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spt correlations to su

spt correlations to su

I am trying to find the equations of G.F. Sowers -1979 for the SPT N-value and the su. I have the figure but not an equations I specifically need the equations for the SC-ML category

RE: spt correlations to su

If you have the figure, plot it on Excel and tell it to format trendline and pick the type that fits best.   

RE: spt correlations to su

Pretty obvious if you have the figure, you can determine an equation - in Singapore, it is common to use Su = 5N (but couldn't find out if the N is corrected or not . . . will try to find out).  I used to make my own correlations in given locales - site specific is always better than the "general" ones given their very fuzzy nature . . .

RE: spt correlations to su

Just be careful about what's being correlated with what.  Not sure about Sowers, but the Terzaghi and Peck correlation is N (not adjusted for any of the things we now adjust for) and unconfined compressive strength, qu, not Su.  

There exists a little-known correlation of N with field vane shear, which has some advantages (primarily less concern about sample disturbance), and which I believe I have referenced on another thread recently.  The disturbance may have something to do with the fact that, for a given N, Sowers gives higher qu with more plastic material.


RE: spt correlations to su

The equation for correlating SPT N to Su is

Su = f1 N (After Stroud and Butler, 1975)

N is uncorrected

f1 = 4.5 to 5 (depends on Plasticity Index - use N=4.5 for PI>40% and N=5 for PI<40%).

RE: spt correlations to su

Delijosi and Big H - What's the unit of measure for Su, kPa?   

Also, what strength test was it referenced to?

RE: spt correlations to su

Unit of measurement for Su is kPa.

SPT N value was referenced to lab Su.

RE: spt correlations to su

The correlation:

Su/Pa = 0.06*N

Is given by Terzaghi & Peck, Pa is atmospheric pressure expressed in the unit you wish to use, and, according to Kulhawy & Mayne, 1990, it has been developed from unnconfined compression tests.

As dgillette implied, originally it was qu =12N in kPa (as cited in Bowles' handbook

Itoo have always wondered wether the N values are corrected or not but, unless specified as N1(60), or N1(70), I'll assume they are not.

The Stroud & Butler correlation is so similar to the one which relates constranend modulus to NSPT and PI that it makes me suspicious, even because it would yield pretty low Su values in low plasticity clays.

The Terzaghi correlation I often feel may underestimate strenght, I have no data to support it though, anyway it's located in a cautious region in between the various suggested correlations.

RE: spt correlations to su

Hi McCoy.  The correlation referenced to VST is

Su = 600 lb/ft^2 + 140 N  +/- 500 lb/ft^2.

(I fit that equation to plotted data from a USBR research report.)  Those tests were done in the early 1960s, and were almost certainly not adjusted for hammer energy or anything.  Probably a doughnut hammer, but that's far from certain.  I knew the guy who did the tests, but I can't ask him because he's dead now (and, coincidentally, he's buried at one of the test sites, which was at that time a new cemetery being built).  The VST data have not been adjusted a la Bjerrum or Aas, LaCasse, et al.

I don't know how to find the Stroud reference (hint!), but in 1975 they probably were not making quantitative adjustments.


RE: spt correlations to su

Dave - Su = 5N (kPa) - as indicated by Nick Shirlaw (Golders) - seems to be the "norm" for Singapore clays. In Toronto, we used the typcial Su ~ 6.25N (as given in most books for very soft, soft, firm, stiff, very stiff).  Allowable bearing pressures were typically taken as qall = 10N (makes sense qu ~ 6Su; qu ~ 6x6.25N ~ 40N; SF=3; qall ~ 13N; use 10 for ballpark).   

RE: spt correlations to su


I converted your VST corelation in atmospheres:

Su = 0.28+0.066N +/- 0.24

An N value of 10 would result in a Su = 94 +/- 24 kPa, versus the 60 kPa Su as per Terzaghi's correlation, 50 to 62.5 Su of BigH's singaporean correlation.

Your correlation is apparently more suited to the kind of clays I usually come across over here, and I love the presence of the error value.
I often feel the Terzaghi correlation underestimates Su in medium and low plasticiy silty clays.

Is the source for the USBR correlation available anywhere?

I like the story of the guy who did the tests and now has a pretty direct feel of the very ground he tested.

Stroud's corelation: I'm suspicious in the sense the cited correlation would appear to be the one for constrained modulus, not Su, the reference is Stroud 1974. ESOPT:"The SPT in insensitive clays and soft rocks"

and Stroud & Butler 1975 : THE SPT and engineering properties of glacial materials.

