IMO any granular fill immediately below a concrete slab should be compacted. Tamping would be acceptable if the sand layer is not thick and any lower sub-grade is either undisturbed or coarse fill has already been machine compacted.

Agree with Ren and would add that sand is a poor choice of granular fill below slab unless its just being used as a thin protection layer on top of the gravel to protect the vapor retarder. Sand is not a very good capillary break.

Sand on top of a compacted gravel base is used here to provide a barrier from termite mud tunnels. All layers MUST be machine compacted. If the inspector can insert a piece of rebar vertically into the base without hammering it in, then you fail inspection and can't pour until that's corrected.

What is the nature of the undisturbed soil, the type and thickness of all fill layers and the type and location of the vapor retarder?

The thickness is around 3-4ft of sand over 3500psi undisturbed soil. The walls are poured with backfill around. My builder says tamping now will damage the foundation. He says the rain will settle it so concrete can be poured. Since we are putting radiant into the garage, I am very concerned about cracking of the concrete slab. Thanks for any info!

I wouldn't be comfortable with that fill. At all. Why wasn't gravel used?

I think Bus has it right. I would not worry about the foundation with a plate compactor. I have way to many hours on the wrong end of one of those and it doesn't have the power to drive that foundation.

As you are talking about adding radiant tubing tot he garage I would be very careful with the elevation of the sand. You are likely going to want a couple inches of polystyrene insulation under that slab. More importantly you need to isolate the edge of the radiant floor from the foundation. Otherside you will create a themal bridge that will draw the heat from the garage floor and substantially increase operating costs. I would get my heating guy in to view this set-up and get this comments prior to the pour. I've seen slabs without the isolating slab edge insulation and if yu take a thermal image of it the slab edge is bright red.

I don't see how that much sand can be machine compacted as it is. Wetting it will increase compaction but the slab should be designed to settle a bit. At least it's not your house floor.

Sand fill can be compacted by either vibration or impact. However, that would have to be in "lifts", that is, in layers of six-12 inches thick.

Before I pour garage floors, we first dump in any leftover clean fill--broken bricks, boulders etc.--then water it like crazy if it's available. Then one or two lifts of compacted gravel, then sheets of steel mesh.

I'm not sure that a little random rain on four feet of sand will achieve much compaction.

This post was edited by worthy on Wed, Jul 30, 14 at 12:36

Good points. I think you would need lots and lots of water to make a difference with that size of sand pile.

Hopefully there are plans for insulation under the radiant tubes. Radiant tubing on grade without insulation below and at slab edges is incredibly wasteful.