Why Normally consolidated clay showing sandy behavior in CU test?

[aaumed]

Why normally consolidated clay shows sandy behavior in CU test, what I mean C=0 and phi>0 ,while it must be completely opposite of that (C>0 and phi =0 because it is cohesive soil)

 

[LRJ]

It's not sandy behaviour: the test if an effective stress test, so there will be a friction angle. In a total stress test you have no friction angle if the soil is fully saturated.

What's more concerning to me is that on the graph furthest to the right your friction angle is lower for a dense sand than a loose sand.

 

[aaumed]

LRJ you are right about phi of loose and Dense sand.
But for first part of your answer I understand you but my question is why that happen? Why same soil may have C =0 and C >0 for different type of test?
What's the theory behind all that?

 

[LRJ]

It's an effective stress path: the pore-water pressure is changing during undrained shearing, so σ' is changing. Total stress wouldn't consider the PWP.

 

[bdbd]

I understand your question, I hope I can explain:

The main difference between sandy and clayey behavior is short term-long term behavior. In short term, loads are carried by excess pore water pressure in clays. However, in long term, when excess pore water pressure dissipates there is no difference: Both NC Clays and sands use friction to carry shear.

But OC Clays? There is another term in long term behavior of OC Clays, APPARENT COHESION. This is not cohesion. But, experiments show that even at 0 stress, OC Clays can carry some shear. This is defined as apparent pressure.

To understand this, we need to look closer to mineral structure of clays. OC Clays are consolidated through geological conditions or previos loadings. This bonds the material in a way that even you do not apply any stress, it holds together. I know I couldn't explain it clearly.

So, the question you should ask is not why NC clays behave like sands. It is normal, we shouldn't expect to soil particles hold each other without any stress. What is weird is OC clays, this is why this c' is discussed very much.

 

[Okiryu]

I asked a similar question some time ago and got an answer from a more experienced engineer. After got that answer, I could understand better the theory behind this, so I will try to remember that answer:

Similar to bdbd, the main rationale is that soil materials are analyzed for short or long term conditions. For short term analysis, the strength of saturated clays are based on their cohesion rather than their friction (c>0, phi=0). For long term conditions, for example during several cycles of wetting and drying, the cohesion of clays (the bonding strength between clay particles) decreases and they act more as a frictional material (c=0, phi>0). Drained triaxial tests take time to conduct in order to not allow excess of pore pressures during shearing. These tests are "slow" tests and may be simulating long term conditions.

Then you have NC and OC clays. OC clays are "older" than NC clays so, some (small) cohesion (bonding between particles) still remaining even during long term conditions.

I understand that this may be confusing, but most of all soil mechanics books explain this in detail so, you can try looking at any soil mechanics book you have already available to get a better understanding of this.

 

[fattdad]

Normally consolidated clays return strength values that are basically, "Fully-Softened." Fully-softened clays typically do not have a cohesion intercept that you can depend on - so, convention is that we just don't use the cohesion intercept.

That doesn't mean there isn't one. . . It also doesn't mean that there is one, either.

I think the research (S. Wright, T. Stark, T. Brandon) shows that the failure envelope actually doesn't have a straight line. Rather, there is a straight line component and then the failure envelope is curved towards the ordinate.

Not to confuse. . .

f-d

ípapß gordo ainÆt no madre flaca!

 

[LRJ]

Agreed, fatdad. The envelope is only applicable at intermediate stresses - at very low and very high stresses it is nonlinear.

 

[aaumed]

I read all comments and I thankful for responds but I have different opinion maybe we can discuss:
the mohr-envelop theory in reality applicable for all types of material not soil only [this issue discussed in Mechanic of Materials Texts], and for all type of material failure there is no such thing (C= cohesive and Ø= angle of friction) in reality C & Ø represents path of failure for material under certain Load condition and they do not related to the property of material like Cohesiveness or Friction ) .
That’s my opinion I would like to discussed with you.