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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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