Soil properties and seismic stability of old and new Fujinuma dams

The old Fujinuma main and auxiliary dams totally collapsed by the 2011 Off the Pacific Coast of Tohoku Earthquake (i.e., the East Japan Great Earthquake Disaster, 11 March 2011). They were restored to much more stable ones, completed January 2017. The soil types and compacted dry densities of the collapsed old dams and those of the new dams are reported. The results from a set of monotonic undrained triaxial compression tests were performed before and after cyclic undrained loadings developing different strains performed on the samples retrieved from the old dams and those of the materials planned to be used for the new dams compacted to different dry densities. By the Newmark-D method and the pseudo-static non-linear FEM using these data incorporating the continuing degradation of the undrained stress-strain properties by cyclic undrained loading, the collapse of the old main dam was simulated and a high seismic stability of the new main dam was confirmed. The results of the analyses indicate that the old dams collapsed due mainly to that: 1) the compacted state of the whole dams was generally poor; 2) the top fill consisted of particularly poor compacted sandy soil would have exhibited a significant reduction of undrained shear strength from a low initial value by cyclic loading and fast erosion; and 3) prolonged strong seismic motion. It is shown that the new dams are much more stable by better compacting better soil types in addition to a better structure.