Liquefaction Assessment | SPT 1

[bookowski]

Question about assessment for liquefaction based on SPT from Fig 1813 in IBC. What percentage of points would have to fall beyond the threshold to consider that there is potential for liquefaction?

My situation:
- On our site we have two borings, within the range of the water table to the 50ft. extent of the chart we have 18 data points of which only 1 falls over the line. Our geotech generally feels that this data point can be discarded but is looking into it more.
- The neighboring site is a recent build and we have their borings as well. They have about 60 data points in the relevant range, of which 5 fall just over the line. The goetch on that job has classified it as potentially liquefiable and had them do compaction grouting.

Based on both sites it looks like somewhere between 5% to 10% could cross the threshold.

(Note that this is in an area of generally low seismicity)

The geotech will be making the call - I'd just like to be more informed about what is correct/standard.

 

[dgillette]

One caution on the "threshold": If you are getting the threshold N values from the 1997 NCEER report (also published as Youd et al in the 2001 ASCE Journal of Geotech Engrg.) or most other sources, the cyclic resistance on the curve represents about 15% probability of liquefaction, not ~zero. That important fact got lost in almost every publication on the subject prior to about 2003. It was like the leading researchers knew it, and just assumed the readers would also. If you need a reference, I can dig up a web link for you.

That said, the answer to your question is in large part geologic - does the mode of deposition cause you to believe that there could be continuous layers of loose stuff, or is it just an isolated pocket here and there? If I were trying to answer that question for my own project, I would be drawing cross sections and trying to look at the low N values spatially. I'm not aware of any accepted rule on the topic.

Cheers!
DRG

 

[EireChch]

Hi I am a not sure of what you mean by threshold ? Is it a SPT-N value of less than X amount is generally liquefiable? Is this a common method for assessing liquefaction without doing an actual analysis?

It could be a case that you have some looser layers with lower N values that may be liquefiable. Depending on depth this may not be a big issue.

Could you supply the bore logs and SPT values? DRGs comment is valuable too, I would draw cross sections an plot the stratigraphy between boreholes. Assess the geology and develop the geotechnical model better.

 

[fattdad]

is there some reason the OP doesn't provide the soil profile and the actual city of the study?

f-d

ípapß gordo ainÆt no madre flaca!

 

[bookowski]

Thanks for the input. As I noted I am the structural and I was just trying to get some more input to help in understanding the geotech's interpretation. Eire - I was referring to the IBC graph delineating 'potentially liquefiable' vs 'unlikely'.

The end result was that the goetech gathered additional borings and decided that the number of points in the 'potentially' category was small enough to dismiss.