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Field N-values vs N60 values
2

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.

RE: Field N-values vs N60 values

I seldom ever did - but then again, I almost always used catheads - which is what the 60% is supposed to be approximately too.  One point, in geotechnical reports, the geotechnical engineer, when reporting N values should always state whether they are uncorrected or corrected and corrected to what.  Some use N55, others N60.  Seldom, though, do geotechnical reports indicate such.

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

(OP)
drile007,

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

2
N60 recognizes that there is incomplete effeciency in the delivery of the energy of a 140 pound hammer falling 30 inches.  For a conventional safety hammer the overall effeciency is considered to be 60 percent.  For the automatic hammer, the effeciency is considered to be 95 percent and for the doughnut hammer it is considered to be 45 percent.  If I have an N-value of 25 for a safety hammer you would re-equate that to an N60 value of 40 (i.e., 95/60).  If I have an N-value of 25 for a doughnut hammer you'd re-equate that to an N60 value of 19 (i.e., 45/60).

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

ditto what everyone said. never do for general assessment other than liquefaction assessment.

RE: Field N-values vs N60 values

fattdad wrote "If I have an N-value of 25 for a safety hammer you would re-equate that to an N60 value of 40 (i.e., 95/60).  If I have an N-value of 25 for a doughnut hammer you'd re-equate that to an N60 value of 19 (i.e., 45/60)."

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

Quote:

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)." ?

yes.

f-d

¡papá gordo ain’t no madre flaca!

RE: Field N-values vs N60 values

Automatic hammer calibrations I have seen recently come to about 90% efficiency. There is a standard test method, ASTM IIRC, that describes how to obtain this number.

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

we hsd it run on our rigs and i believe it came out 80-100% across several different blow materials.

RE: Field N-values vs N60 values

We've measured >90% with the CME hammer, which is unfortunately sensitive to the speed of the hammer.  I don't recall if it runs off a hydraulic pump or the rig's PTO, but I do recall that small changes in engine speed made a difference.  Apparently, the hammer gets thrown upward or something.

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

i say yes, but then the confining pressure bumps the numbers up. that's sort of the principle for the N60 if i'm not mistaken...take in to account several different aspects of the process as well as the material properties to get down to a normalized n-value.

RE: Field N-values vs N60 values

Yes and no.  The confining pressure adjustment used for liquefaction assessment is Cn, discussed here a few days back; it adjusts for confining stress, but the blowcounts are only adjusted for the energy transmitted to the rods by the hammer blow (what's measured by the PDA used for the measurement).  There are no data that I've ever heard of that account for the static weight of the hammer and mechanism sitting on top of the rods.

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

"what are you doing working on saturday"...kid is watching dora the explorer so i'm trying to amuse myself for an hour or so...however i did have to do some paper work this morning. either way we must really love to work for free.

RE: Field N-values vs N60 values

Yeah, I work on Saturdays too - 60 hour work week every week and also on stat holidays.  oh, well - you get this overseas.  One thing that has always "puzzled" me over the years is the phlethora (hope the spelling is correct) of correction factors that are being/trying to be introduced with the SPT.  Can't we leave a simple test simple?  It gets to a point as to what point one stops the correction factor process.  Years ago, no one corrected - almost everyone used the cathead and doughnut hammer - things worked out . . .  Again, I don't much pay attention to many corrections (unless for seismic) as I don't precisely worry if the N = 21 or 24.  I'm using it for general guidance for the most part.  There is still much fuzz obviating such precision - for most cases.

RE: Field N-values vs N60 values

i agree and that's my firm's stance. we give soil descriptions based on uncorrected n-values as well as other stuff. if we happen to run across a 7 bpf auto n-value 3 feet below the water table, then we might discuss it as such due other higher effeciency. we'll keep auto versus r&c in the backs of our minds but we don't convert every single sample over. for seismic, we do convert at least from auto to r&c. sometimes, the best "conversion" is simply talking to your drillers and picking their brain.

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 don't think the great push for multiple adjustments got started until people started using SPT for quantitative assessments of liquefaction resistance, where the difference between 21 and 24 can make a difference in the predicted outcome.  CPT is better because it requires fewer adjustments that aren't supported by very many data.  

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

i'd forgotten to follow up with a response to something earlier related to my response.
"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

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.

RE: Field N-values vs N60 values

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.

RE: Field N-values vs N60 values

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

RE: Field N-values vs N60 values

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

RE: Field N-values vs N60 values

ever tried ReMi (refraction microtremor) to evaluate Vs?

RE: Field N-values vs N60 values

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

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