## 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%).

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

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 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

## RE: Fines content FC and cohesion in clayey sand

f-d

ípapß gordo ainÆt no madre flaca!

## RE: Fines content FC and cohesion in clayey sand