in placerville.

[niyad]

4. Not anymore. I lived in Reno at the time.

[mopinko]

5. ah.

brother built a paralympic training camp in tahoe.
was the caretaker of a place right on tahoe for a long time.
retired from teaching and started a wilderness guide business w a buddy. they got focused on disabled athletes, and off he went.

to me he's still the same asshole that made my childhood miserable. but he's done good shit.

[mopinko]

2. my soil is very sandy. it moves A LOT.

i live in what was once the bed of lake michigan.
there is a very slight slope, and the ground water is very close to the surface.

you especially can get top soil moving and the sand sitting still.
my back yard is about 4-5" higher that it was when i moved in in 87.