Question about effective cohesion and effective friction angle

[pietro82]

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

 

[GeoPaveTraffic]

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

 

[LRJ]

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.

 

[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

ípapß gordo ainÆt no madre flaca!

 

[AdityaDeshmukh]

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.

 

[LRJ]

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

 

[pietro82]

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?

 

[LRJ]

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.

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.

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).

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.

 

[fattdad]

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!