Hi all,
Please check whether my understanding is correct under this topic and if I'm wrong or missed points, provide the explanation, thanks.
For the medium dense to dense soil, due to the shearing effect particle will expand which result in the increase of friction angle which in-turns increase the shear strength of the soil (generally dilatancy angle is taken as friction angle - 30 where phi > 30).
Any reply please, thanks.
Nitha,
You are correct in that dilatancy is a part of the soil frictional strength which will result in volume increase during shear.If you're modeling undrained behavor the diltancy will result is negative pore pressures (suction) which will increase effective stresses and hence increase effective strength.
The 30 degree you show is a good rule, because soil with less than 30 degrees usually contract rather than dilate during shear. Be sure not to double dip and consider the dilantancy angle twice! Don't use the full phi AND a dilatancy angle of phi-30. Use the phi of 30 and the part of phi greater then 30 as the dilatacny. But in my opinion this an advanced refinement that is useful in some very specific cases to analyze dilatant soil behavior. In most analysis I rarely consider diltancy, and most often use the full phi. If you're not sure don't use it and use the full phi which includes dilatancy
Martin
Answer Verified By: Nitharshan
Dear Mr. Martin,
Thanks for the reply
So as per the table above, if we consider gravel, the peak is 34 and apparent is 32, then if we consider the dilatancy effect, phi = 32 and dilatancy = 2. For the dilatancy, we need to consider whether the peak is greater than 30 or the residual is greater than 30?
Normally for the slope stability analysis, not the immediate open cut slope but the existing slope, it is advised to use apparent friction angle instead of the peak as the soil particles are in the position of slide and particles are re-arranged. In that case, we use only phi =32, not dilatancy =2 am I correct?
"most often use the full phi" => you are referring to peak friction angle am I correct?
Yes, most often for geotechnical design the deformations are not great enough to get to residual. Unless you're analyzing a landslide that has moved a lot where the residual strengths might occur. I think that it is more appropriate to consider the dilatancy as the difference between the peak and a critical state strength which is not exactly the same as a residual strength. Although the difference is somewhat confusing to me. The dilatancy has more to do with consideration of volume changes during shear, which is why I said if you're not focusing on the volume change and pore pressure computations you're best off sticking with a simpler analysis and using the peak strength with zero dilatancy.
Noted, thanks.