Field N-values vs N60 values
Field N-values vs N60 values
(OP)
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.
I realize that some liquefaction anslysis specifically calls for this conversion, but I'm more interested in "day to day" foundation recommendations.





RE: Field N-values vs N60 values
RE: Field N-values vs N60 values
Sorry for this "stupid" question, but as a structural engineer I will dare to ask:
What is the difference betwen N and N60? What is N60?
Drile007
RE: Field N-values vs N60 values
N60 is a method of "standardizing" N values based upon the drilling methods. N-values recorded during the drilling explorations are used in simpler evaluations of soil strength and susceptibility to settlement. Some drillers have automatic hammers which are safer and provide more repeatable energy levels while other drillers have the older "safety hammer" which operates off of a rope and cathead.
The energy delivered by an automatic hammer is greater than the rope and cathead system as the rope and cathead system experiences losses through friction and is not as repeatable.
Other parameters which can be corrected include overburden stresses, boring diameter, anvil size, sampler type, and rod lengths. Energy and overburden have the greatest influence on the correction.
RE: Field N-values vs N60 values
For liquifaction analyses, you typically normalize the data to N-1-60 values, which account for a confining stress of 1 tsf. So even if you have all N60 values you'd do another assessment to recalculate the N60 values for what they'd be under 1 tsf of confining stress. There is a normalizing factor "Cn" that does this. Can't find it in my files.
f-d
¡papá gordo ain’t no madre flaca!
RE: Field N-values vs N60 values
RE: Field N-values vs N60 values
Should that say "If I have an N-value of 25 for AN AUTOMATIC hammer you would re-equate that to an N60 value of 40 (i.e., 95/60)." ?
msucog: For any analysis other than liquefaction, I would probably not bother trying to adjust it for energy UNLESS I'm using the CME auto hammer or something of the sort, or a hammer I know to perform very badly. The correlations are mostly based on unadjusted N with unknown hammer and operator characteristics, so leave them alone ordinarily - it would just be trying to sharpen the pencil too much. (Remember, it's just pounding a piece of pipe into the dirt - not exactly a precision instrument.) It could be important with the CME hammer, however, because the difference is so large; you might see a problem where there isn't one.
BTW, Cn is approximately (1/sigma'v)^1/2 for sigma' in tsf, atm, kg/cm^2, or bars. (Those units are all within a few percent of each other, close enough for dirt.)
Regards,
DRG
RE: Field N-values vs N60 values
yes.
f-d
¡papá gordo ain’t no madre flaca!
RE: Field N-values vs N60 values
http://nsdl.org/resource/2200/20061003061828253T
f-d
¡papá gordo ain’t no madre flaca!
RE: Field N-values vs N60 values
Jeff
RE: Field N-values vs N60 values
The last instrumented and measured response of an automatic hammer that I saw (2003) ranged from about 68 to 76 percent. The work was done for using the rig on a COE levee project, and the COE wanted to know exactly what the efficiency was going to be for that particular rig.
RE: Field N-values vs N60 values
RE: Field N-values vs N60 values
Another question I'v never had an answer to is: What is the effect of having a 200-300 lb dead weight sitting on top of the rods in addition to the hammer blows? Does it artificially lower the blowcount below what it "should" be?
RE: Field N-values vs N60 values
RE: Field N-values vs N60 values
In addition to that, there are adjustments to the SPT for hammer energy, [constant ID sampler or sampler with liner] vs [sampler with space for a liner but no liner], wave transmission in very long or very short rods, drill hole diameter, and about everthing else except the phase of the moon and whether the driller is left-handed.
By the way, what are you doing working on Saturday? And what am I doing working on Saturday?
RE: Field N-values vs N60 values
RE: Field N-values vs N60 values
RE: Field N-values vs N60 values
i'm not oversees...home every single night. but i do work 50-60 hours a week with 90% of that billable so my weeks are rough. i'm not required to do that but maybe it's just in my blood (i was once a contractor working 50-70+ hours a week..except i was paid more by the hour plus overtime). sad to say i took a pay cut to be a geotech. but then again, i dictate my own hours (ironic how i still work myself to death). i find my job now much more enjoyable for the most part since i get to learn something new every single day and don't have to answer to someone cutting corners. if you've seen me around on this board, you know i'm not particularly positive about contractors. i worked for absolutely one of the best supers out there...but i still lost sleep at night. in other words, never trust a contractor...no matter how good you "see" them doing. with my posts on this site, i'm probably pushing 70 hours a week. at least i'm able to pass along my trials and tribulations to others to help them avoid the same mistakes. cheers all.
RE: Field N-values vs N60 values
I can't work 60 hrs/wk for very long, rarely much over 50. Too many other things need to get done, including sleep and the occasional attitude-adjustment activity, and my wife likes to see me at home sometimes. I start getting grumpy if I work much over 50/week.
RE: Field N-values vs N60 values
"What is the effect of having a 200-300 lb dead weight sitting on top of the rods in addition to the hammer blows? Does it artificially lower the blowcount below what it "should" be? ----that's sort of the principle for the N60 if i'm not mistaken"
for liquefaction, i've got a paper proposing to account for "short" rods. when you're calculating N1,60 it's N1,60=N1*CR*CS*CB*CE where CR is the correction for short rod length, CS=non standardized sampler configuration correction, CB=borehole size correction, CE=hammer efficienty correction. essentially for the short rod length, the correction is <1 for anything shallower than ~45'. in other words, (and i'm not 100% sure) i estimate that this is taking in to account the absence of the rod weight just prior to the blow. so at say 15', CR=0.85. so if N1=7, then N1,60=6. i'll put you on the paper and see what you think. there's several other possible corrections discussed for spt, cpt, and Vs methods.
"recent advances in soil liquefaction engineering: a unified and consistent framework" from the earthquake engineering research center by seed and several others (EERC 2003-06)
RE: Field N-values vs N60 values
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.
RE: Field N-values vs N60 values
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.
RE: Field N-values vs N60 values
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
RE: Field N-values vs N60 values
RE: Field N-values vs N60 values
RE: Field N-values vs N60 values
RE: Field N-values vs N60 values
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i've been impressed so far as compared to downhole and general comparison to spt and/or "known" field conditions
RE: Field N-values vs N60 values
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