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foundation on rock

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Thank you all for your comments about my foundation on rock question.

I am looking more for a discussion about the CONCEPT of a non-embedded
foundation on rock rather than comments specific to my project. Overall site
stability is outside the scope of my question at this time.  

The code appears to require a minimum 12 inch embedment for footings
supporting one floor, and most soils engineers will require a minimum of 18
inches.  One of my objectives with this question is to understand all the
reasons why embedment would be required or desired so that I can see if
there are other ways to design around some of these issues. 

To answer some of your questions:  the pad is currently level.  The building
(30'x32')will be located entirely within a "cut" area (no fill).  The pad is
entirely on "natural grade" that is extremely competent for bearing.  The
pad was leveled by a combination of explosive blasting, digging, and
scraping with very heavy equipment. Zone 4 seismic, but no mapped fault
lines, nor recorded seismic events within at least a 30 mile radius.
Adequate drainage away from the house will be provided.  Not the entire pad,
nor the entire rest of the site is "undiggable", but enough of the pad area
is so hard that some blasting would be required for a foundation trench in a
fixed footprint.  

The "mat" concept was NOT for differential settlement issues.  The concept
was for mass (to help with friction and structure stability), and to create
adequate footings for the walls without trenching and without having to
bring in extra fill for the non-bearing slab areas.  It seems to me that the
extra concrete material would be cheap in comparison to the extra earthwork
in time, blasting permit frustrations, and money.

Thank you for the comment about moisture under the slab and the ensuing
reduction in friction.  I also feel generally uncomfortable about relying
solely on friction for anything and would also be inclined to want some
concrete keying into the natural grade or some other method of pinning.

I would also be interested in hearing from anyone with experience with other
types of foundation systems in/on rock that are cost effective and sound, or
those with direct experience with foundations in the far southeast hills of
San Diego County.

Karen E. Casano, SE

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