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# Estimating friction angle of clay2

## Estimating friction angle of clay

(OP)
Hi

Is it possible to estimate friction angle of clay for drained stability calculation? I heart that there is some software which is able to estimate friction angle based on clay-size fraction and some other parameters.

### RE: Estimating friction angle of clay

From BS8002 - Code of practice for designing retaining walls. Φ' can be estimated based on Atterberg data.

[img ]

### RE: Estimating friction angle of clay

Look in geotech books for plots of phi angle vs. plasticity index of the clay.

### RE: Estimating friction angle of clay

Somewhere between 15 and 25!

### RE: Estimating friction angle of clay

temee, see attached spreadsheet from Professor Stark (you can download it freely from his website). This is for calculating fully softened and residual friction angles based on LL and clay fraction.

I also agree with Ron on those values !

### RE: Estimating friction angle of clay

(OP)
Okiryu, thanks for spreadsheet. I can't open attached graph from Lambe and Whitman book.

### RE: Estimating friction angle of clay

Okiryi - That second link you posted (Page 18), I am not sure if that is a good reference to use. I checked a PI of 20% which gives a SinΦ=0.5 - Sin (inverse) of 0.5 is a Φ of 30 degrees. I dont think that is reliable? Maybe I am doing it wrong? Am i missing something?

When i check a PI of 20% with the BS8002 equation I posted above it gives a Φ of 26 degrees.

Be interested to check the L and W graph with the BS8002, the link you posted doesnt work. Could you post it again.

### RE: Estimating friction angle of clay

EireChch, here is the L & W chart. It is basically the same to the one shown in the link. Your calc is correct - for LL=20, the phi' is 30.

I think that since the equation from BS8002 is based on critical state soil mechanics, from the stress-strain curve, the strength at constant volume is referred at a point of "lower strengths" (end of the curve) and this may be the reason why the BS equation results in lower phi' values.

### RE: Estimating friction angle of clay

Okiryu, i think you may be right on that.

Ive checked it again for a PI of 30%

L & w gives a Φ = 27°
BS8002:2015 (my first equation) gives a Φ = 24°
BS8002:1994 (pic below) gives a Φ = 25°

Not much in it really...

### RE: Estimating friction angle of clay

EireChch, anyway, the BS are good references...but I think that for critical projects, best thing to do is to run triaxial tests to see the actual values...

### RE: Estimating friction angle of clay

Okiryu - yep definitely, in a perfect world.

I would be interested to see if anyone uses any correlations between Cu and Φ....

### RE: Estimating friction angle of clay

I've used both Stark's spreadsheet and L&W's chart. However, I have always capped the phi angle for clays at 28 degrees. Has always worked for me, but certainly not magic. In the end, it comes down to experience; hopefully experience in the local area of the project.

Don't forget that your groundwater assumptions and design factor of safety are just as important as your strength assumptions.

Lest of luck.

Mike Lambert

### RE: Estimating friction angle of clay

Without testing, the typical maximum assumed phi' for our medium stiff residual clays is 25 degrees. However, CU triaxial tests showed sometimes high values such as 30-32 degrees. When I compared these results with the L&W chart, it correlated well. So, I decided to run triaxial tests when the budget allows, it is worth to spend some money and then get more economical designs.

### RE: Estimating friction angle of clay

Like mentionned above , Lambe and Whiteman chart is useful . Anyway, it cannot got under 15° but it may in some extreme cases drop below that for residual strength analysis. You can also consider 25° as a maximum since sand starts from 30°
Make sure however you are not facing a residual strength situation

### RE: Estimating friction angle of clay

I'll just echo the post of Okiryu. I'd start with Stark's correlations.

in the instance of clay, you have softening effects. Such effects are rarely captured in in-situ testing or in laboratory testing of undisturbed samples. In those instances, the strength is informed by conditions that may not be available during the design life.

Softening effects occur from freezing/thawing and wetting/drying. Such changes can take years (decades?) to develop. Using softened strength is prudent in long-term design.

If I had a project where such strength is critical, I'd likely use Stark's correlations. I'd also likely take a bulk sample, hydrate it to the liquid limit, normally consolidate it to three confinements and run my own fully-softened DDS test. Stark's correlations are based on the torsional shear test and I'd want to know if my relations to clay and PI related to the DDS as Dr. Stark's correlations related to torsional shear.

I'm a skeptic that way. . .

f-d

ípapß gordo ainÆt no madre flaca!

### RE: Estimating friction angle of clay

Now the question - are the phi vs Ip values on these charts and tables based on Triaxial tests or direct shear tests??

### RE: Estimating friction angle of clay

(to BigH: Tim Stark's research, if that's what you are referencing, is based on torsional shear correlations.)

f-d

ípapß gordo ainÆt no madre flaca!

### RE: Estimating friction angle of clay

Thanks fattdad - but as you imagine, one must be careful about how the phi values were determined; they are not all the same! Trust all is well!

### RE: Estimating friction angle of clay

Thanks Okiryu. I've attached a few charts that I was given in a lecture by Wesley (Okiryu - same as I sent to you privately). Wesley (from Kiwiland) worked quite a bit in Indonesia and with residual soils. He puts forth the concept of "where does the Atterberg limit of a sample fall relative to the "A-line" - i.e., in a parallel sense to it. He figures this is more important in deriving behaviour than just plotting on the chart. I think that his graphs show this.

### RE: Estimating friction angle of clay

BigH, thanks for the charts. I was also able to interchange some emails with Dr. Wesley. He was very responsive and helpful. Thanks again !

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