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RE: Design for Liquifaction (or mitigation thereof)

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Dave- 

You're right. Without utilizing some form of soil improvement technique, the
liquefaction potential still exists whether the proposed foundation is a
grillage of grade beams with an interconnecting structural slab or a mat
foundation. After an event, there should be an expected significant
differential settlement. This would include an excessive amount of
settlement (still up to 8") and, as importantly, non-uniform settlement
producing an undesirable amount of "tilt" in the floor slab. If I go with
this solution, I will warn/advise the owner (via the architect) that this
condition exists and that costs to repair (i.e., injection grouting) this
situation should be planned for. Further, utility lines leading into the
building should have flexible connections which will accommodate this
movement.

However, there should be minimal structural damage since the "raft" will be
designed to float during the event. The geotechnical engineer is currently
in the process of developing design criteria for this condition which will
provide a distance of loss of support or reduced support. With this
information, I can design either foundation system to resist the changes of
support stiffness. In the end, the foundation should act as a rigid plate
with the building riding on top. FYI, the building is rather lightweight;
one story wood framed structure with wood framed roof.

It was interesting that the gentleman I spoke with at Earth Systems seemed
to think that a mat foundation, even including the cost of injection
grouting after an event, would be cheaper than stone columns.

Regards,

T. William (Bill) Allen, S.E. (CA #2607)	
ALLEN DESIGNS	
Consulting Structural Engineers	
http://www.AllenDesigns.com	
V (949) 248-8588	 .	 F (949) 209-2509	
-----Original Message-----
From: Gale45man [mailto:gale45man(--nospam--at)yahoo.com] 
Sent: Sunday, December 12, 2004 2:59 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Design for Liquifaction (or mitigation thereof)


So...if you go with a mat, it sounds like you may not
design it to resist the expected differential
settlement from liquefaction (based on Hayward Baker's
recommendations).

If that is the case, I guess the rest of the structure
and any heavy architectural elements had better at
least tolerate that large differential (didn't you say
up to eight inches? over what distance?)...to the
extent that that nothing falls and kills someone. 
That could involve some interesting calculations...

d a v e  e v a n s


 



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