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Bridgestructures (Structural) (OP)
14 Mar 04 16:09
What is the relationship between he undrained shear strengh(Su) and the unconfined compressive strength(qu) of a soft clay?  AASHTO gives simplified pressure distributions with values based on the undrained shear strength for cohesive soils.  My boring data gives 0.7 tsf for qu and SPT N values ranging fom 2-4.  I have been told that one can assume that Su=0.5qu, but this is also the value for cohesion (c).  Any help is appreciated.
Helpful Member!(2)  Focht3 (Geotechnical)
15 Mar 04 11:48
Under many circumstances,

Su = c

 - but not always.  You have to look at the soil type, stress history, type of load, load application and duration, soil creep, etc.

Why are you dealing with this very geotechnical engineering issue, and not the project geotechnical engineer?



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.

Helpful Member!  BigH (Geotechnical)
21 Mar 04 16:36
Your N value of 2 to 4 is rather low for Su of 1400psf.  Usually N = 2, in cohesive soil, is taken as about 250psf; N = 4 is taken as 500psf.  For Su = 1400 psf, I would have looked for the N value to be in the order of 11 to 12.
Check on the N values - check the other properties (Atterberg limits and the natural moisture content compared to the LL and PL).  Also, discuss this with your geotechnical engineer. Focht3 is right - this is the "job" for the geotechnical engineer.  What AASHTO chart/table are you looking at for pressure distribution vs Su?????
Helpful Member!  jdmm (Geotechnical)
7 Apr 04 0:18
The symbol Su or Cu is generally reserved for undrained strength.  This is not to be confused with c or c' that is generally used to represent the cohesion intersept for drained shear conditions.  Drained and undrained strengths are very different and as far as I know one can not be determined from the other.

Worst than that, SPT, although a result of the soil strength is a function of so many parameters that it really shouldn't be correlated with anything except relative density of sands ( not gravels, not silt, not clay).

To answer your first question, the undrained shear strength is, by theory and practice, equal to one half of the unconfined compressive strength under undrained conditions.  This relates back to the Mohr-Coulomb failre surface so it is not an emperical correlation.

As far as correlating any strength conditions of clayey soils to the SPT I suggest that you don't take this approach. The vane is a simple test that gives much more reliable information.

I assume that the qu you refer to is from a pocket penetrometer or did you conduct a test on an undisturbed sample?
BigH (Geotechnical)
7 Apr 04 1:44
jdmm is right about the cautioning of the use of the SPT for clay's undrained shear strength.  Still, it has been traditionally used for so many years . . . It is a valid estimation in my book given correct judgment on the final use of the numbers.  I have developed correlations of Su and N for a number of clayey soils - it's deviation or scatter is no worse than for many other correlations.  Many times, too, you have no recourse.  The drillers here do not have vanes with them - if you want them it is a real hassle.  They are only set up for SPTs and thin-walled tubes (that they "tap" in, of course!! - but that's another point).  But, most of all, with all correlations and most "test" results you need proper understanding of your problem and good judgment.
Focht3 (Geotechnical)
8 Apr 04 15:42
This is not to be confused with c or c' that is generally used to represent the cohesion intersept for drained shear conditions.

While this represents a more desirable circumstance (by cleaning up our nomenclature), it isn't true - yet.  We still have many remnants of the use of "c" in design procedures - for example,

fs = α·c

for axial pile capacity analyses.  Total stress, not effective stress, concepts still dominate this area of geotechnical design.  And ½·qu is still referred to as 'c' in many design offices.



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.  See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

dirtguy4 (Geotechnical)
4 Jun 04 4:52
I am not a firm believer in the unconfined compression test, even though I use it all the time as does everyone else in our industry.  It is cheap, quick and easy to perform.  The problem is that it measures the wrong parameters.

Shear strength, by definition, is the effective cohesion (c') plus the effective stress times the tan of the effective angle of internal friction. (Bishop)  Our testing should then be aimed at obtaining c' and the effective angle of internal friction.

The unconfined compression test assumes that the internal friction angle is zero, which is generally not correct and thus our undrained shear strength is 1/2 the deviator stress which is not entirely accurate either.  

I agree with jdmm on this one.  An accurate laboratory vane, this is not a field vane or hand vane but a stationary laboratory, bench mounted lab vane apparatus that can measure the shear strength on shelby tubes samples will generally give better results, especially when testing clays and clay tills.  The beauty of the laboratory vane is one can get several tests throughout the length of one shelby tube sample.  For example take one lab vane test, push out a little sample, take another lab vane test and so on.  Averaging the results will give a far more reliable result that an unconfined, especially if the till is quite rocky.  
Focht3 (Geotechnical)
4 Jun 04 8:46
That's fine in weak soils.  What do you do when "c" or Su exceeds 4 ksf in 98 percent of the soil samples you test (and frequently exceeds 8 ksf), and the sample permeability is typically smaller than 10-8 cm/sec?

I'd love to run CU's with pore pressure measurements all the time (ah, the fees!) but it won't happen.  CPT and DMT won't work, and SBP is limited to the upper 15 feet (~5 meters) of soil in many areas.  (G increases rapidly with depth around here.)  Our depth to constant soil moisture is 15 to 20 feet, sometimes more.

The unconfined compression test is here to stay, at least until we have a better-cheaper-faster alternative -



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.  See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

dirtguy4 (Geotechnical)
4 Jun 04 22:05
An excellent point Focht3 , trying to push a tube in soils with Su values greater than 4 ksf would become a bit expensive and time consuming and probably pointless.  I resort back to BigH's correlation of Su and N for the clay soils in our area.  I also agree that until we can develop a testing method that is better, cheaper and faster than the unconfined compression test, we will be using it for many years to come.
BigH (Geotechnical)
5 Jun 04 8:05
To add to a point in DirtGuy4's earlier post about laboratory vane and doing the vane at several locations in the thin-walled tube, in the course of some of our work in Ontario, we found that the strength in the middle of the tube is somewhat higher than at each of the ends.
Focht3 (Geotechnical)
5 Jun 04 10:02
We can't use true Shelby tubes - the damn things crumple too easily.  So we compromise and use a thick walled tube with a sharpened edge.  While it doesn't meet the ASTM thin walled tube sampler guidelines, it's better than SPT samples.  Of course, we end up with a lot of those also...



Please see FAQ731-376 for great suggestions on how to make the best use of Eng-Tips Fora.  See FAQ158-922 for recommendations regarding the question, "How Do You Evaluate Fill Settlement Beneath Structures?"

BigH (Geotechnical)
5 Jun 04 16:57
BridgeStructures has never gotten back with any data on his Su=1400psf (as per my original post).  Can you please provide it so we can continue . . .

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