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Re: gravel under slab on grade

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Keep the gravel.

The gravel serves the purpose of allowing ground water to flow
horizontally rather that swell up the soil (not so likely with sand).
The vapor barrier keeps the moisture out and the sand allows for a
cushion from expansive materials.

If he bid the job with the, have him put it in, sand or no sand base.
If you want to take it out, have the a soils engineer make that
decision, (the contractor should pay for this if no soils engineer on
the job initially).  If it is warranted by the soils engineer to take it
out, ask the contractor how much he is saving by eliminating it, then
tell him you want that much credit back for the owner.


Jeff Smith wrote:

> I have a situation where I have specified a 5" reinforced residential
> garage slab over 2 inches clean sand, 6 mil vapor barrier and 4"
> crushed
> aggregate. As it turns out the existing subgrade is sand, probably
> dune
> sand from the area that was brought in as fill 100 years ago. The sand
> is
> fairly clean, very deep and appears cohesionless. The contractor wants
> to
> pour the slab without the gravel base. He wants to moisture condition
> the
> existing sand and compact it with a light roller, then place the V.B.
> directly over the sand and place the 2" sand cushion over the V.B.
> He believes the gravel is not necessary due to the existing sand
> subgrade.
> We all know he simply wants to save money, but is the gravel omission
> justified. Any thoughts on this?
> Regards,
> Jeff Smith. S.E.
> phone: (415) 543-8651
> fax: (415) 543-8679
> email: smthengr(--nospam--at)
> Smith Engineering
> 27 South Park
> San Francisco, CA 94107