Roger Bould, of Waikanae, asks :-
When I walk along the beach at low tide, each time I put my foot down, the sand around it goes drier than it was before my foot landed. Logic makes me believe my weight would compress the sand particles. This would force the water out from between them. So why don’t I see water around my foot? I noticed that when I lifted my foot again, the footprint I left behind was watery.
Simon Cox, a geologist with GNS Sciences in Dunedin, responded.
You are right to be thinking of the drying and wetting in terms of deformation.
Beach sand tends to be very well-sorted, so the equidimensional grains of sand tend to behave a bit like small spheres and deposit in a variety of packing arrangements. Wave and water action tends to organise them into a close packing arrangement where the volume of the voids between the grains is at a minimum.
On the beach the voids are mostly full of water. When you put your foot down the grains and water under your foot are subjected to a compression (what we call plain strain) where your weight is held by the strength of grain-grain contact and incompressible water It does not change volume - just shifts downwards.
Beyond your footprint , the area of sand does actually deform slightly by a shear strain which rearranges the grains ever so slightly from the best packing setup. So this area 'dilates' (increases volume) and appears to dry out (water can't flow into the voids as fast as they are being re-organised).
As you lift your foot water flows into the depression caused by your weight.