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RE: gravel under slab on grade

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If it's dune sand (therefore wind-blown) it will be fine and of uniform
grain size, therefore hard to compact.  I hope the Contractor intends to
use a vibratory roller, otherwise his compaction may be only cosmetic.
Assuming you're in a seismic area, liquefaction of loose fine sand is a
real concern.  One positive note, the vapor barrier and cover will help
avoid disturbing the surface of the native sand due to workmen's feet,
etc., once it is compacted.

> ----------
> From: 	smthengr(--nospam--at)[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 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