Corrected N-values for seismic Site Class?

[DRB71]

I have never been clear on whether N-values should be corrected for hammer type, rod length, etc. when determining the seismic site class. So, we recently purchased a copy of ASCE 7-10 hoping it would shed some light. As I read ASCE 7-10, it clearly states that uncorrected N-values should be used for determining the site class, and refernces ASTM D1586.

ASTM D1586 states that variations in N-values of a 100% or more have been observed with different SPT equipment and drillers in adjacent borings conducted in the same soil conditions.

The inherent variabilty in SPT is no surprise, and we routinely correct N-values in other evaluations.
So, why are we not correcting them when determining site class?

Any thoughts/clarrification is appreciated-thanks.

 

[moe333]

I have the same issue with site class determination - it should be based on corrected N-Value. It is a problem that needs to be resolved by IBC.

 

[dgillette]

Disclaimer: The following comments are based on no actual factual direct knowledge of site-classification procedures:

The issue of correcting N or not depends on being consistent with what was done in the first place, in developing the method, which I would suppose is based at least in part on correlation between blowcount and SWV. For a given soil at a given density, N (unadjusted) and SWV (unadjusted) both vary with overburden, so in liquefaction analysis, both are adjusted by overburden to provide an index of density (and therefore an index of liquefaction potential). However, it is the actual value of SWV that matters in the response analysis, and presumably in the site classification. My guess, therefore, is that you should not apply Cn. The short-rod correction can be a very big deal near the surface, but do the top 10 or 20 feet even matter much for the site classification? Unfortunately, it may not be easy to dig up much information on how/whether things like short-rod corr were incorporated.

Even if there were no adjustments made in the original work, you still might want to adjust sometimes. For example, the method may have been developed from safety hammers with rope and cathead. If you are using an old doughnut hammer or a CME auto hammer, you might want to adjust for energy to get it closer to the original.

Can you get site classification from CPT instead? That automatically makes hammer energy and short rods go away.

 

[DRB71]

Thanks dgillette, I did not consider previous methods which were likely used to develop a correlation between N and SWV.

I feel that hammer corrections are warranted. However, I suspect that the committee responsible for developing this standard must have considered this issue. Unfortunately, ASCE 7-10 does not provide commentary on this particular topic.

Seismic CPT is a great option. In general, CPT is not widely used in my area (New England), yet.

Thanks for your comments.

 

[moe333]

It makes sense that the classification method would have at least in part been based on correlation between N Value and SWV. I agree that Cn probably shouldn't be applied since it is common to both SWV and N Value. But I think the short rod and hammer energy corrections can make a significant difference in the calculated site classification.

I'm not sure how the classification method was originally developed. My guess is that it is based on N60 but I may be wrong.

 

[msucog]

From my understanding and interpretation, I only correct for autohammer to rope & cathead hammer. I don't remember the specific language but that was my interpretation. Besides after that, I think you're trying to put too many decimal places on something that should not depend on one single borehole. There is much conservatism built in to the n-value method anyway simply due to the variation that can occur material to material. As a quarky comparison, it's sort of like driving a barge while looking through a telescope in my opinion. This is why I prefer geophysical methods and specifically surface methods so that you're looking at the conditions over a larger area.

Try FEMA451b. around p.1399. This link is a good source of multiple documents so try many of them.

http://www.wbdg.org/ccb/browse_org.php?o=39

I seem to recall that it's vaguely stated either in ASCE7 or IBC. I'd have to look again and see since I don't remember for certain where I came to that conclusion.