Estimating friction angle of clay 2

[temee]

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.

 

[EireChch]

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

 

[PEinc]

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

http://www.PeirceEngineering.com

 

[Ron]

Somewhere between 15 and 25!

 

[Okiryu]

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 !

 

[Okiryu]

I just also remembered about the attached graph from Lambe and Whitman book...

 

[temee]

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

 

[Okiryu]

See page 18 of this link:

http://www4.hcmut.edu.vn/~cnan/Prin... engineering (Fifth Edition, Das)/311-363.PDF

It is a similar graph to what is shown in L&W.

 

[EireChch]

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.

 

[Okiryu]

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.

 

[Okiryu]

The L & W chart here again...

 

[EireChch]

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...

 

[Okiryu]

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...

 

[EireChch]

Okiryu - yep definitely, in a perfect world.

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

 

[GeoPaveTraffic]

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

 

[Okiryu]

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.

 

[killswitchengage]

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

 

[fattdad]

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!

 

[BigH]

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

 

[fattdad]

(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!