Liquefaction Factor of Safety
Liquefaction Factor of Safety
(OP)
Hi All,
I am taking a poll to see what people are using for a factor of safety against liquefaction in California. We are in the Central Coast region, and it seems that many soils engineers use a factor of safety of about 1.1 to 1.3 for single family dwellings. The local jurisdiction requires a FS=1.5 for liquefaction. I have not been able to locate any FS requirements in the 2007 CBC at this time (please let me know if there is one..)
Is this overkill, or just a conservative requirement? What is the professional consensus for liqefaction FS?
Thanks in advance!
I am taking a poll to see what people are using for a factor of safety against liquefaction in California. We are in the Central Coast region, and it seems that many soils engineers use a factor of safety of about 1.1 to 1.3 for single family dwellings. The local jurisdiction requires a FS=1.5 for liquefaction. I have not been able to locate any FS requirements in the 2007 CBC at this time (please let me know if there is one..)
Is this overkill, or just a conservative requirement? What is the professional consensus for liqefaction FS?
Thanks in advance!





RE: Liquefaction Factor of Safety
I'm in the Bay Area and basically everything Holocene liquefies if it's saturated and not clay. We look at differential settlements (1/2 to 2/3s of the expected total displacement - though I've seen people use the difference in liquefaction settlements between adjacent borings as the differential settlement).
RE: Liquefaction Factor of Safety
I haven't found anything about this in the 2007 CBC and SP 117 suggests some values, but with no firm recommendations. Any help is appreciated.
RE: Liquefaction Factor of Safety
Your factor of safety should really come from the return period of the earthquake you're considering (IMO). If you calculate the settlement from liquefaction due to the 475 year earthquake, take half of that and apply it across the column line distance (or center of mat to end of mat for slabs) and the structural engineer indicates he can live with that, I'd propose that to the local building official. If the structural engineer can't live with it, but indicates it won't cause the building to collapse and liquefaction mitigation costs more than the building would, then I'd argue it would be cheaper to replace the building than mitigate the settlements.
Or you could propose 50-year earthquake return period and 1.5 factor of safety.
It's the 00's baby, everything's negotiable.
RE: Liquefaction Factor of Safety
That said, the simplified liquefaction triggering analysis has about a dozen sources of significant uncertainty, and H.B. Seed never considered the triggering curve to be a hard boundary. FS=1.1 may not provide much of a margin, but don't ask me what number is appropriate. If not for the code, you would look at the probabilities and the consequences. My guess is that they are low enough that an expensive foundation treatment could not be justified by any kind of risk calculation.
RE: Liquefaction Factor of Safety
RE: Liquefaction Factor of Safety
RE: Liquefaction Factor of Safety
RE: Liquefaction Factor of Safety
Factor of safety for liquefaciton is generally defined as the ratio of Cyclic Resistance Ratio (CRR) (mostly a function of density of the sand) divided by the Cyclic Stress Ratio (CSR) (mostly a function of acceleration and in-ditu stresses in the stratum of interest). That makes no sense to me, but that's how it's defined. Ray Seed and his co-workers provided various curves of probability of liquefaction - which while interesting, are also of little practical use.
Settlements are all based on lab tests on clean sands which you don't see much of in real life either. Generally, the numbers you get make you think that any stratum more than 1000 years old would be so dense it could never liquefy again - but that doesn't appear to be the case in real life.
Structures don't suffer (much) from just settlement. It's differential settlements that cause much of the damage and there isn't any sensible guidance for determining how much differential settlement occurs over what distance.
If you have a 20-foot thick liquefiable strata 30 feet down with a 20 foot cap of stiff clay over it, is that a problem? I don't personally think so, but my opinion isn't going to help you much with a building inspector who wants a factor of safety of 1.5 against liquefaction.
I think you have to use judgment. Which unfortunately allows clients to shop around for a geotechnical engineer who's judgment is for sale.
RE: Liquefaction Factor of Safety
RE: Liquefaction Factor of Safety
I have no idea how the building code enforcers feel about all of this - a lot of people seem to view Seed-Lee-Idriss as deterministic. (For most of my work, I have the luxury of ignoring building codes, as long as I can get it past a board of heavyweight reviewers.)
RE: Liquefaction Factor of Safety
For those of you who have Day's book, how do you feel about it? Personally, it was $120 that I wish I had back. Every time I pull it out looking for something, I never find it. So, not only did I waste the $120, it's wasted several hours of my time too.