×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Why Normally consolidated clay showing sandy behavior in CU test?

Why Normally consolidated clay showing sandy behavior in CU test?

Why Normally consolidated clay showing sandy behavior in CU test?

(OP)
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)

RE: Why Normally consolidated clay showing sandy behavior in CU test?

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.

RE: Why Normally consolidated clay showing sandy behavior in CU test?

(OP)
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?

RE: Why Normally consolidated clay showing sandy behavior in CU test?

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.

RE: Why Normally consolidated clay showing sandy behavior in CU test?

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.

RE: Why Normally consolidated clay showing sandy behavior in CU test?

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.

RE: Why Normally consolidated clay showing sandy behavior in CU test?

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!

RE: Why Normally consolidated clay showing sandy behavior in CU test?

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

RE: Why Normally consolidated clay showing sandy behavior in CU test?

(OP)
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.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources

White Paper - Effects of AIA Quick Guide to UL 489 or UL 1077
The function of a circuit breaker is to provide overload (thermal) and short-circuit (magnetic) protection to a circuit and its downstream components. A circuit breaker functions like an airbag in a car, protecting circuit components and people by tripping the circuit to interrupt the current flow if it detects a fault condition in the control system. Download Now
White Paper - Guide to Integrate Large-Format Additive
As with any new technology, getting into large-format 3D printing begins with investigation. The first question may be a simple one: what does “large-format” mean? For 3D printers, “large” is a relative term. Many extrusion-based (FFF) 3D printers are referred to as desktop machines, because they fit on table space. Some of these have very respectable build volumes – but when it comes to “large-format,” the machines will need their own dedicated floor space. Large-format 3D printers have significant build volumes and are most often found in professional settings, like manufacturing facilities and R&D centers. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close