walking in wet sand I notice that about 6" around each foot the sand appears to dry out under the pressure of my weight. Is it just a case of the water being squeezed out, or is there a more subtle answer? How deep would the effect in the vertical direction? I presume such effects must be important when putting large structures on moist soil or clay.
What’s happening, I’ve been told, is that the sand re-arranges itself when you step on it. The new arrangement isn’t as efficiently packed as before, so there’s more space between the sand grains, so the water doesn’t fill it as completely, so the very top of it doesn’t have water in it anymore. You aren’t squeezing the water out of the sand, in other words – you’re expanding the sand space so the water doesn’t fill it anymore.
There’s an entry on this in my favorite weird physics reference, Jearl D. Walker’s The Flying Circus of Physics, and I think there are a few references.
(Think of it! There are actually references on this bizarre physics question!)