Undrained behaviour (NF) of dense YS sand and DEM ellipsoids after isotropic and K ₀ consolidations: (a) in the q–p′ space (triaxial tests); (b) in the q–ϵ _q space (triaxial tests); (c) in the q–p′ space (DEM); and (d) in the q–ϵ _q space (DEM)