Seismic damage mechanism of high concrete face rockfill dams

Based on the case study of Zipingpu concrete face rockfill dam (CFRD), the visco-elastic model for rockfill materials and the model for their residual strain are employed to investigate the seismic responses of high CFRDs, and the mechanism of their damage behaviors is studied combined with the field observations. The numerical results show that the input accelerations are considerably amplified within the top zone and the surface of the dam, and the so called whipping effect does exist in high CFRDs. Therefore, the rockfill materials near the crest and the downstream slope shake to be loose and some particles even fall along the slope. Strong shaking also results in evident shear contraction of rockfill materials and leads to considerable settlement of the dam. As a result of the contraction of the dam shell, the third stage concrete slabs separate from the cushion layer and the frictional forces between them disappear, and the shear failure occurs in the horizontal construction joints and tensile crack occurs on the surface of the cantilever slabs under the combined action of gravity and seismic inertia forces. During the earthquake, the rockfill materials move towards the center of the canyon, resulting in strong compressive stresses, and thus, the compressive failure within the slabs is near the center. In contrast, the vertical joints near the bank is generally suffered by tensile failure. Both the numerical results and the field observations show that the damage behaviors of high CFRDs subjected to earthquakes are closely related to the excessive residual deformation of rockfill materials and large difference in rigidities of concrete slabs and rockfill materials. Therefore, for building a high CFRD in earthquake regions, the compaction criteria for rockfill materials should be properly enhanced to reduce its possible earthquake-induced deformation.