## 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

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

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

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

f-d

ípapß gordo ainÆt no madre flaca!

## RE: Question about effective cohesion and effective friction angle

Hope this helps.

## RE: Question about effective cohesion and effective friction angle

## RE: Question about effective cohesion and effective friction angle

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

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.

See above.

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).

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

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!