Field N-values vs N60 values 2

[garrettk]

Out of curiosity, how often do you as a geotech engineer convert field n-values to N60 values? If "it depends", what typical criteria do you use for making the decision to convert or not convert?

I realize that some liquefaction anslysis specifically calls for this conversion, but I'm more interested in "day to day" foundation recommendations.

 

[dgillette]

Thanks for the reference. I'm familiar with it, having "discussed" some of its other points with the lead author. (BTW - Others have shown you cannot rule out liquefaction based on liquid limit as it says in that paper. PI is much better. See Bray and Sancio in the ASCE JGGE a couple years back.)

The principle of N60 is simply adjusting the blowcount in direct proportion with the hammer energy (as a percentage of the theoretical max = 30" x 140 lb). The dead weight of the CME hammer does not show up in the energy measurement, which is done by wave-equation analysis using a Pile Dynamics, Inc. pile-driving analyzer. My question pertains to the effect of the dead weight, which doesn't exist with a regular old doughnut or safety hammer and is not accounted for by converting raw N to N60 as is normally done. I THINK the ASTM standard has a maximum dead weight requirement, but I don't know how much science was involved in setting it.

The jury may still be out on the short-rod correction. Not everyone is on board with it, and whether what the PDA is measuring by wave equation really describes what's going on when the integration time = 2*Length/(wave speed) is so short. (Two trite cliches in two short sentences!) To me, it still seems counterintuitive.

 

[msucog]

yep, i believe i've got the ones you reference. i also think that sancio is the one that recently came to atlanta to discuss liquefaction from the silty fines often encountered around here.
i agree that it seems far fetched to get down to the nitty gritty analysis for just about anything based on spt. when i say that, i mean where someone might think their in the "accuracy" range of 1 or 2 bpf. small adjustments in procedure or materials encountered can cause those kinds of shifts in the numbers. however, as long as you keep the sampling procedure in perspective with what you're trying to do, it should work well. if i happened to be on a site where i thought liquefaction was critical, i'd definitely try to use something other than spt.
have you ever used Vs (either downhole or surface wave methods) to evaluate liquefaction? if so, where you comfortable with the finding? i'm in to geophysical surveys so i'm always interested in it's limitations for the more critical projects or on projects in areas that liquefaction is a problem.

 

[dgillette]

I've used Vs (mostly cross-hole) at a number of different sites. At many, but not all, we got good agreement between Vs and SPT, CPT and/or Becker hammer. Where we didn't get good agreement, more often than not, the Vs indicated greater liquefaction resistance than did the penetration methods. There are several possible explanations, including weak cementing and "faster" layers through which a refracted wave outruns the direct wave in a crosshole test. We think we got good results from the Oyo Corporation suspension logger at one site.

At sites where it will work, I lean toward CPT in large part because of the CR*CS*CB*CE that you mentioned. Not everyone feels that way, however, and SPT has its loyalists.

DRG

 

[microseismic]

I only convert it for seismic analysis in border line strata where accuracy matters.

 

[msucog]

ever tried ReMi (refraction microtremor) to evaluate Vs?

 

[dgillette]

Have to admit that I've never even heard of it.

 

[msucog]

http://www.optimsoftware.com/products/

http://www.seismo.unr.edu/ftp/pub/louie/home/home.html

i've been impressed so far as compared to downhole and general comparison to spt and/or "known" field conditions

 

[msucog]

this is the link i was looking for since it has a lot of good info

http://www.seismo.unr.edu/ftp/pub/louie/papers/disper/refr.html