Wondering what people think of omitting gravel as a base under a radiant slab?
Our sand is pretty damp and we would like to get our slab poured ASAP. Given that we have pure sand my thinking is that the water will drain down (very good drainage on our site). We will add foam and vapor barrier before pouring our slab so there will be even less of a likelihood of the water moving into the slab.
Another question I have is this: once we have compacted the damp sand, will it settle very much as the water drains out?

I have used as much as 5' of sand under a slab.  The sand was dry, contained by block walls and compacted.  Never a problem.  My guess is that if the sand is quite wet there is a chance of some settlement when the sand drains.  I cannot be sure about this because the mositure might just be between the sand particles and not actually adding to the volume like it would with clay.  Since concrete has very good spanning abilities over small areas, there may never be a way to know if the sand did settle.  I have not worked with all of the sand being wet, so my thinking is not based upon experience.
 
I have placed sand on top of the vapor barrier and added limited water to it to help it pack and also to keep it from robbing the water from the concrete without any problems.  This practice is now discouraged because of concerns about molds.

If you are concerned about large areas settling, then I suppose a rebar grid would support the concrete and also provide for a way to tie the radiant tubes in place.  I would much prefer to use a rebar grid than wire mesh to reinforce a slab.

I know the rule about whether dirt is too wet for compaction but I have not heard of such a rule for sand.

I'd want to know why the sand is so wet in the first place. If it's just due to weather exposure and would drain/dry if under cover, that's one thing. If it's due to high water table, I might be inclined to build up the slab area with crushed stone (aka "gravel" in some places) before adding the foam and VB. If the area is on a generally sloped lot, a perforated pipe within the stone to drain the area to daylight downhill somewhere would be good.

As to putting sand, wet or not, directly under the slab, yes, that's a no-no. Buildingscience.com has a lot of good articles on the subject. Here's one:
http://www.buildingscience.com/documents/insights/bsi-003-concrete-floor-problems/?searchterm=no%20AND%20sand%20AND%20layer%20AND%20under%20AND%20slab*

Just to be clear, a layer of plastic solves the issues with sand under the slab. Ie, sand, plastic, concrete is OK except that it doesn't provide for radon removal (which may not be a problem in your area).

you say your sand is "wet' so if at all possible you need a drain at a lower elevation to carry away any water before it gets to your floor. If the water gets to yoru floor elevation, it could be a problem, so now is your only opportunity to keep it away permanently.

The sand is damp mostly, from being damp when it was put into the basement and now the top layer is starting to dry, although it is still winter here (Quebec) so not a whole lot of heat to help with that! The water table is much lower than the level of the floor of our basement. We will definitely lay down some plastic before the foam.
Thank you all for the input.

Sand has a much higher capillary draw than gravel, and can seep into & fill perforated drain pipes, etc. Directing roof drains well away from the house is the first line of defense, but I personally wouldn't feel right about not having perimeter drain at the footing as well, even with sand that seems to drain well on it's own.

Absolutely. We have a perforated drain around the perimeter of the house and will be doing the best we can to direct water away from the house (we have generous overhangs as well). My main concern right now is whether the sand (under the poly, foam and cement slab that we will be putting down in a week) will settle as the dampness moves down. My thinking is that the sand is damp now because it was damp when it was put in the basement in late November and that there has not been adequate heat to evaporate the moisture. We put in 10 inches of sand, by the way.