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How to find correlations

how to find correlations

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

BigH (Geotechnical) 8 Dec 09 07:38

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

dgillette (Geotechnical) 8 Dec 09 11:47

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

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

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

Mccoy (Geotechnical) 15 Dec 09 12:19

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

dgillette (Geotechnical) 15 Dec 09 13:01

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

BigH (Geotechnical) 15 Dec 09 22:08

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


40N; SF=3; qall

13N; use 10 for ballpark).

RE: spt correlations to su

Mccoy (Geotechnical) 16 Dec 09 05:07


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

dgillette (Geotechnical) 16 Dec 09 11:04

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

KVgeo (Geotechnical) 16 Dec 09 14:54

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

dgillette (Geotechnical) 2 Feb 10 17:14

Better slow than never. Finally got around to scanning it last night when scanning other things.

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?

RE: spt correlations to su

dgillette (Geotechnical) 2 Feb 10 19:27

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

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