Dilatancy

Question

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).

Martin Hawkes · Accepted Answer

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

Dilatancy

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