Buckets of sand.

[Uncwilly]

The sand that is already moist doesn't expand per se. Rather it fails to compress when one packs it down. The water does not move out of the way. Dry sand can be packed more tightly and sand is more dense than water.

The 2 beers story is a good illustration.

This does not apply to dirt (soil). Water in the soil provides cohession and acts as a softening agent that allows dirt clods to deform.

[cheesehead]

Originally posted by Uncwilly
Rather it fails to compress when one packs it down. The water does not move out of the way.

That depends on the size (and perhaps shape) of the sand particles, the ratio of water to sand, and the compression pressure, at least.

I remember picking up handfuls of wet beach sand, squeezing them to make water come out, then seeing the sand reabsorb the water (that is, the part that had not yet dripped out of my hands) when I relaxed the pressure. Similarly, one can fill a bucket with wet sand, compress it to squeeze some water out and pour that water out of the bucket, then add more wet sand to refill the bucket - repeating until no more water can be squeezed out. Then whether the bucket of compressed wet sand outmassed the bucket of wetted dry-compressed sand would depend on the details listed above.

Consider sand particles as big as marbles (~1 cm diameter). Clearly a bucket of compressed dry marbles (case 1) will mass less than a bucket of compressed wet marbles (case 2), since adhering water will remain in the spaces between the wet marbles, and water is denser than air. For this size of sand, this will be true even if a thin layer of water remains between the wet marbles, preventing them from absolutely touching each other so that the wet marbles are microscopically less densely packed than the dry marbles.

Now if one imagines continuously reducing the size of sand particles from marble-size, eventually one reaches a size at which the effect of water film preventing wet particles from absolute contact overcomes the effect of water remaining in the larger spaces. That is, at that size the mass of water remaining in the spaces between sand particles will equal the lower mass of sand particles in the wet-sand bucket due to the wet particles being slightly less densely packed than the dry particles. Then at still smaller sizes, the water's film effect to lower compressibility of particles will be greater than the interstitial water mass, and case 1 mass will exceed case 2 mass.

Question: at what particle size (roughly, perhaps assuming spherical particles) will the two opposing water effects be equal? If this size is in the range most people would call "sand", then the puzzle answer could differ.

[Maybeso]

Uncwilly's mention of dirt and dirt clods got me to thinking:
It appears from the posts that everyone is presuming "clean" sand -- if there were anything soluable in the sand it would behave more like dirt.
So for clarity I will state the assumption the sand is completely insoluable -- right Uncwilly?

Here's another possibility to make you think. What would be the difference between these two buckets:

Bucket 3: Dry Sand packed down then wetted.
Bucket 4: Dry Sand packed down then wetted while still under pressure.

That is, would bucket 3 expand slightly so it was more than full, requiring you to scrape some wet sand off the top?
As the water flows into the sand, the capillary effect would try to force the particles apart.

When they have sand sculpture contests on the beach, do the teams:
add sand to the forms, then pack, add water, pack, add sand, pack, ...?

This discussion implies that would produce the most dense sand. And the least likely to crumble during the sculpting.

[Uncwilly]

Maybeso,

Yes, clean sand. AFAIK, Density=sablity or styrength.

Cheesehead,

I don't have number for you, but yes sand does fit the range where water foces should be factored in.