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RE: Liquefaction and Slabs-on-grade

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Dear Howard,
 
Liquefaction potential occurs in saturated loose sands and silts and defect zone of liquefaction is the top 15m. The geotechnical engineer should study these case via site investigations, borings, SPT and CPT tests. I saw your sentence saying "water table shouldn't;t be an issue".  Are there any tests performed for monitoring the ground water level including all seasons [highest and lowest levels]? If there is saturated loose material and liquefaction potential, then ground water level should be studied. As Hunt said negative skin friction will adversely affect the pile capacity during an earthquake.
 
Consider execution of stone columns against liquefaction instead of piling [piling is an efficient method against earthquake, but not for liquefaction] . This method is an old, classical but very effective method, and still one of the most popular methods all around the world against liquefaction. Jet-grouting can also be considered.
 
Regards,
 
Yavuz Seymen
Dar Engineering and Consultancy 
 
 
 -----Original Message-----
From: Tripp Howard [mailto:tripphoward(--nospam--at)yahoo.com]
Sent: 16 December 2003 22:36
To: seaint(--nospam--at)seaint.org
Subject: Liquefaction and Slabs-on-grade

I need a sanity check on a method I'm proposing to use for the foundations of a new building I'm designing.  The building will be located in Charleston SC.  The geotech report indicates that the site may settle "2-inches or less and be relatively uniform in nature" due to liquefaction.  Furthermore, the building could settle up to 3-inches due to the weight of fill needed to raise the building pads.
 
The report recommended 2 options for foundations.  The first would be to surcharge the site.  This would alleviate the 3-inches of dead load settlement due to the fill, but would still allow 2-inches of settlement due to liquefaction.  The second option would be to support the building on pile foundations which would eliminate both the dead load settlement and the liquefaction settlement. 
 
The method I've proposed to use is a combination of these 2 options.  The main structure (i.e. columns, walls, etc.) will be supported on pile foundations.  However, I'm proposing that the building pads be surcharged anyway so the floor slab can be supported on grade instead of piles. 
 
My reasoning is twofold.  First, even with the extra cost of the surcharge, I think a grade supported slab has got to be cheaper than a pile supported one.  Secondly, even if it does settle 2-inches in a large earthquake, it wouldn't pose a life safety issue.  Some repairs might have to be made to the slab after a large earthquake, but it won't be the only thing needing repair.  The new building is going right next to another soon-to-be-constructed building (separated by an isolation joint) which also has a grade supported slab.
 
Anybody see anything wrong (i.e. code violation, non-standard, etc.) with allowing the slab to settle during a large earthquake?
 
Any and all comments are welcome, especially from some of you west coast seismic guru's out there.
 
Thanks for the help.
 
Tripp Howard, P.E.
 


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