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RE: Design for Liquifaction (or mitigation thereof)

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None of the options mitigates liquefaction itself, and then you have to predict (crystal ball technique) the soil movement. Some of the techniques will minimize liquefaction, but they do not mitigate it.

What about stone columns? They are cheap and mitigate the problem. If the geotech does not know about these, get a new geotech. There are many structures around the world that use stone columns to mitigate liquefaction. For your soil conditions they may not be appropriate, but I would at least consider them.

Regards,
Harold Sprague




From: "Bill Allen, S.E." <T.W.Allen(--nospam--at)cox.net>
Reply-To: <seaint(--nospam--at)seaint.org>
To: <seaint(--nospam--at)seaint.org>
Subject: Design for Liquifaction (or mitigation thereof)
Date: Tue, 7 Dec 2004 15:44:13 -0800

Dear Colleagues;



I'm looking at a new project where liquefaction is an issue. According to
the soils report, total induced settlements, should liquefaction occur, are
estimated to be approximately 3 to 8 inches.



The geotechnical engineer has suggested that I consider the following five
options:



1.	Densified potentially liquefiable sand/silt layers at 10 to 40 feet
depth by use of vibro-compaction, vibro-replacement, compaction grouting, or
deep dynamic compaction.
2.	Deep dynamic compaction of the upper 40 feet of soil by use of
falling weights.
3.	Foundations that use grade-beam footings to tie floor slabs and
isolated columns to continuous footings (conventional or post tensioned).
Flexible connections for utility tie-in required.
4.	Structural flat-plate mats, either conventionally reinforced or tied
with post-tensioned tendons.
5.	Deep foundations (drilled piers, geopiers, stone columns or piles)
founded at a depth of 40 feet.



The geotechnical engineer has no experience with option 1. He thinks option
2 will be unacceptable because the project site is in a populated area and
dropping 2.5T weight from 40 feet might be objectionable. We both believe
option 5 would be cost prohibitive for this project.



For some background, the structure is a one story medical office building in
UBC/CBC country. The site is 15.6 km from a Type A fault. Groundwater was
encountered at a depth of 9 to 15 feet. The soil is classified as 5 feet of
sandy silt over 10 feet of silty sand over 5 feet of sand.



A foundation I recently designed for a similar structure can be found here:



http://www.allendesigns.com/projects/20325/20325S11.pdf



The geotechnical conditions were different in that this foundation was
moderately expansive and was designed using UBC section 1815. However, you
can see the types of loads from the foundation design. The exterior walls
are bearing walls and there is a center girder line with columns and pad
footings. The proposed structure will be framed similarly.



If it helps, the roof framing plan can be found here:



http://www.allendesigns.com/projects/20325/20325S21.pdf



In talking with the geotechnical engineer, I told him that I would have to
charge the client extra $$ if I designed using either option 4 or 5. I only
included conventionally reinforced UBC 1815 type footings in my Basic Scope
of Work. He said he thought that a grade beam foundation would be acceptable
and he referenced a subgrade modulus in his report of 200 pci and an
allowable soil bearing pressure of 2,000 PSF.



One of the problems I'm having is that, for a uniform pressure of 2000 PSF,
based on k = 200 pci, I'm only getting a deflection of 0.07 inches. I guess
I was expecting something like 50 pci or an allowable soil bearing pressure
of 500 PSF during a seismic event. Fortunately, I only have to consider dead
load since this is a one story building. FYI, the proposed building is
approximately 141 feet x 225 feet.



Now my question(s):



1.	If I choose a grade beam system similar to the job I recently
completed, what should I use for methodology/criteria in designing the grade
beams?
2.	Any other observations/recommendations?



TIA,




T. William (Bill) Allen, S.E. (CA #2607)


ALLEN DESIGNS


Consulting Structural Engineers


 <http://www.allendesigns.com/> http://www.AllenDesigns.com


V (949) 248-8588

.

F (949) 209-2509






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