In the present paper, the nonlinear seismic behavior of a concrete gravity dam- reservoir-massed foundation system is studied focusing on the foundation material nonlinearity. For this purpose, a finite element program utilizing the elasto-plastic formulation for the foundation medium is provided considering the tensile and shear failure modes. Hoek-Brown and Mohr Coulomb criteria are used as the yield and plastic potential function of the shear failure mode, respectively; and in the tensile failure mode, various levels of the tensile strength are applied to the foundation. Bearing capacity is studied considering the localized plastic elements and forming the sliding path by plastic elements is studied for investigating the sliding stability of the foundation. It is found that the foundation nonlinear model and the magnitude of tensile strength of the foundation can have significant effects on the failure mechanisms happened within the foundation rock.
