I haven’t thought of this for decades, since I was young enough to build a sandcastle myself, so I don’t know what made me think of it now. But I remember that if you build a structure out of damp sand, and then sprinkle over some dry sand and tamp it in, the structure seems to become much harder and much more solid.
Can any of the civil engineering types tell me why that works, and if the same principle is used in full-scale building projects, in any way.
Because the sand takes up the excess water and thus eliminates the lubricity of the other wet sand is my guess.
I think I read someplace that the ideal water to sand ratio was something like 1:8. That is actually pretty dry sand. You were probably using too much water to begin with, and as Tuckerfan says, the dry sand absorbed the excess water.
IANACE though.
Mmm, lubricity.
It should be a city in Arkansas (for those unfamiliar, cities have signs up proclaiming to be the "Boyhood home/birthplace/residence of Bill Cinton) so we could make slogans.
Lubricity - Home of the World’s Largest Slip’n’Slide!
Lubricity - Birthplace of Ron Jeremy
Lubricity - Host of the Female Ejaculator Olympics
French Lick, Indiana beat 'em to it.
But, you can’t build a castle out of completely dry sand, either. In a given building material, what determines the best degree of moisture?
Is that a sister city to Big Bone Lick, KY? (Totally worksafe link, BTW.)
That’s determined by the composition of the sand. For doing green sand casting (which isn’t all that different than making a sand castle), the rule of thumb is that if you squeeze the sand and get any significant moisture out of it, it’s too wet. To correct this, you mix in either more sand or binder (which can be anything from flour to complicated chemicals, depending upon how sophisticated your operation is).
There’s certainly optimum ratios out there, but I don’t know what they are, having only learned the “check it and see” methods.
Optimum Moisture Content is the term you’re looking for. This is different for different soils.
One method of determining this is the Standard Proctortest. In specifying density, usually I will say “compact soil to 95% Standard Proctor”. That way, I’m not specifying a moisture content but a density. It’s up to the dirtwork contractor to get the moisture right.
You may also be interested in the Atterberg Limits. These limits mark the moisture contents at which a soil changes from one phase to another (solid, semi-solid, plastic or liquid). Different liquid limits and plastic limits tell us whether or not a soil is suited for use as a building material (compactibility, permeability, strength).
I figure this is why concrete starts as a more liquidish substance, and then hardens to the rock-like material we all know and love.
I have a question, how dry does concrete get when it dries? Does it still contain a reasonable amount of moisture, or is it pretty good and dry?
Doesn’t the curing of concrete and cement have more to do with bonds being formed by hydration than the material drying out?
Yes.
It does still contain some moisture. I have not been able to find a cite for how much - I don’t have my concrete book anymore.
Correct. It is a chemical reaction, not a “drying out”.
For a sand castle, you want just enough water that surface tension holds the grains of sand together, but not so much that the grains can float about.
Thanks for all the informative answers.
Regarding ‘curing’, when I toured the Hoover Dam I remember the guide saying that it would take ab 1500 years to cure fully. It was interesting to see corridors, work rooms, and offices inside the damn, just like any other industrial facility, except in this case the structure is resisting the pressure of millions and millions of gallons of water.
Fascinating stuff.
The optimum water content is the best water content for making sand castles in? I don’t believe that’s true. I would think the best would be well on the dry side of optimum. Sand castles can be built because of capillary action and the associated suction. Negative porewater pressure is much higher in soil that is on the dry side of optimum than it is in soil at the optimum moisture content.
With respect to sand castles, the ammount of clay in the sand, the grain size along with the size range and regularity of the of the grains can have quite a significant effect in determining how secure the structure will be and the water content required to make it thus. Coarse, uniform, regular grained sand with minimal clay content is very difficult to make anything reasonable out of regardless of water content. IANASCE.
My answer was in response to this question:
Notice he asks “in a given building material”, not just sand - hence my link.
Agreed. In a way, the capillary action does for its stability what cohesion does for a soil containing clay.
I took a quick look to refresh my memory on this and all I could find was a study for Indiana DOT which mentioned using a well-graded sand at 12% to 15% moisture content, with optimum being 15%. On the other hand, I don’t remember covering sand castles in-depth in soil mechanics class. 
Ah, sorry.
What was the use of this sand?
In any case, recall that the capillary pressure is inversely proportional to the radius of the meniscus formed between two soil particles. If there is more water the meniscus will be larger and the suction smaller.
The research study was on ‘Highway embankments and subgrade’. From [here](The field portion of the study consisted of construction and instrumentation of a 30-foot high embankment. The embankment was divided into three sections: a reference sand control section, a clay control section, and the green sand test section.).
I’ll just have to look it up if I need to (or ask you). I’m using my brain memory space for useful stuff like fluvial morphology.