Capturing the transition from diffuse to localised failure in constitutive modelling of partially saturated soils

•The interaction between responses inside and outside shear bands governs the macro behaviour of partially saturated soil specimens undergoing localised failure.•Incorporating this interaction along with thickness, orientation and coupled hydro-mechanical behaviour inside shear band allows correct description of both pre- and post-localisation behaviour.•The material outside the localisation zone is assumed to undergo reversible deformation and saturation, while irreversible behaviour inside the localisation zone is the main source of dissipation.•The material behaviour inside the localisation band is described by a constitutive model with a single yield surface in stress-suction space governing the interdependence of two evolution rules for plastic strain and irrecoverable saturation degree.•The proposed formulation results in a size-dependent constitutive structure capable of describing the transition of behaviour of partially saturated soils from homogeneous to localised stages.