Gradient-dependent generalized plasticity mechanics (GGPM) in which the yield functions not only depend on stress tensor and an invariant plastic strain measure

but also depend on spatial derivatives of the latter quantity

is based on the generalized plasticity mechanics(GPM). It can describe the strain localization characteristics and the basic mechanical characteristics of geomaterials at the same time. In order to derive the equilibrium equations and the boundary conditions

an incremental functional in which the displacement

plastic multiplier and are independent variables

is constructed to build up the variational principle of GGPM. The stress boundary conditions as well as the plastic factor boundary conditions are obtained by natural boundary conditions

so a numerical model can be ensured for geomaterial strain localization.