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RE: Design for Liquifaction (or mitigation thereof)[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Design for Liquifaction (or mitigation thereof)
- From: "Bill Allen, S.E." <T.W.Allen(--nospam--at)cox.net>
- Date: Wed, 8 Dec 2004 11:05:16 -0800
Yes, he is (located in the desert). It’s LandMark. We worked together on a similar project in Mecca, but that condition was a UBC 1815 foundation.
Now he’s suggesting to over-ex 10 feet to reduce displacements. The liquefaction zone is between 8 and 20 feet. He is saying if we over-ex 10 feet, the settlement would reduce to 2-1/2”; if we over-ex 13 feet, the settlement would be 2”; if we over-ex 15 feet the settlement would be 1-1/2”. Of course, since ground water may be as high as 8-1/2 feet, dewatering may be required. He seems to prefer this over stone columns (which he calls an “exotic” solution). It seems to me that over-excavating this much, dewatering and recompacting might be more expensive than piles or stone columns.
In any case, I told him that I would need design values for the foundation (GBs to span “X” w/o support and/or reduced allowable soil bearing pressure, etc.). He’s working on those values now.
Somewhat related, I asked him if I would have to design the slab for a hydrostatic pressure if the footings were not pile supported or if I had to design the slab to span between grade beams if the slab was pile supported during an E/Q event. He said “yes”, so I asked him to also provide a design hydrostatic pressure. Of course, if this were a mat foundation, this issue would be insignificant.
Joe, did/do you design your slabs as structural slabs spanning between grade beams?
Was the Geotech locate to the desert area ?
If not I would consult with one of the local Geotech in this area.
You may want to talk with Greg McCombs S.E. at Riverside County Building Dept. in Indio
he may be reviewing your project. Or it may be Kack Sung P.E. in Riverside.
As Dave stated, back when I was doing soils work we use to recommend grade beam grids of 8 - 10' oc both ways and total loss of support within any given grid. From my experience most Geotech have a poor understanding of structures.
From what I understand in this country we don't have any bodies(bldgs) to judge by.
Most of the bodies we have are usually related to broken water lines beneath structures and you find out when you see water or settlement. But that occurs over time days - weeks-months not a 2 min. earthquake.
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