In a few words, Stroud 1974 and Stroud and Butler 1975 published correlations for the NSPT versus the constrained modulus, not for the NSPT versus Su, as far as I know.

I did not look the above articles up though, so I might not be perfectly accurate in my statement.

RE: spt correlations to su

The USBR correlation is in an obscure research report which I have not been able to find in our electronic files.  I will scan it and post it here.

RE: spt correlations to su

What are you using the correlation for?

Is it a critical project?

Then I would suggest that this approach is not appropriate and that you should attempt to recover undisturbed samples and do triaxial/shear box testing.


Perform oedometer testing and calculate OCR values and use a standard overburden pressure vs su assumption.


Estimate su from field vane testing which records stress vs strain throughout the duration of the test.

The SPT test is not an undrained test. The rate of loading and the input energy is highly variable and as such the test is not reliable for anything other than pre feasibility level guesstimates of su and definitely not suitable for design purposes.


RE: spt correlations to su

Dave - many thanks - I'll attach one I made of clay soils in India's Calcutta area (have to locate it first).  Can you rescan page 1?? or did the original cut off the first inch of text on the left?  cheers

RE: spt correlations to su

The "original" really did get cut off when it was printed from the microfiche.  Sorry.  I have emailed some research-types who cited it in a report, in the hope that they have a cleaner copy.


RE: spt correlations to su

the best way to estimate undrained shear strength from SPT is to jam a pocket penetrometer into the recovered SPT sample.

I completly agree that correlations between blow counts and undrained shear strength are iffy.


¡papá gordo ain't no madre flaca!

RE: spt correlations to su

fattdad - I would suggest a torvane.  have done it before - there will be arguments about "disturbance" - but it will give a lower bound, anyway.

RE: spt correlations to su

1. In addition to Delijosi post about the relation of Stroud (1975) some more information on values of f1 (all in kPa):
     f1≈4.5 for PI>30%
     f1≈5 for PI=25%
     f1≈7 for PI=15%
2. According to Sowers in kPa (I found the relation from graph of NAVFAC DM7.01 with the adequate conversion):
     f1=3.6 for clays of low plasticity and clayey silts
     f1=7.2 for clays of medium plasticity (close to Terzaghi value)
     f1=12 for clays of high plasticity.
3. As one can note, those two approaches contradict. With increasing plasticity, the first gives lower Su and the second higher Su for the same blow counts!
4. Consequently those relations should be used with caution.
5. Don't forget that SPT is a quite good tool for sands but not for clays.

RE: spt correlations to su

There isn't necessarily a contradiction there, and there may be an explanation.  

Go back and look at the strength tests that the SPTs were correlated with.  In the case of T&P, and I believe also Sowers, the reference test was the unconfined compressive strength.  Less-plastic materials suffer more disturbance in sampling and handling, which may explain why they (at least on average) give lower qu for a given N.  Regarding Stroud and Butler, Delijosi wrote "SPT N value was referenced to lab Su," but was not specific.  Assuming that was something other than unconfined, the effects of disturbance are compensated to some extent; hence, there could be less of a trend or an opposite trend in Su vs PI, for a given N.

How good were the fits?  [I haven't seen the actual data for either T&P or Sowers (or Stroud).]  Did they control for all the relevant variables that would create apparent trends where none exist?  For example, did all of their high-PI data come from a particular stratum that happened to be at a greater depth or had higher Ns?  One correlation I remember, but can't find, shows the slope of Su vs N decreasing with increasing N.  Was that effect involved?  Were the same hammers used at all the sites, or did some get tested with doughnut hammers while others had CME auto hammers?  Does sensitivity affect it, and can that sensitivity be linked with PI in such a way as to explain the trend?  Were enough different sites and materials included to show that the trends are valid in general and not just between site A and site B?

Considering the mechanism by which clayey soil resists the penetration of split-barrel sampler, is there any theoretical reason why a more (or less) plastic clay with a given Su should provide greater penetration resistance?

BTW - where was Stroud and Butler 1975 published?   

RE: spt correlations to su

Some more information regarding Stroud relation for the aid of conversation:
1. The tests referred to stiff to hard fissured clays and weak rocks.
2. The blow counts were correlated with undrained strenghts from UU triaxial compression tests in 102 mm diameter specimens.
3. The standard splitspoon sampler was used.
4. Generally Stroud highlights the importance of fissuring in clays. According to "Soil Mechanics in Engineering Practise" (3rd edition), the results of Stroud showed a lower value of f1 with increasing clay hardness for the same value of plasticity. This is attributed to the weakening effect of fissures, which is more important in the lab tests.

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