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RE: foundation on rock

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Compacted decomposed granite has a pretty good subgrade modulus so I
wouldn't anticipate much differential settlement.  Especially since you said
it was a small structure.  That's a pretty strong slab, too (assuming you
reinforce it.)  If settlement of the D.G. vs the rock is an issue, you could
replace the D.G. with a lean concrete mix.

I love foundations on rock!

Bob Garner, S.E.
(Sandy Eggo, too)

-----Original Message-----
From: Casano, Karen [mailto:Karen.Casano(--nospam--at)dgs.ca.gov]
Sent: Tuesday, February 08, 2005 2:14 PM
To: 'seaint(--nospam--at)seaint.org'
Subject: foundation on rock


Fellow Engineers (especially those with a geotechnical bent):

What potential performance problems might you anticipate with placing a 12"
thick mat foundation (for a small structure) ON TOP OF an existing natural
grade?  Existing natural grade is mostly rock (very dense granite which
would require blasting to remove or trench into), some sedimentary rock and
compacted DG, no frost issues (southern CA). Compacted fill would be placed
around the finished perimeter of the mat to provide drainage.  Lateral loads
resisted by friction.

Karen E. Casano, SE
San Diego


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