Dear,

Thak you for the answer.

you can take any type of soil that is why I didn't mention. now coming to triaxial testing you can perform triaxial compressive testing and triaxial extension testing for the soil samples right. I want to know that peak and residual shear strength in these two conditions will be same. ultimately soil strength depends on the conditions it is put forth.

The answer would be "No". The shear strength of the soil is primarily depends on the state of the soil which is determined by the loading and drainage conditions. Hence, the mobilized shear strength for a same soil would be different depending on the state of the soil. 

In simple terms, lets say, sand is being loaded in compression. During this process, the interparticle contact area increases, voids reduce and soil densifies. Under this conditions, the mobilized shear strength of the soil is dependent on the frictional component of the shear strength. While in tension, opposite occurs, the interparticle contact area reduces, voids increase and soil loosens. In this condition, primarily the strength is dependent on cohesion component. So the governing component for mobilization of shear strength are different for compressive and tensile loading. Without no cohesion, the tensile strength of the sand is near to zero. Things get complicated when the soil is cohesive and under different drainage conditions. However, the basic understanding is similar, that the governing mechanisms contributing to the shear strength of the soil would be different in compression and tension. Hence, the shear strength won't be the same. 

I hope that answers your question. 

Regards,

Sravan MV

Dear Sravan Muguda Viswanath,

Thank you for the answer.

you are conforming that ultimately soil strength depends on the conditions it is put forth. I agree with this.

One thing that the tensile strength of sand is zero can not be accepted as we have conducted triaxial extension test on sand and we got stress strain curves. Also you can refer Triaxial extension test on sand in litterateurs.

Refer

1)Vaid, Y. P., Sivathayalan, S., and Stedman, D.,

“Influence of Specimen-Reconstituting Method on the

Undrained Response of Sand,” Geotechnical Testing Journal,

GTJODJ, Vol. 22, No. 3, September 1999, pp. 187–195.

2) Y. P. VAID, E. K, F. CHUNG and R, H. KUERBIS PRESHEARINGANDUNDRAINEDRESPONSE OF SAND-SOILS AND FOUNDATrONS Vol.29,Japanes eSociet yof Soit Mechanics andNb.4,

49--6 1,Dee. 1989

Thank you for sharing the literature. Yes I understand it won't be zero. That's why I have not mentioned as absolute ZERO. The usual consideration tensile strength  would be of very low value in dry condition. However, as I mentioned in my reply, the shear strength depends on the loading and drainage conditions. When investigated in deeper sense, the particle surface texture, orientation, particle gradation and presence of adsorbed water would definitely contribute to the tensile strength of the sand.

For soil shear strength behaviour completely different under tension or compression, see for example Briaud L.L. Ménard Lecture 18th CSMGE The pressuremeter test: Expanding its use , and bibliographic ref. on Ménard Pressuremeter Test.

Dear Claudio Lorusso,

I think you are mentioning phase angle between axial load and maximum principal stress where in compression is zero degree and while in extension test is 180 degree.

yes, you are right. But I will check it. Thank you.

Dear all,

The shear strength is generally a function of the cohesion and the confining pressure for C-Phi soils....In the special case Velusamy is considering, which is sand, the effective C component is theoretically zero.... For the particular question he is asking, we have two cases:

(1) Compressing the sample, which involves extra confinement.   In this case the shear loading comes from the difference between the extra vertical compression and the initial, presumably isotropic, confining pressure. (Not necessarily isotropic though). On the other hand, the soil shear strength increases due to the associated increase in the confinement pressure which, in turn, improves the shear strength. 

(2) The extension of the samples does not take place from zero confinement pressure, and that is why, the shear strength still exists for the extension test due to the reserve confining pressure. The extension reduces the isotropic confining pressure. If the extension process reaches the point where the confining pressure on the potential failure plane becomes zero, the shear strength will theoretically vanish. (You may not reach this state in your test design)

It definitely follows that the compression can never result in the same shear strength as the extension do, with the latter be the weaker if both tests start from the same initial confinement pressure.

It has to be noted that the consolidation being a reason why the shear strength of soils under compression get improved is a different issue, and not directly related to samy's question. (It can surely be there and its magnitude is dependent on the type of soil, state of stress and the drainage condition)

I believe the above provides full explanation and uncouples the two different issues of the state of stress and the material non-linearity, which are sometimes confusing.

Kind regards,

Hesham Elshazly

Hi Velusamy Gobinath,

I think the answer is No! Normally shear strength of soils come from cohesion and angle of internal friction using the very famous equation;  S=c+ sigma tan phi. c is independent of loading whereas phi is load dependent.

I hope that answers your question

If extension is due to confining pressure then shear strength phenomenon is same in comparison.

It depends on the type of test but generally strength in compression is larger tan strength in tension.

Hello Velusamy Gobinath

As had been mentioned the strength of a soil is greater in compression than in tension

But no quantitative values have been specified for these strengths. Assuming a Mohr-Coulomb failure criterion

a sample of soil under all round stress of σ3 will have a strength given by

σ1 = σ3Nφ + 2c' √Nφ where Nφ = tan2(45o + φ'/2)

A soil that has zero cohesion also has zero tensile strength

The tensile strength of the same soil under all round stress of σ3 will be

σ1 = σ3/Nφ - 2c'/ √Nφ

under the sign convention that compressive stresses are positive and tensile stresses are negative.

In this last case if σ1 is +ve the strength is compressive while if it is -ve it is tensile

Example: consider a soil for which c' = 50 KN/m2, φ = 30o, and σ3 = 100 KN/m2 , It can be shown that the compressive

strength of the soil is 473 KN/m2 whilst its tensile strength is 24. 4 KN/m2

you can find a comprehensive answer to your question here:

https://www.youtube.com/watch?v=vywkjAfdP6U&t=758s