Regarding the undrained shear strength

[geotechniqa]

1-You get the undrained shear strength from the Vane shear test. Is this equivalent to the one we get from the undrained unconsolidated shear triaxial test.
If yes , which one does it represent.? the peak or the residual one.
2-By the way, can any one refeer me to a source where shear stress-shear strain curves are reported for the undrained unconsolidated tests. I want to see the trends of these curves for different clays

 

[BigH]

geotechniqa - please refer to the Bjerrum correction values for vane shear tests in clays. See his original paper or many many texts that reference it. Bowles presents it plus gives his take on an "improved" correlation of the vane test result vs the CU test. Of course, by procedure, when doing the field vane test, the first value is the "peak" vane test value; then you turn the vane 25 or so revolutions and do the test again - this is the residual value. Have you ever done a field vane test? Many texts have stress-strain curves. Terzaghi Peck Mesri, Lambe and Whitman, Muir Woods, Head's book on laboratory testing (V3); Bishop's classical book on triaxial testing . . .

 

[geotechniqa]

Thanks BigH
1-So to confirm: your answer is that both peak and residual can be obtained from Vane shear,but it depends on the way you do the test.
But you did not say which undrained is it the one coresponding to UU test or CU test

2- I am talking about shear stress-shear strain curve obtained from the UU test (PHI=0) and not CU test. This curve is not routinely reported.

 

[BigH]

Suggest you might want to read relevant sections of David Muir Wood's book "Soil Behaviour and Critical State Soil Mechanics". I have, hopefully without angst, attach a few relevant graphs for you.

 

[Mccoy]

Just wondering about the post peak plateau value which is in between the peak and the residual (large strains), also called phi_constant_volume, or phi_critical state.

Is there a way to capture it by FV tests, as in example, limiting the strains (# of revs?)

Would that make a difference in the soft clays investigated by FV?

 

[geotechniqa]

BigH thanks a lot for your answer.

I believe that these values you attach represent Peak value of the Undrained Shear Strength.
Note still there is no shear stress-shear strain curve shown for the UU triaxial shear. This doesn’t seem to be common output.
I think it is not reported commonly, because the UU is routinely performed on soft clays and thus the peak will differ slightly (if not) from the residual. There is no need to report it . However, for clays with OCR > 3 , it should be reported.

 

[dgillette]

McCoy - Our VST device records a continuous trace for up to a full rotation, maybe more. That doesn't exactly give us good stress-strain relationships, but it gives a post-peak/fully softened value before we spin it 360 and 1080 deg, with measurements after each for remolded.

geotecnicqa - I do not agree that there is no need to report post-peak values except when OCR>3. Stress-strain curves (or stress-rotation curves in the case of VST) should be standard for ALL tests.

 

[geotechniqa]

McCoy.. are you aware of any shear stress-shear strain curves recorded by the unconsolidated undrained shear (not CU test)test .
If yes please refer me to a source. My understanding that you cannot capture the stress-strain relation by even the most advanced VST.. correct me if i am wrong please

 

[dgillette]

geotechnicqa - You are correct that you cannot get a stress-strain relationship from VST, because you never know what the strain really is (and it's far from uniform. What one gets from VST is stress-ROTATION curves, which can provide you with post-peak a.k.a. softened strength, remolded strength, and a qualitative sense of how rapidly the strength drops off after the peak (brittleness).

 

[geotechniqa]

dgillette : you said we should repost it:
But are you aware of any shear stress-shear strain curves recorded from the unconsolidated undrained shear (not CU test)test ...?

 

[BigH]

See Terzaghi Peck and Mesri (1996): Article 20, figure 20.28. Shows stress vs strain for unconfined (UC), unconsolidated compression (UU) and triaxial compression (TC). You might find Article 20 a good read.

 

[Mccoy]

I've always regarded the VST a great field test, too bad it's not much used over here. Lab tests have that common affliction of disturbance, especially in soft soils, unless extreme care is taken in sampling. Which alas, happens very rarely

 

[BigH]

Our company years and years ago actually used the Swedish Foil Sampler in some investigations - of course, I was probably still in Junior High at the time . . .

 

[geotechniqa]

BigH thanks .. I will look at the source