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Re: Liquefaction and Slabs-on-grade

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A couple of things you should discuss with your geotech.  To have liquefaction potential you need to have saturated sandy soil.  This does not necessarily mean at the surface but could be a wet sand layer perhaps 20 feet below grade.  If this sand layer is the cause of the liquefaction settlement then this could cause some significant down drag and loss of skin friction on your piles during a seismic event.  I would look closely at what is exactly causing the liquefaction and where is the suspect soil layer(s).  Surcharging the soil is unlikely to help much with your liquefaction potential.  I would also discuss with your geotech how much differential settlement they expect between your pile foundations and your floated slab.  I worked 20 years in a building with this system and the floors ended up like swiss cheese from all the holes required for pressure grouting to keep the floor level.  It may not be a life safety problem but can sure piss off the client.

Thomas Hunt, S.E.
ABS Consulting

Tripp Howard <tripphoward(--nospam--at)>

12/16/2003 02:45 PM

Please respond to

Re: Liquefaction and Slabs-on-grade

Thanks for your reply.  To address your comments:
1. Equipment will be pile supported if needed.
2. Slab will be separted from structure with isolation joint
3. Water table shouldn't be an issue
4. I asked geotech if surcharge would eliminate the settlement due to liquefaction also.  He said it really wouldn't help that at all.  I don't know if "super-surcharging" would help but at some point the cost of piling on more dirt will surpass the cost of just using a pile foundation.
I'm going to a review meeting on this project tomorrow and I guess I'm a little worried that someone's gonna ask "Hey, does the code (IBC2000) allow us to do that"?  I can't see where it forbids it, but I'm looking for a little reassurance.
Tripp Howard, P.E.

Padmanabhan Rajendran <rakamaka(--nospam--at)>

I don't see anything wrong with your approach. However, there may be a few items that need to be addressed.
1. If the slab supports any sensitive equipment, that equipment may be supported on a pier or pile.
2. The slab must be separated, with a separation joint, from the walls and the columns and from any other equipment foundation.
3. If rise of water table is possible, the slab/wall and slab/column joint will require waterstops.
4. Potential for liquefaction implies that the soil is predominantly sand or silt. If sandy, most of the settlement due to loads will occur immediately after construction.  If soil is silty, compact the soil below the slab at a pressure much higher than what the surcharge pressure would be. That should take care of load settlement, at least, to some extent.

Tripp Howard <tripphoward(--nospam--at)>

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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