A finite deformation multiplicative plasticity model with non–local hardening for bonded geomaterials

The paper presents a finite deformation, isotropic hardening, non–associative elastic–plastic constitutive model (FD_MILAN model) for describing the mechanical behavior of a wide range of bonded natural geomaterials such as stiff overconsonsolidated clays, porous soft rocks or bio–improved soils. The formulation of the model is based on the multiplicative split of the deformation gradient and on the assumption of hyperelastic behavior. To deal with the occurrence of strain localization, typically observed in this class of geomaterials, the model has been equipped with a non–local version of the hardening laws. This approach is capable of regularizing the pathological mesh dependence occurring in the post–localization regime when adopting classical plasticity models. In view of its application to practical geotechnical problems characterized by large displacements and deformations within a hydro-mechanical coupled environment, the model has been implemented in the recently developed Particle Finite Element code G–PFEM for geomechanics applications. To demonstrate the effectiveness of the numerical implementation, a series of numerical simulations has been performed considering two representative boundary value problems: the modeling of shear localization in plane strain biaxial tests and the simulation of CPTu tests in a saturated porous soil. The results of biaxial test simulations have highlighted the role of the characteristic length in controlling the thickness of the localized zone and the effect of the confining pressure in determining the pattern of shear band formation. An interesting feature emerging from the partially drained CPTu simulation results is the progressive formation of persistent shear bands, which originate from the cone tip and propagate outwards along the entire penetration depth.