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Re: foundation on rock

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On 8 Feb 2005, at 14:13, Casano, Karen wrote:

> 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

Elevation issues and lateral load issues are the two things that I would
worry about most (assuming that the rock is sound). 

As James Lutz asked, "Is it level?"  If the site is not level and you have
to fill over the rock to achieve a flat building pad, then I would have some
concerns about how that was accomplished.  If the site has to be leveled,
then what is done with the surrounding rock that is at a higher elevation
than the building pad, or is that pad at the crest of a hill?  Drainage is
my primary concern with elevation, especially where friction forces are
to resist lateral loading.  If you've got water between the foundation
and the rock, then you're lubricating that contact and reducing the
friction force.  I would rather see the foundation keyed or otherwise
anchored to the rock so that lateral loading won't cause sliding.

Lateral loading issues are the other concern, with resistance to
sliding being top on the list.  Seismic wave velocities through
rock can attain speeds around a mile per second.  This will
make for high momentum and impulse on the rock/foundation
interface.  Can the friction resist this?  Also, can the structure handle
the loading being transferred up into it from the foundation which
will be moving at the same rate as the rock?

Hope this doesn't muddy the water too much.

Take care,
Lloyd Pack, P.E.

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