Nomenclature for Triaxial Test

[geotechguy1]

I've heard some engineers and textbooks refer to triaxial tests as 'triaxial shear' tests. I have also heard the test referred to as 'triaxial'. I've also had several senior engineers attack me for using the phrase 'triaxial shear' because, as he put it, 'there is no shear in a triaxial test'.

Personally I don't see anything wrong with calling it a triaxial shear test. Obviously there must be some shear in the soil within the test, otherwise how on earth would you get shear strength parameters from it?

 

[Ron]

It is clearly a shear test. It is difficult to discern the shear planes in a triaxial soil specimen (the soil "mushes" together); however, if you look at an unconfined compression test in concrete (essentially the same as a triaxial test, just that the lateral confining pressure is zero) you see that the failure plane is a cone-shear failure.

The correct terminology is a triaxial shear test. Triaxial refers to the loading directions and shear refers to the failure mode/planes.

 

[LRJ]

One stage of a conventional triaxial test is a shearing stage.

Regardless, this is all semantics and it's not like anyone would misunderstand you.

For what it's worth, I would go with 'triaxial test' purely for brevity: there's no point adding the word 'shear' since it adds nothing to the description besides extra letters.

 

[fattdad]

There certainly is shear in a triaxial test. Heck, the very mohr-coulumb failure envelope has, "Shear" on the vertical axis! You are dealing with pedants that are claiming there's some refined point to make and they'll show you! B.S.! Yes, all the applied loads are, "Normal" to each other. That doesn't mean that the stresses in the column of soil is all compressive though! It makes such little sense to take the time to type about this, I should stop.

Sometimes, you need to find better senior engineers to work with - ones that don't attack - ones that actually understand geotechnical engineering. As far as I'm concerned, it'll be unlikely that they will change their perspective, 'cause now they're dug in!

In case the subject returns back to you - yes, you can also derive other paramaters from the triaxial shear test. However, who'd order a triaxial strength test if you only are interested in one of the other parameters?

What do your senior engineers say about the direct shear test?

What do you senior engineers think about stress paths?

Have they offered you their copy of Bishop and Henkel?

f-d

ípapß gordo ainÆt no madre flaca!

 

[geotechguy1]

He claims that Norbert Morgenstern himself told him it was a triaxial compression test not a triaxial shear test, and Mr. Morgenstern is practically god in these parts so I think I'm SOL.

 

[fattdad]

Sure about Morgenstern.

Sure, the applied loads in a triaxial test are normal to a sample surface.

Your OP claims, 'there is no shear in a triaxial test'. {sic} But, there is. . . There's shear on the failure surface, which is not normal to the applied load (i.e., sigma 1 or sigma 3). The claim, "There's no shear in a triaxial compression test" is just so wrong, it almost doesn't matter what Morgenstern says. There is shear, it's measured, it's plotted and it's used to derive the friction angle, which is our nomenclature for stress-dependent shear strength.

Maybe I'm missing something?

f-d

ípapß gordo ainÆt no madre flaca!

 

[LRJ]

I can understand clarifying whether the triaxial test is compression or extension. However, as fattdad says, in both cases there is shear.