Influences of constitutive model for rockfill materials on calculated stress and deformation of concrete-faced dams

Choice of constitutive model for rockfill materials is the most influencing factor that affects the computed stress and deformation results of concrete-faced dams (CFD). In this study, the Duncan's E-B (EB) nonlinear elastic model and the Nanshui (NS) double-yield surface elastoplastic model are used for rockfill materials to simulate the construction and impounding processes of a typical CFD by using three-dimensional finite element method. The differences in the displacement and stress of rockfill zones and concrete slabs are compared, and the reasons were analyzed. Two most important differences are found. First, the displacement quantities such as the dam settlement and slab deflection predicted by the NS model are smaller than those obtained by the EB model, and this difference can be attributed to the higher minor principal stresses and therefore higher deformation moduli within rockfill zones by the former. Second, the EB model predicts a tensile zone along the upstream slope near the bottom of face slabs, while the results by the NS model shows that the slope stresses of face slabs are completely compressive. The tensile stress predicted by the EB model is due to its isotropic elasticity nature, i.e., the cushion materials expand along the slope under the huge hydrostatic pressure, leading to a displacement upward towards the dam crest. The NS model, on the other hand, predicts an additional plastic volume contraction within the upstream rockfill zones due to the expansion of F₁ yield surface, which completely counteracts the elastic expansion along the slope and results in a downward displacement of the cushion layer.