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RE: Design for Liquifaction (or mitigation thereof)

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Could Harold or somebody expand on "stone" columns?
I've never heard of them, but then this relatively
low wind and seismic country.
Gary

On 8 Dec 2004 at 9:39, Bill Allen, S.E. wrote:

> 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
> >
> >
> >
> >
> >
> 
> _________________________________________________________________ Is
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