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RE: gravel under slab on grade

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-----Original Message-----
From:	Jeff Smith [SMTP:smthengr(--nospam--at)]
Sent:	Thursday, October 09, 1997 8:08 AM
To:	seaoc(--nospam--at)
Subject:	gravel under slab on grade

I have a situation where I have specified a 5" reinforced 
garage slab over 2 inches clean sand, 6 mil vapor barrier and 4" 
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 
directly over the sand and place the 2" sand cushion over the 

He believes the gravel is not necessary due to the existing sand 
We all know he simply wants to save money, but is the gravel 
justified. Any thoughts on this?


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

I have the same situation here in the desert around Palm 
Springs. I have eliminated the gravel requirements after 
conversations with local soils company's since compaction is not 
an issue and drainage is excellent in a sand base. In our area, 
vapor barriers are used primarily to prevent the growth of root 
systems from coming up through cracks in a slab. There is never 
an issue with moisture since water never stands long enough to 
vaporize through the crack and sand is great for drainage.

Check with a local soils engineer for a recommendation. Remember 
too, not all areas within one known for great sand or even 
decomposed rock is consistent. There may be area's where clays 
exist and these areas need protection. Below my home is 
decomposed granite and sand. Yet just down the hill is an area 
of expansive clays that has created problems in construction. Be 
cautious and consult with your local soils experts.

Dennis Wish PE