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RE: Design for Liquifaction (or mitigation thereof)

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Harold -

Thanks for your response. BTW, I for one appreciate your contribution to the
list. All of your posts have a high content to noise ratio but still manage
to include entertainment (i.e. as in sand pounding).

I agree that a mat or grade beam solution does not inhibit the effects of
liquefaction and I'm not sure if the owner (County of Riverside) will be
willing to accept a project that might change elevation 3"-8" after an
event. 

I'm only obliquely familiar with stone columns. From a structural
standpoint, they work like piles, right? IOW, the foundation spans between
the stone columns during a seismic event, right? Aren't they at least 3 feet
in diameter? If so, then it seems to me that a grade beam grillage would not
make as much sense (grade beams would have to be about four feet wide,
right?) as a mat where the mat would act as a two way slab sitting on a grid
of stone columns, right?

FWIW, the geotech did list stone columns in his report. In option 5, he
listed drilled piers, geopiers, stone columns or piles founded at a depth of
40 feet as an option. It was more than likely me who discounted the cost
effectiveness of stone columns for a project like this. If CIDH piles 24" in
diameter w/ 3 foot wide grade beams or precast drilled piles 18" in diameter
w/ 30" wide grade beams were used, do you believe that stone columns are
still more cost effective on a project of this size? One of the problems I
have with grade beams with or without piles is the slab. Is it designed as a
structural slab spanning without soil support between grade beams due to
either design live load (down) with piles or some level of hydrostatic
pressure (up) without piles.

Thanks,
	
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: Harold Sprague [mailto:spraguehope(--nospam--at)hotmail.com] 
Sent: Wednesday, December 08, 2004 8:50 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Design for Liquifaction (or mitigation thereof)

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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