Title. I saw a video showing it happening, but I don’t understand why.
In: 51
I’m a geotechnical engineer, so I’d say I’m pretty well qualified to answer this.
In ELI5 terms – clay soils when they dry up, shrink in size (several reasons for this, which is part of a theory called total stress, but that’s beyond the scope of ELI5). The water content of the soil gives it some volume, less water, less volume. This causes the tiny little holes in the soil body to close up. This is where the water lives in clay soils, in these tiny little holes. As you add more water it becomes softer, take away the water, it becomes harder.
Rehydrating the soil, it needs time to “soak” to absorb the water back to a happy moisture content. If you pour water over the top it tends to flow over because it cannot absorb that quickly, due to the little holes being less prevalent.
Because of this principle, you tend to get a phenomena called heave, or shrink swell which is when clay soils can move (in some degrees measurable in 10s of millimetres) over the course of a year, and can lead to cracking or other structural issues with buildings and structures.
There’s more to it, but that’s as basic as I can make it without getting into total and effective stress, cohesive and granular soils etc.
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