Relation between friction and dilation in Mohr-Coulomb zone model

I wonder why the Mohr-Coulomb zone model in 3DEC/FLAC3D/FLAC doesn’t allow the dilation angle to be larger than the friction angle?

Good question. It is possible for the dilatancy angle to be greater than the friction angle for certain type of soils.
@cheng do you know why?

From papers I have read, it appears that earlier versions of Itasca programs admitted the dilation to be larger than friction.