phi angle of clays
phi angle of clays
(OP)
I am working on a project where I need to determine the phi angle of an overconsolidated clay for retaining wall design. I have been informed by colleagues that the best way is to infer this data is from correlations with PI and that triaxial testing on clays typically yields phi angles of around 0 representing short term conditions.
Is this correct and if so is there a more direct way to determine a long-term phi angle in a clay?
Is this correct and if so is there a more direct way to determine a long-term phi angle in a clay?





RE: phi angle of clays
RE: phi angle of clays
RE: phi angle of clays
When talking about friction angles and clays you MUST ALWAYS indicate if you are talking about drained or undrained strengths. Undrained friction angles in clays are often taken as 0 degrees. Drained friction angles are often estimated using coorelations between drained friction angle and plasticity index. Drained friction angles are more acurately determined using drained triax tests or undrained triax tests with pore pressure measurements.
Mike Lambert
RE: phi angle of clays
How can I interpret the drained friction angle using pore pressure measurements from an undrained test?
RE: phi angle of clays
RE: phi angle of clays
RE: phi angle of clays
RE: phi angle of clays
www.PeirceEngineering.com
RE: phi angle of clays
Typically fully-softened shear strength is less than peak and greater than residual.
f-d
¡papá gordo ain’t no madre flaca!
RE: phi angle of clays
RE: phi angle of clays
We agree that peak maybe an overestimation because of existing fissures. But phi_residual maybe an underestimation because it describes the soil frictional resistance with very large deformations involved (about 1 meter would be the relevant scale).
So we are left with phicv, which can be determined by torsional ring apparatuses like fattdad describes and by correlations with PI as already described. There is one more choice: some generic post-peak friction angle, greater than phicv which by the way is Greater than phires in clays.
What I stress is that we should not forget to define precisely which beast are we talking about within the managerie of the different friction angles.
RE: phi angle of clays
As for fully softened strengths, while I certainly understand the concept and agree that they can occur; I believe that they are being over used/applied. I've seen people argue that fully softened strengths should be used when that calculations show factors of safety below 1 for existing slopes with no indications of instability. I've worked in residual soils, most of which are overconsolidated by desication; for over 25 years. Fully softened strengths would indicate that 80 percent or more of slopes that are existing would have factors of safety well below 1.
Unless there is a history of slope movements, it is unlikely that the fishers will line up enough to significantly reduce the soil mass strength below peak strenghts. Slopes designed using peak strengths and appropriate factors of saftey, 1.4 to 1.5, have shown little chance of failure. Failures happen, but in all cases I've looked at the failures have been caused by poor or no analysis or inadaquate investigations that missed shales or clay shales (caes where the weak planes do line up over long distances.
Overall, I have not found a use for fully softened strengths and will be happy when people finaly stop thinking they should be used for all clays for all situations.
Sorry, all; I'll get off my soap box and go back to my drink :)
Mike Lambert