## Fully Softened Shear Strength

## Fully Softened Shear Strength

(OP)

Hello

For a slope stability project I am involved in, we ran CU tests with pore pressure. We also ran hydrometer tests for the same sample. The drained angle of friction i am getting from the CU envelope is smaller than the drained fully softened angles that i get using the correlation between Fully softened angle, liquid limit and Clay fraction (Stark et al.). Am I missing something here? Should not the angle obtained from CU envelope (peak angle) be larger than the fully softened angle? Any insight will be appreciated.

Thanks

For a slope stability project I am involved in, we ran CU tests with pore pressure. We also ran hydrometer tests for the same sample. The drained angle of friction i am getting from the CU envelope is smaller than the drained fully softened angles that i get using the correlation between Fully softened angle, liquid limit and Clay fraction (Stark et al.). Am I missing something here? Should not the angle obtained from CU envelope (peak angle) be larger than the fully softened angle? Any insight will be appreciated.

Thanks

## RE: Fully Softened Shear Strength

## RE: Fully Softened Shear Strength

Mike Lambert

## RE: Fully Softened Shear Strength

Gamez, J. and Stark, T.D., “Fully Softened Shear Strength at Low Stresses for Levee and Embankment Design”, ASCE Journal of Geotechnical and Geoenvironmental Engineering, June. 2014, pp. 06014010-1-06014010-6.

Some of the angles from CU lab tests are quite low. I may have to ask the lab to rerun.

Geotechguy1, i have not checked the SD of the correlations yet. will check. I suspect it might be a case of erroneous lab test though.

## RE: Fully Softened Shear Strength

## RE: Fully Softened Shear Strength

For Stark's FSS calculations, you can download a spreadsheet from Professor's Stark website:

http://tstark.net/geotechnical-software

You will notice that the strength envelope is not linear as it depends on the effective normal stress.

## RE: Fully Softened Shear Strength

Mike Lambert

## RE: Fully Softened Shear Strength

I don't fully follow what you are referring by "which phi angle". There are three equations relating fully softened angle with liquid limit and clay fraction. I used the relevant equation based on LL and clay fraction. Am i missing something?

Okiryu,

I downloaded the spreadsheet and it gives me the same results. The sheet essentially incorporates the equations that are in Stark's paper. Samples are medium stiff to very stiff. There are drained cohesions reported. Should you get zero cohesion from the tests? I have assumed zero drained cohesion for my analyses though.

Thanks

## RE: Fully Softened Shear Strength

Stark's equations give you a shear stress at a specific normal stress. Depending on the equation you are using, you get either 3 or 4 pairs of shear and normal stress. If you plot those points, they do not form a straight line. To get a phi angle you must have a straight line. So my question to you is how are you drawing the straight line that gives you a phi angle?

Options to draw a straight line include:

1. Best fit between the 3 or 4 points. If you do this, you will end up with a phi angle and a cohesive intersecpt.

2. Straight line with zero intersecpt through one of the other 3 or 4 points. Each of the points will give you a different phi angle.

Mike Lambert

## RE: Fully Softened Shear Strength

## RE: Fully Softened Shear Strength

Check out Lade's 2010 Paper I've attached. Castellano's 2014 Thesis is also probably the current state of the art for Fully Softened Strength https://vtechworks.lib.vt.edu/bitstream/handle/10919/56631/Castellanos_BA_D_2014.pdf

## RE: Fully Softened Shear Strength

## RE: Fully Softened Shear Strength

Not to take away from Tim's correlations; however. But, if you have a lab, run the test and record the value. I don't believe you will get there with a CU-bar or a Shelby-tube sample.

So, mush the sample up with DI to the LL and run a DDS. (give those incrementation steps time to consolidate; however!)

f-d

ípapß gordo ainÆt no madre flaca!

## RE: Fully Softened Shear Strength

## RE: Fully Softened Shear Strength

https://erdc-library.erdc.dren.mil/jspui/bitstream/11681/4109/1/ERDC-GSL-GeoTACS-TN-13-1.pdf

## RE: Fully Softened Shear Strength

## RE: Fully Softened Shear Strength

## RE: Fully Softened Shear Strength