Drained parameters - c' and phi'

Hi guys, just wondering if anyone uses c' and phi' parameters together for design (retaining wall, slope stability etc). If so how much cohesion would you use and why if not why not.

Regards
Rob.

 

[GeoEnvGuy]

Over consolidated high plastic clay can be modelled that way, typically only the friction angle is used rather than relying on cohesion for a long term failure.

In my slope models I used drained strengths for compact sands or rockfill.

 

[RGKVI]

Yes typically Cu values only for undrained short term modelling and phi values only for drained long term values. However effective stress triaxial test results on OC clays will often give you a c and phi value; in which case would you just use the phi value or would you redraw the failure envelope to go through the zero cohesion intercept and get a higher phi value?

 

[GeoPaveTraffic]

RGKVI,

I have used c' and phi' for slope stability analysis. In my experience the c' measured in the laboratory can be quite high and does not agree with back-calculated values from landslides.

So when I used a c', I would typically use a "nominal" value of 25 to 50 psf along with the phi' from the laboratory testing. That "nominal" cohesion had a relatively small effect on deeper failure surfaces, but it helped keep the shallow surfaces out of the model results. This is a method I used for many years.

More recently, I have been using fully softened strengths based on the work of Stark and others. While there is much that I do not like about the way Stark developed his correlations, they compare very well back-calculated values from landslides. In case you are not familiar with fully softened strengths, in short they are a curved strength envelope with higher phi' angles at low confining stresses. So in large part they mimic the strength envelope you get by using a small c' with typical phi' values.



Mike Lambert

 

[GeoEnvGuy]

The discussion in Fell's geotechnical engineering of dams 2nd edition regarding whether to use the peak, fully softened, or residual is helpful as different situations justify a different strength selection. Such as the strength selected to an existing slide plane beneath a dam or the strength that should be assumed for an excavation.

 

[RGKVI]

Thanks Mike,
That's very helpful and I'm going to look into the use of fully softened strength parameters which I confess I've never come across before. Every day is a school day.
Regards,

Rob.