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Fines content FC and cohesion in clayey sand

Fines content FC and cohesion in clayey sand

(OP)
1. General question: Are there recognized correlations for cohesion - either effective or total - as a function of % passing 200 (0.075mm) for a clayey sand. Say, 5% to 15% FC, with the balance of the soil non-cohesive.
2. For earth pressure evaluation, when is it justified to use cohesion together with the friction? That is, c-phi behavior for either long term or short term.
3. Same question slope stability evaluation.
I understand that traditionally, phi-only for long term slope stability.
The reason for the question: recent technical literature for seismic earth pressures, like the Caltrans funded UCLA Shamsabadi tech paper and NCHRP 611, show that small values of cohesion (100psf = 5kPa) can significantly reduce the seismic active earth pressures. Given the uncertainty of cohesion, it would be useful to define a “safe” lower bound cohesion value based on FC or PI. Say, a relationship with 3 sigma (99.7%).

RE: Fines content FC and cohesion in clayey sand

I expect you will find quite a bit of variation in answers/thoughts. Here are my general thoughts, but site/project specific answers could vary.

1. Not that I have ever seen, and I doubt that such a correlation could be developed since the stress history of the soil would be very important.
2. I rarely use cohesion in earth pressure calculations and never when calculating long term behavior.
3. I will occasionally use very small cohesion values, ~25 psf, when running long term stability calculations just to prevent shallow surfaces.

Now getting 100 psf cohesion under seismic loading, I expect that anything other than clean sand and gravel has a shot at that much undrained strength during an earthquake.

Mike Lambert

RE: Fines content FC and cohesion in clayey sand

in the realm of seismic analyses, I have no opinion, really, other than it seems the loading would be too quick to dissipate much pore pressure and likely to be total stress - so, I'd think undrained shear strength?

In the realm of static analyses, we use undrained shear strength (Su=c, phi=0) AND drained shear strength (c=0, phi>0). I only use long-term strength when dealing with soil strength 10 or 20 ft below the ground - i.e., drained strength on a drilled shaft or such.

f-d

ípapß gordo ainÆt no madre flaca!

RE: Fines content FC and cohesion in clayey sand

f-d, am I correct if I use your shear strength approach only for saturated soils? For unsaturated soils, I have seen c-phi values for UU triaxial tests. For unsaturated soils, can long and short term shear strength parameters be considered similar? Maybe not for highly plastic clays, but for silts? Maybe?

RE: Fines content FC and cohesion in clayey sand

drained and undrained loading is independent of 100 percent saturation. Yes, in unsaturated soils there can something other than a phi=0 condition for undrained strength.

f-d

ípapß gordo ainÆt no madre flaca!

RE: Fines content FC and cohesion in clayey sand

thanks f-d. I can see that soil mechanics was developed considering that soils are saturated, however it becomes a little confuse when we deal with unsaturated soils...I am trying to read Prof. Rahardjo's book on unsaturated soils but not have time so far...

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