Definition of Significant Settlement (CA) 1

[cbear1]

Hi all,
I have a question regarding the definition of "significant settlement" for foundations in California. Basically, a soils report has defined a liquefiable layer of soil from about 10' below grade to 21' below grade. In a seismic event they anticipate about 1.3" of settlement due to liquefaction. They have not suggested any foundation mitigations since they say that the bearing capacity should not be lost due to the 10' of non-liquefiable materials over the liquefiable layer (although I don't see how you can assume that settlement will be uniform with the two borings they did - it seems local discontinuities could be a possibility) and that the structural engineer has deemed this settlement "not significant". My questions are these:
1. What would a structural engineer deem a "significant" settlement for a residential structure with a conventional spread footing? (and has anyone found this in the 2007 CBC?)
2. How close should borings be placed to ensure uniform settlement? (or should you just assume a conservative 2/3 differential settlement and design accordingly?)
Thank you in advance for your help!

 

[fattdad]

The biggest problem with settlement is differential settlement. Total settlement can affect utility connections if it gets too great, but design solutions can address this concern.

If you have a compressible layer at 10 to 21 ft below the foundation it's unlikely that the nature of the settlement will be diffential. I'd expect the nature of the problem to be a concern for total settlement.

Here's the problem with greater number of borings: You may just get alot of data, but it won't be very refined data. For the case of liquifaction, the issue is to "determine" variations in the void ratio of the soil in the liquifiable layer. Standard borings just don't do this. Rather the engineers use somewhat crude correlations to arrive at liquifaction potential. I don't fault this process as the "actual" study is just too complicated.

I'm not sure I'm helping, but if you have trusted engineer and there is local experience on this soil layer (or similar soil layers) in light of the design earthquake, you're likely o.k.

This is somewhat easy for me to address from my sofa in Virginia, but that's some thoughts. . .

f-d

¡papá gordo ain’t no madre flaca!

 

[DALIAMF]

thank you

 

[muuddfun]

From my experience here in southern california where everything liquefies if it is granular and below the water table, 1.3 inches of settlement is not that much, especially if you have 10 feet of good material over it to maintain some bearing. It is just not worth it very often to do a lot of closely spaced borings to get a better handle on the differential settlement potential because the cost is hard to justify to clients if the variablility in the SPT etc is going to exceed the margin of settlement you are going to calculate. Even if you assumed a differential settlement of 100 percent of the total settlement you are only bearlly going over what is usually considered tolerable for conventional foundations, and if you take it at 2/3's it gives you about 0.85 inches, I am assuming in 40 feet, it's not that hard to deal with.

If the settlements get larger then it makes sense to go in with CPT, because the reality of what you can get for settlements and bearing of liquefiable layers is better by getting the more continous data, the SPT generally gives the more conservative answer mainly due to the spacing of the samples.

As for the spacing of borings, there is always the possiblilty of discontinueties in the underlying strata, and the spacing needed is totaly a function of the particular site and what you are going to build. Two borigs for a small site may be just great or you could miss everyting entirely, that is the chance we have to take in this business.