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.

 

[BigH]

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.

 

[drile007]

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

 

[garrettk]

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.

 

[fattdad]

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!

 

[msucog]

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

 

[dgillette]

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

 

[fattdad]

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!

 

[fattdad]

More information: You can refer to the link below and there are equations to calculate the value of Cn.

http://nsdl.org/resource/2200/20061003061828253T

f-d

¡papá gordo ain’t no madre flaca!

 

[jdonville]

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

 

[theCorkster]

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.

 

[msucog]

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

 

[dgillette]

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?

 

[msucog]

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.

 

[dgillette]

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?

 

[msucog]

"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.

 

[BigH]

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.

 

[msucog]

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.

 

[dgillette]

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.

 

[msucog]

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)