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!

*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.

Jobs

Question about effective cohesion and effective friction angle

Question about effective cohesion and effective friction angle

(OP)
Hi all,

I'm just reviewing on a book few concepts of effective stress. I know that the Mohr's cicle in terms of effective stress has the same radius of the one in terms of total stress, but it is translated due to the pore water pressure, as you can see in the next figure, that is for a cohesionsless soil:

http://s27.postimg.org/wrh6vs7lv/sigma.jpg

So, the effective cohesion and effective friction angle dependent by the pore water pressure, so is it also dependent by the soil loading condition?
thanks

RE: Question about effective cohesion and effective friction angle

I'm not certain what you are asking with ".. so is it also dependent by the soil loading condition?"

Drained strengths of clays and some silts are generally represented by an effective phi angle and an effective cohesion. You must be very careful with the effective cohesion.

Mike Lambert

RE: Question about effective cohesion and effective friction angle

Agree with the caution regarding interpretation of 'effective cohesion': in most cases it's not real and is only a line-fitting parameter. At low effective stresses the effective friction angle will increase and the intercept will usually be very close to zero (depending on fines content). I prefer to use the term 'apparent cohesion' when referring to the 'c' intercept for CD triaxials.

To answer the OP's question, yes the parameters are dependent on loading condition. This is a fundamental principle of soil mechanics, i.e. the difference between undrained and drained (and partially drained) behaviour.

RE: Question about effective cohesion and effective friction angle

the radius of the Mohr's circle in effective or total stress is not the same! Depends on critical state soil mechanics!

f-d

ípapß gordo ainÆt no madre flaca!

RE: Question about effective cohesion and effective friction angle

It does not depend on the loading conditions. Effective parameters mean how much the soil skeleton can take irrespective of the water present in pore. The water in pores experience pressure and bear part of load. If you subtract that, you get effective parameters which tells you about the soil strength, friction angle, etc.
Hope this helps.

RE: Question about effective cohesion and effective friction angle

On that note I'd be interested to know what the OP meant by 'loading conditions' as we appear to have interpreted it differently.

RE: Question about effective cohesion and effective friction angle

(OP)
Hi all,

thanks for your reply. As loading conditions I mean the values of axial and lateral sigmas applied to the soil.

whithout considering the critical state, as it is stated here the radius of the Mohr's circle expressed using effective stresses is the same of total stresses, because it is just translated to the lower sigma in function of porewater pressure, isn't it? In case sigma1 and sigma2 are applied fast and then removed, total stresses increases while effective stressare are zero right? In this case, how will it be the Mohr's circle? In case sigma1 and sigma2 are applied and they are kept for a period, the porewater pressure is dissipated in function of time and then Mohr's circle expressed using effective stresses traslates through the time, right?



RE: Question about effective cohesion and effective friction angle

Quote (fattdad)

the radius of the Mohr's circle in effective or total stress is not the same! Depends on critical state soil mechanics!

f-d

This is incorrect: the deviator stress is the same for effective and total stress, therefore the radius of the Mohr circle is the same.

Take this example: s1 = 1300, s3 = 1000, u = 600

This gives: s'1 = 1300 - 600 = 700; s'3 = 1000 - 600 = 400

q = s1 - s3 = s'1 - s'3 = 300

q is the diameter of the Mohr circle, so t = q/2 is the radius, which is the same for effective and total stress.

The difference between effective stress and total stress is in the stress path.

Quote (pietro82)

whithout considering the critical state, as it is stated here the radius of the Mohr's circle expressed using effective stresses is the same of total stresses, because it is just translated to the lower sigma in function of porewater pressure, isn't it?
See above.

Quote:

In case sigma1 and sigma2 are applied fast and then removed, total stresses increases while effective stressare are zero right?
Not necessarily - the pore-water pressure response is not that simple, e.g. very dene sand would tend to dilate (negative pore-water pressure) when shearing in an undrained manner. Also to note, sigma1 and sigma3 are the more commonly used expressions (sigma2 = sigma3 in triaxial test conditions).

Quote:

In case sigma1 and sigma2 are applied and they are kept for a period, the porewater pressure is dissipated in function of time and then Mohr's circle expressed using effective stresses traslates through the time, right?
I'm not sure what you mean. But yes, pore-water pressure does dissipate over time if the material is allowed to drain.

RE: Question about effective cohesion and effective friction angle

To LRJ and the OP,

If I misspoke, it was unintentional. I refer to the OP to Bishop and Henkel, "The Measurement of Soil Properties in the Triaxial Test."

f-d

ípapß gordo ainÆt no madre flaca!

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


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